Terrestrial Carbon Removal and Sequestration
Definition of Terrestrial Carbon Sequestration
Terrestrial carbon sequestration is defined here as the increase in the amount and maintenance over time of organic carbon (OC) in biological stocks, driven by plant assimilation of CO2 from the atmosphere. Biological carbon stocks are largely controlled by actively cycling processes, that is, assimilation (carbon uptake from the atmosphere) and respiration (carbon loss to the atmosphere). The carbon stocks of interest are those that can accumulate and persist over multidecadal timescales, namely woody biomass and coarse woody debris and soil organic matter (SOM). More ephemeral carbon stocks, including herbaceous biomass and plant litter with short residence times (<1 y), are generally ignored in the context of carbon sequestration because they do not represent a persistent removal of CO2 from the atmosphere. Harvested woody biomass used for long-lived wood products may also accumulate and persist but is also subject to release of stored carbon as product use is discontinued—unless sequestered in a landfill. While the committee recognizes that technologies to increase carbon uptake by forestry and agriculture may cause increased emissions because of decomposition and disturbance of recalcitrant carbon in soils, virtually all the data reported are for net carbon uptake, determined from stock changes over time. Hence decomposition and disturbance impacts on carbon losses are included in the net values.
As a general principle, the overall balance of these carbon stocks is driven by the difference between carbon inputs (via plant CO2 assimilation) and carbon losses to the atmosphere (via decomposition/microbial respiration as well as from fire/combustion). Thus, the standing stocks of carbon can be increased by increasing the rate of input of carbon, decreasing outputs of carbon, or both. For woody biomass, this implies growing more trees with more biomass per unit area, and maintaining that biomass over a longer time span, and/or decreasing loss of woody biomass through tree mortality, fire, and harvesting operations. For soil OC, this means increasing the rate of input of plant-derived detritus to the soil and/or reducing the rate at which
the organic compounds added or already in the soil decompose and mineralize to CO2. Achieving and maintaining increases in biological carbon storage requires active management practices that manipulate system carbon balances, for which a variety of existing as well as potential future options exist.
Rationale for Terrestrial Carbon Sequestration
Global terrestrial biotic carbon stocks include ca. 600 Gt C in biomass and ca. 1,500 Gt C in soil to a depth of 1 m (ca. 2,600 Gt C to 2 m); annual fluxes from and to the atmosphere and terrestrial ecosystems, on the order of 60 Gt C yr-1, are roughly balanced but with an average net residual carbon uptake by terrestrial ecosystems of 1-2 Gt C (Le Quéré et al., 2016). However, historically the conversion of native ecosystems to managed land, particularly cropland, has been a large net source of CO2 flux to the atmosphere with a significant depletion of biomass and soil carbon stocks, particularly over the past ca. 200-300 years as human population and land transformation exploded. Recent estimates of the anthropogenic-induced losses of terrestrial carbon stocks are on the order of 145 Gt C from woody biomass and soils from 1850 to 2015 (Houghton and Nassikas, 2017), 133 Gt C from soil carbon stocks over the past 12,000 years (Sanderman et al., 2017), and 379 Gt C of biomass over the past 10,000 years (Pan et al., 2013). These estimates of historical losses indicate a hypothetical though highly impractical upper bound for restoring terrestrial carbon stocks through adjustments to land management.
The rationale for pursuing terrestrial carbon sequestration as a CO2 removal strategy is at least three-fold. First, the “technology” for carbon capture and storage already exists, via the uptake of CO2 by plants and its storage in longer-lived biotic carbon pools, providing opportunity for storage that is energetically and economically competitive with other carbon removal and mitigation options. Second, there is a significant storage capacity based on the magnitude of historical carbon stocks compared with much lower contemporary stocks. To a large extent, terrestrial carbon sequestration approaches can be seen as a reversal of previous ecosystem degradation—changing land use and management to favor greater biomass and soil carbon stocks. Third, increasing biomass and soil carbon stocks may confer additional ecosystem service benefits, including watershed protection, increased biodiversity, and improved soil health and fertility. In contrast, ecosystem service benefits, including biodiversity, may be reduced if the carbon sink–creating activities change disturbance regimes or other landscape characteristics that affect species. These are discussed in more detail below.
APPROACHES FOR TERRESTRIAL SEQUESTRATION
The committee’s assessment of terrestrial carbon sequestration options centers around the predominant types of managed ecosystems: forestland, cropland, and grassland, each with multiple management “levers” to modify carbon stocks. Other intensively managed land such as urban/peri-urban landscapes contain biotic carbon stocks in trees and greenspaces (e.g., parks, lawns), for which some of the sequestration practices described under the forestland and grassland category could apply. Wetlands are of particular importance because they contain the highest carbon stocks per unit area of any ecosystem. Where they have been drained and converted to crop, grazing, or forest production, approaches to recover carbon sequestration capacity will be discussed under each of the three main land-use types. Coastal ecosystems (i.e., “coastal blue carbon”), which exist on the edge of terrestrial and ocean ecosystems, are covered in Chapter 2. Bioenergy with carbon capture and storage (BECCS) is likewise covered in Chapter 4, with references to this chapter regarding land requirements for all approaches.
In describing technical options, the committee distinguishes between “conventional” and “frontier” technologies or practices to increase carbon stocks via CO2 removals. Conventional practices refer to management practices that are already in use, to a limited degree, and have an existing body of applied research knowledge. Increasing carbon stocks via these practices mainly requires a much greater degree of participation by land managers and refinement in their applicability and support services to facilitate broader adoption. In contrast, frontier technologies are still in the basic research phase and have not been tested for widespread deployment (e.g., perennial grain crops) or involve practices (e.g., “enhanced wood preservation”) that fall outside the framework of current land management practices.
Conventional Forest Practices
Forestry practices that can act to remove CO2 and reduce emissions fall into three categories of action: (1) avoiding conversion of forest land to other land uses (deforestation) that store less carbon and have lower rates of removal, (2) converting nonforest land to forest (afforestation/reforestation) thereby increasing carbon removal and stocks, and (3) modifying forest management practices to either increase carbon stocks or increase net removals from the atmosphere (balance of gains and losses of CO2), or both. Modifying forest management involves an array of practices, such as accelerated regeneration of nonstocked forest after disturbance, which increases carbon removal in the near term; restoring degraded forests to healthier and sustainable
conditions that maintain removal capacity, and extending the rotation length (age of forest at harvest); which maintains removal capacity, avoids emissions associated with wood harvest, and directs more biomass into long-lived wood products that store harvested carbon. Altering management and wood product design to foster preservation of more long-lived wood products that store the harvested biomass carbon is a fourth category that is treated later in this chapter as a frontier technology.
Assessment of the carbon consequences of forestry activities is complicated by the many different direct and indirect effects on the carbon cycle. Forest ecosystems are composed of three main carbon pools that respond differently to harvesting, management, and other disturbances: live biomass (above- and below-ground); standing and down dead wood; and SOM including surface litter, humus, and mineral soil layers. These carbon pools respond to disturbances over periods of years to decades such that accounting for impacts is an ongoing process. Besides ecosystem impacts, harvested wood products have multiple effects on the carbon cycle including their function as temporary storage of removed carbon while in use or disposal, substitution of wood for other construction materials that require substantial quantities of fossil energy to produce (avoided emissions), and use of wood for biofuel, which may reduce net emissions relative to burning fossil fuels (discussed in Chapter 4).
Although avoiding deforestation typically has the most significant and immediate impact on a per-hectare (ha) basis by reducing emissions and sink capacity, it is not explored in depth in this report because it is mainly an emissions reduction or avoidance activity rather than an activity that increases CO2 removal. Avoiding deforestation is a potentially significant activity, with avoided emissions ranging from 56 to 116 Mg C/ha/y in the United States and 96-103 Mg C/ha/y globally (EPA, 2005; Griscom et al., 2017).
Afforestation/reforestation has been extensively studied and implemented in the United States and around the world, so may be considered immediately deployable for carbon removal purposes. Afforestation/reforestation involves planting trees or facilitating natural regeneration of trees on land that has been in a nonforest use condition for some length of time. The committee considers regeneration of a harvested forest as a forest management practice even though tree cover may have been temporarily reduced to a level typical of nonforest use. Foresters have planted new forests or reforested nonforest lands for many decades and have good knowledge of which species are likely to be successfully established and which areas are suitable for
planting or assisted natural regeneration (Sample, 2017). In regions where tree planting is associated with timber production, such as the U.S. Southeast and Pacific Northwest, provenance trials, tree breeding programs, and genetic improvements have progressed to identify trees that have good survival rates, grow fast, and have properties that are most suitable for wood products (Burley, 1980). However, to date, carbon removal rates have not been the target of research about growing stock improvements; instead, improvements in planting stock have been targeted to increase timber production (above-ground biomass) while the effects on below-ground biomass and soil carbon remain largely unknown (Noormets et al., 2015). As a result, there is a need to consider selections of trees that would increase whole-tree biomass as a means to increase their function as carbon removal organisms (e.g., Muchero et al., 2013). Data on tree growth and carbon removal rates are readily available, as are guidelines for successful planting and natural regeneration. Table 3.1 illustrates some of the more common and representative carbon removal rates that span a wide range of values because of heterogeneous climates, sites, and forest types.
Rates of ecosystem carbon increase with afforestation/reforestation in the United States range from 0.7 to 6.4 Mg/ha/y for a period of 50-100 years or more (Table 3.1). The carbon removal rate is variable over this time period, generally following a growth
TABLE 3.1 Published Estimates of Biomass Carbon Sequestration Rates for Various Forestry Activities
|Activity||Net stock increase
(Mg/(ha y) C)
|Afforestation/Reforestation||2.8-5.5||Griscom et al., 2017|
|Afforestation/Reforestation||3.4||Smith et al. 2016|
|Improved forest management||0.2-1.2||Griscom et al., 2017|
|Activity||Net stock increase
(Mg/(ha y) C)
|Improved forest management||1.4-2.5||Denef et al., 2011|
curve with a diminishing rate of increase as the forest reaches maturity (Figure 3.1). However, if the activity is initiated over a period of years on different parts of the landscape, which would be likely given infrastructure and funding constraints, the idealized pattern of incremental carbon storage may be more linear except for the early years (Figure 3.2). Estimates of the potential carbon sink from afforestation/reforestation include changes in biomass and soil carbon without consideration of impacts of future harvesting or natural disturbances. Disturbances would release stored carbon from the ecosystem. In the case of harvesting, some of the carbon lost from the ecosystem would be retained in wood products or landfills. Increases in harvested wood products may also reduce emissions through substitution of wood for other types of construction materials that require more energy to produce. Emissions reductions from increasing harvested wood in place of other materials are not estimated here even though potentially significant, because these activities do not represent increases in carbon removal.
Improved Forest Management
Improved forest management has been extensively studied in developed countries with respect to impacts on carbon removal, feasibility, and cost; therefore, this technology may be considered as immediately deployable. Forest management involves a very wide array of practices that have been tailored for regional and local conditions based on experience and extensive silvicultural experiments, many of which have been ongoing for decades (Malmsheimer et al., 2011; McKinley et al., 2011). In general, promising forest management practices to increase carbon removal include the following:
- Accelerating regeneration in areas that have had major disturbances;
- Restoring forests that have been converted to “unsustainable” forest conditions, which includes both increasing carbon stocks by returning a forest to its original vegetation type that is better adapted to site and climate, and reducing carbon stocks of overstocked stands to levels that are less likely to have intense wildfires that convert the forest to the much lower carbon density of nonforest;
- Extending harvest rotations to grow larger trees and sustain carbon removal rates (not to mention avoid emissions associated with harvest);
- Maintaining healthy forests by treating areas affected by insects and diseases or preventing conditions that foster outbreaks; and
- Thinning and other silvicultural treatments that promote overall higher stand growth compared with untreated conditions.
Changing forest management practices such as extending timber harvest rotations and improving stocking and productivity could store an additional 0.2 to 2.5 Mg C/ha/y for several decades based on U.S. and global estimates (Table 3.1). These estimates include changes in biomass and soil carbon but exclude changes in the stock of harvested wood products. Emissions reductions from increased use of harvested wood in place of other materials are not estimated here even though potentially significant, because these activities do not represent increases in carbon removal.
The range of potential benefits from improved forest management is very wide because of the need to account for different land use and management histories as well as the many different climates, site conditions, and forest types. Generally, the per-hectare values are significantly less than observed for afforestation/reforestation because the growing stock of trees in an existing forest in the absence of management will still take up significant quantities of CO2; therefore, the additional uptake of CO2 is less than if comparing forest with no forest (McKinley et al., 2011).
Frontier Forest Practices
Increase Harvested Wood Use and Preservation
According to many studies, the main climate benefit of increasing the use of harvested wood products is emissions reductions from the substitution of wood products for materials such as concrete and steel that require more fossil fuels for production (e.g., Hashimoto et al., 2002; Perez-Garcia et al., 2005; Sathre and O’Connor, 2010). However, increasing the preservation of harvested wood by improving preservative treatment methods (Song et al., 2018) or advanced landfilling could be a significant CO2 removal approach with additional economic benefits. Zeng (2008) proposed harvesting live trees and other biomass from managed forests and burying the logs in trenches or otherwise storing them to prevent the carbon from being released. Improving the preservation of wood products from existing harvest operations and potentially increasing harvest with high levels of product preservation could be viable approaches to increasing carbon removal.
Wood removals from the world’s 4 billion ha of forests average around 3 billion m3/y or 0.65 percent of the growing stock, of which about half is for timber products and
half for fuel (FAO, 2015b). During harvest, a significant portion of the live biomass is left in the forest as logging slash (not including tree roots), ranging from roughly 30-40 percent of the pre-harvest biomass at the national scale, with much higher variability at more local scales (Oswalt et al., 2014; Winjum et al., 1998). Some of this logging slash is used for fuelwood and charcoal. Much of industrial wood removals comes from approximately one-half of the world’s forests designated for timber production or multiple use (FAO, 2015b).
Much of the biomass removed from forests for timber products is emitted during primary processing into products, with losses ranging from roughly 20 percent to 60 percent depending on conversion efficiency (Bergman and Bowe, 2008; Ingerson, 2009; Kline, 2005; Liski et al., 2001). After the end of their useful life, wood products are typically deposited in landfills that are often designed for relatively rapid decomposition, or subject to other fates such as deposit in dumps that emit their stored carbon (Skog, 2008). According to several estimates, about 0.5 to 0.7 Gt/y CO2 of the harvested carbon is sequestered in use or landfills after accounting for the inputs from current harvests and inherited losses from past harvests (Hashimoto et al., 2002; Miner and Perez-Garcia, 2007; Pan et al., 2011; Winjum et al., 1998).
Conventional Cropland and Grassland Practices
The vast majority of agricultural lands around the world are not managed optimally for increased soil carbon storage. Most annual croplands in temperate climates have bare-fallow conditions outside of the main crop-growing season, and intensive tillage practices are still widespread. Some annual cropland is on marginal lands that are subject to continuing soil degradation. Many pastures and rangelands employ unimproved grazing systems and suboptimal forage management. However, numerous conservation management practices are available that can increase carbon stocks in soils and are successfully practiced by progressive farmers and ranchers. In many cases these practices have been well studied, with long-term field experiments and comparative field observations. Table 3.2 lists several categories of management practices, classified according to their main mode of action in either increasing carbon inputs to soils and/or reducing carbon losses from soils.
Following implementation of improved practices, soil carbon accrual rates can continue over several decades but attenuate over time as soil carbon contents tend toward a new equilibrium state with no further carbon gains unless additional carbon-accruing management practices are adopted. Furthermore, because of the dynamics of mineral-organic matter interactions that largely control the residence time of
TABLE 3.2 Examples of Conventional Agricultural Management Actions That Can Increase Organic Carbon Storage and Promote a Net Removal of CO2 from the Atmosphere
|Management Practice||Increased carbon inputs||Reduced carbon losses|
|Increased productivity and residue retention||X|
|No-tillage and other conservation tillage||X||X|
|Manure and compost addition||X|
|Conversion to perennial grasses and legumes||X||X|
|Rewetting organic (i.e., peat and muck) soils||X|
|Improved grazing land management||X||X|
SOURCE: Adapted from Paustian, 2014.
carbon in soil (Lehmann and Kleber, 2015), soils with initially low carbon concentrations have a greater propensity to gain carbon compared to soils with already high carbon concentrations. Thus there is effectively a “saturation limitation,” which varies as a function of soil texture and mineralogy (Stewart et al., 2007) and hinders further carbon accrual for mineral soils with very high organic matter contents.
Improved Annual Cropping Systems
On annual croplands, farmers may adopt several cropping choices that increase inputs of carbon into soils: replacing winter bare-fallow with seasonal cover crops, planting crops that produce large amounts of residues, promoting more continuous cropping (reduced summer fallow frequency) in semi-arid environments, and increasing the proportion of perennial grass/legume forage crops within crop rotations. Such cropping choices can maximize the time during which live vegetated cover is maintained on the soil and increase the amount of root-derived carbon added to the soil (Rasse et al., 2005). In the past few years, interest has grown in the use of cover crops, which have been strongly promoted by the U.S. Department of Agriculture’s (USDA’s) Natural Resource Conservation Service. Cover crop adoption is rising; however, adoption rates in the United States remain low (<5 percent of cropland area; USDA, 2014). This reflects unfamiliarity for growers, barriers from additional costs, restrictions related to
crop insurance, and possibly immature technology in some areas. Systems to increase cropping frequency and reduce summer-fallow in semi-arid cropland have been successful in increasing productivity as well as soil carbon stocks (Peterson et al., 1998), although large areas dominated by alternate year wheat summer-fallow practices remain. Wider adoption of crop rotations that incorporate 2 to 3 years of grass or legume hay with annual crops (common in mid-20th century Corn Belt agriculture) is limited by the higher prices for main commodity crops (e.g., corn, soybean), which encourages continuous grain mono-cropping. Hence, for a variety of reasons, best cropping practices for sequestering carbon are not widely used on U.S. annual cropland, which means there is ample room to increase adoption rates if soil carbon sequestration becomes a more prominent policy goal.
Farmers use tillage to manage crop residues and prepare a seed bed for crops, and it is the main source of soil disturbance in croplands. Intensive tillage tends to accelerate decomposition rates of SOM (Paustian et al., 2000). Advances in tillage implement technology and agronomic practice have allowed farmers in recent decades to reduce tillage frequency and intensity, sometimes ceasing tillage altogether with a practice known as “no-till.” Reduced tillage systems, particularly no-till, can increase the mean residence time and slow decay of SOM (Six and Paustian, 2014), promoting greater soil carbon storage (Table 3.3). Many field studies show increases in soil OC following adoption of reduced till and no-till, with variations due to soil texture and climate. However, there are instances in which no-tillage does not increase soil carbon relative to conventional tillage, particularly in wet, cool climates where productivity under no-till may be reduced (Ogle et al., 2012) and in soils with already high topsoil carbon contents, where organic matter stabilization efficiency may be lower than for residues mixed deeper into the soil with tillage (Angers and Eriksen-Hamel, 2008; Ogle et al., 2012). No-till and reduced tillage are not widely adopted on most annual cropland in the United States, although there is significant regional variation.
In summary, a combination of an extended vegetated period, increased residue (particularly root-derived) inputs, and minimal tillage systems could be broadly implemented as best practices for increasing soil carbon stocks on annual cropland. Rates vary significantly as a function of climate, soil, and land-use history, but rates of 0.2-0.5 tC/ha/y over time periods of 20-40 years are representative, as observed in numerous meta-analyses of long-term field experiment data both globally and in the United States (Table 3.3).
TABLE 3.3 Examples of Published Summaries and Meta-Analyses for Soil Carbon Sequestration Rates with Conservation Practice Adoption on Annual Cropland
|Practice(s) adopted||Δ SOC (SE) (t/(ha y))||Region||# of field comparisons||Source|
|Cover crop||0.32 (0.08)||Global||139||Poeplau and Don, 2014|
|Cover crops||0.36||US||31||Eagle et al., 2012|
|Improved rotations||0.14-0.18a||US||78||Eagle et al., 2012|
|Improved rotations||0.1-0.21b||Global (Temperate-dry)||13||Ogle et al., 2005|
|Improved rotations||0.17-0.34b||Global (Temperate-moist)||13||Ogle et al., 2005|
|No-till||0.48 (0.13)||Global||276||West and Post, 2002|
|No-till||0.15-0.80b||Global||160||Ogle et al., 2005|
|No-till||0.33||US||282||Eagle et al., 2012|
|No-till||0.30 (0.05)||Southeast US||60||Franzluebbers, 2010|
|No-till + cover crops||0.55 (0.06)||Southeast US||87||Franzluebbers, 2010|
|No-till||0.48 (0.59)||Northcentral US||19||Johnson et al., 2005|
|No-till||0.27 (0.19)||Northwest US||40||Liebig et al., 2005|
a Range in mean values is for rotation improvements from elimination of bare (summer) fallow and adding perennials forages (for 1-3 years) to annual crop rotations.
bThese studies reported annual changes in SOC stock (0-30 cm depth) as percentage changes relative to baseline soil carbon stocks. To convert to an amount (t/ha/y) for a representative range of SOC change rates, the committee used 15 tC/ha and 60 tC/ha (0-30 cm depth) as high and low stock values, respectively. These correspond to low and high values for carbon stocks on permanent cropland amount major soil classes in the Intergovernmental Panel on Climate Change (IPCC) soil carbon inventory methodology (IPCC, 2006). NOTE: Delta SOC denotes mean change in soil OC stocks (t C/ha/y) with standard error (SE) (where reported), or in some cases values are reported as a representative range in mean annual change rates.
Organic Matter Amendment
Organic matter additions such as animal manures and composts can increase soil carbon contents, both by virtue of the added carbon in the amendment itself and through improving soil physical attributes and nutrient availability. Cropland soils receiving substantial organic amendments invariably show increases in soil carbon concentrations derived from the amendment itself. However, this does not equate necessarily with a CO2 removal from the atmosphere, but simply a transfer of carbon from another location (Leifeld et al., 2013). To the extent that the amendments improve soil performance and thus increase in situ plant productivity and residue carbon inputs, then the amendment can in fact stimulate real increased CO2 removals. Hence a full life cycle assessment (LCA) approach in which the boundaries of the assessment extend outside the farm to include the greenhouse gas (GHG) and net carbon emissions with the production and baseline utilization of the amendment is needed for an accurate accounting of net CO2 impacts. Because nearly all raw animal manure is land applied (Wu et al., 2013), increasing rates of manure addition at one location would necessitate an equal reduction elsewhere. Hence, counting the manure carbon added as part of the overall carbon balance is problematic. For other waste streams, such as municipally sourced compost, where the alternative use may be landfilling, an LCA approach that accounts for net emissions associated with a landfilling alternative vs production and land application of compost could be evaluated as a CO2 emission strategy. One study of compost applications to grasslands in California (DeLonge et al., 2013; Ryals and Silver, 2013) estimated a net CO2 removal and GHG reduction of 23 tCO2e/ha for the first 3 years following compost application, because of soil carbon increase (discounting added compost carbon) and net avoided GHG emissions from diverting lagoon-stored manure and landfilled green waste to compost. However, similar studies are lacking for other areas in the United States.
Conversion of Annual Cropland to Perennial Vegetation
Probably the most effective means of increasing soil carbon stocks on annual cropland is to convert to perennial vegetation, either for grazing and forage production, afforestation (see previous section), dedicated energy crops (e.g., switchgrass, miscanthus), or as conservation set-aside. Perennial grasses in particular allocate a large fraction of their carbon assimilates to below-ground production, which enter the SOM pool through root exudation, sloughing, and turnover. Over time, in the absence of long-term soil degradation, SOC stocks can approach or equal those in precultivation native grassland systems. Perennial grasses purposed as dedicated energy crops (see Chapter 4) and established on former cropland typically increase soil carbon stocks,
which can contribute to the net GHG balance of those systems (Field et al., 2018; Liebig et al., 2008). Conversion to perennial woody vegetation (e.g., forest) increases stocks of woody biomass carbon but may also increase soil carbon stocks. In many cases, accrual rates are similar to those for conversion to grassland vegetation (Guo and Gifford, 2002; Post and Kwon, 2000), although some studies suggest that soil carbon gains occur under deciduous species whereas soil carbon gains under coniferous woody vegetation are minimal (Laganière et al., 2010; Morris et al., 2007). Several meta-analyses have documented soil carbon gains on cropland converted to perennial vegetation (Table 3.4).
TABLE 3.4 Examples of Published Summaries and Meta-Analyses for Soil Carbon Sequestration Rates Following Conversion of Annual Cropland to Perennial Grassland or Forest Vegetation
|Practice(s) adopted||Δ SOC (SE) (t C ha/y)||Region||# of field comparisons||Source|
|Annual cropland to grassland||0.9 (0.1)||Global||161||Conant et al., 2017|
|0.12-1.1a||Global||58||Ogle et al., 2005|
|0.28-0.56||Global||76||Guo and Gifford, 2002|
|0.33||Global||46||Post and Kwon, 2000|
|Annual cropland to forest||0.27-0.54||Global||38||Guo and Gifford, 2002|
|0.34||Global||30||Post and Kwon, 2000|
|0.16-1a||Global||189||Laganière et al., 2010|
a These studies reported annual changes in SOC stock (0-30 cm depth) as percentage changes relative to baseline soil carbon stocks. To convert to an amount (t/ha/y) for a representative range of SOC change rates, we used 15 tC/ha and 60 tC/ha (0-30 cm depth) as high and low stock values, respectively. These correspond to low and high values for carbon stocks on permanent cropland amount major soil classes in the IPCC soil carbon inventory methodology (IPCC, 2006).
NOTE: Delta SOC denotes mean change in soil OC stocks (tonnes C/ha/y) with standard error (SE) (where reported), or in some cases values are reported as a representative range in mean annual change rates.
Agroforestry involves the incorporation of trees into agricultural systems, either in combination with annual crops or grazed pastures (the later often referred to as silvopastoral systems). There are many different practices in which trees may be interspersed with crops (e.g., alley cropping) or herbaceous forage, used as border or buffer plantings (e.g., living fences, windbreaks, forest buffers) or used in a time sequence or rotation with annual crops (e.g., improved tree fallows). Regardless of form, the inclusion of perennial woody species in combination with annual crops or pastures typically results in increased soil carbon stocks and woody biomass stocks (Table 3.5).
Improved Grazing Land Management
Grasslands contain some of the highest soil carbon stocks of any managed ecosystems. With the exception of some managed pastures, grazing lands are infrequently or never tilled, and the perennial grasses that dominate in most rangelands and pasture allocate a substantial portion of their photosynthetically fixed carbon below ground, thus supporting comparatively large soil carbon stocks.
Grasslands in the United States can be loosely categorized into pastures, which are typically in more mesic (i.e., moister) environments, with higher productivity, often with selected (sown) species and more intensive soil management such as fertilization, liming, and irrigation. Pastures in some cases may be rotated with annual crops
TABLE 3.5 Examples of Carbon Accrual Rates for Biomass and Soil Carbon from Recent Meta-Analyses and Multisite Assessments of Temperate Zone Agroforestry Practices
|Practice(s) adopted||Δ C stock (t ha/hr)||Years||Region||# of Field Comparisons||Source|
|Alley cropping (wood carbon)||0.65||6-41||France||13||Cardinael et al., 2017|
|Alley cropping (soil carbon)||0.24||6-41||France||6||Cardinael et al., 2017|
|Agroforestry (soil carbon)||0.43-1.88||NR||US||NR||Eagle et al., 2012|
NOTES: Delta carbon denotes mean change in soil OC or biomass stocks (tonnes C/ha/y), or in some cases values are reported as a representative range in mean annual change rates. NR denotes not reported.
and typically have soils that were tilled at some time in the past. In contrast, rangelands are mainly found in semi-arid and arid climates, are usually dominated by native grassland vegetation, often have never been tilled, and usually have no (or minimal) management interventions in terms of fertilizer or irrigation use. Grazing management is the primary management “lever” for rangelands.
Grazing management influences grassland productivity and carbon storage (Table 3.6) in both pastures and rangelands, via the amount, timing, and duration of vegetation removal by grazing animals (Milchunas and Lauenroth, 1993). With excessive grazing pressure (overgrazing), plant productivity and hence carbon uptake is reduced and soil carbon stocks decrease (Dlamini et al., 2016). Therefore, reducing stocking rates and grazing intensity can allow vegetation productivity to recover and carbon stocks to increase. However, other than eliminating overgrazing, there is considerable debate regarding the impact on soil carbon of more intensively managed grazing systems (i.e., rotational grazing, adaptive multipaddock, mob grazing), compared with less intensively managed, continuous grazing systems. It has been suggested that management-intensive grazing—short heavy-grazing periods followed by long grazing-free periods—can increase soil carbon storage (Chaplot et al., 2016; Wang et al., 2015), although many experiments do not show a significant difference between light to moderate continuous grazing vs management-intensive grazing (Briske et al.,
TABLE 3.6 Examples of Soil Carbon Sequestration Rates from Recent Meta-Analyses and Field Studies of Improved Grassland Management Practices
|Practice(s) adopted||Δ SOC (SE)
(t C ha/y)
|Region||# of field comparisons||Source|
|Improved fertility||0.57 (0.08)||Global||108||Conant et al., 2017|
|Legume inter-seeding||0.68 (0.22)||Global||13||Conant et al., 2017|
|Improved grazing||0.3 (0.14)||Global||89||Conant et al., 2017|
|Improved grazing||-0.8 to +1.3||US||13||Eagle et al., 2012|
|Adaptive multipaddock||0.48||US (Texas)||6||Teague et al., 2011|
NOTE: Delta SOC denotes mean change in soil OC stocks (tonnes C/ha/y) with standard error (SE) (where reported), or in some cases values are reported as a representative range in mean annual change rates.
2008). However, the interactions that determine vegetation productivity and soil carbon responses for different grazing systems are complex (McSherry and Ritchie, 2013). In general, grazing systems that maintain plant cover and maximize plant vigor and productivity are most conducive for building and maintaining soil carbon stocks (Eyles et al., 2015). Additional field-based research and improved models are needed to determine the best grassland- and region-specific systems for promoting soil carbon sequestration and their capacity for net GHG reductions and CO2 removal (Conant et al., 2017; Eyles et al., 2015).
Rewetting of Organic Soils
Organic soils (i.e., peat and muck soils), which develop under wetland vegetation and are saturated with water to the soil surface for a substantial part of the year, have extremely high organic matter content (>12 percent of soil mass as OC, for a minimum 20 cm horizon depth; FAO, 1998) that can extend to several meters in depth. When converted to agricultural use (or forest plantations) following drainage, liming, and fertilization, they can be extremely productive soils, but with rapid rates of organic matter decomposition and carbon loss rates as high as 20 tC ha/y (Armentano and Menges, 1986). Consequently, restoring the wetland hydrology and perennial vegetation can reverse the processes driving soil carbon losses and greatly reduce CO2 losses compared to drained organic soils and in many cases can reestablish the soil as a net carbon sink, although increased methane (CH4) emissions following rewetting can decrease the overall net sink. In a global review, Wilson et al. (2016) reported that rewetting managed organic soils led to net annual CO2 removals for most soil classes (usually <1 tC ha/y), varying as a function of thermal regime, site productivity, water table height, and time since restoration. Most lacking are data for impacts of restoration on tropical organic soils.
Frontier Cropland and Grassland Practices
The management interventions described above are well known and used to varying degrees in the United States and elsewhere and have been the subject of previous (and ongoing) research. In most cases, causal mechanisms and relative magnitude of carbon stock responses are known, although important basic and applied science questions remain. In contrast, the committee characterizes several approaches as “frontier technologies” because they are in an early stage of development; many questions regarding rates and capacities for carbon removals remain unanswered, and there has been little or no deployment of these technologies outside of field research
situations. Two of the technologies involve increasing the residence time and stability of OC in soils, and the other two involve developing plants with greatly enhanced capacity to add carbon to the soil.
Pyrolysis (heating in the absence of oxygen) of plant material is an exothermic (i.e., energy-yielding) process that produces volatile compounds and oils (which can be purposed for bioenergy and biomaterials), as well as solid, “charred” organic residues with high aromaticity and reduced O:C and H:C ratios, that are much more resistant to microbial decomposition than the original plant residues (e.g., straw, wood, shells). The longevity of biochar applied to soil can vary substantially depending on the pyrolysis temperature and duration, biomass type, and climate and soil conditions. Highly condensed, low H:C chars derived from wood are highly recalcitrant and can remain in soils over century timescales (Lehman et al., 2015). Naturally occurring pyrogenic carbon (e.g., black carbon, charcoal) comprises a substantial portion of the total soil OC in many fire-prone grassland and forest ecosystems (Skjemstad et al., 2002), and additions of charred biomass by Neolithic farmers have resulted in soils with greatly increased OC stocks (e.g., Terra Preta soils of the Amazon; Glaser and Birk, 2012).
Pyrolysis is a bioenergy pathway (predominantly for the production of liquid biofuels) that could be part of a bioenergy production strategy (see Chapter 4). Pyrolysis BECCS aims to both displace fossil-based fuels as well as produce a biochar coproduct that can be added to soils for long-term carbon storage. Alternatively, purposed biochar production, with the pyrolysis liquids and volatiles combusted for process heat and/or energy generation, can produce biochar as the main product for soil application to sequester carbon and improve agronomic performance. The extent to which biochar production and addition to soil represents a net CO2 removal depends on a full lifecycle consideration of all GHG emissions associated with the biomass sourcing, its production and use, and fossil fuel offset values (Roberts et al., 2010; Woolf et al., 2010). In addition, indirect impacts of soil application of biochar on other biogenic GHG emissions and plant carbon uptake factor into assessments of its potential to reduce net GHGs. Recent meta-analyses suggest that biochar applications may reduce nitrous oxide (N2O) emissions from soil on the order of 10 percent (Verhoeven et al., 2017), although the response varies between soils, with some biochar additions increasing N2O flux. Sánchez-García et al. (2014) suggest that biochar decreases N2O emissions from the denitrification (anaerobic) pathway but increases emissions from the nitrification (aerobic) pathway, possibly accounting for some of the differences between soil types and soil conditions in biochar effects on N2O flux. Biochar applications also
appear to decrease CH4 emissions from flooded (e.g., rice) soils and in acid soils (Jeffery et al., 2016), whereas application to nonflooded soils with neutral or alkaline pH may reduce rates of CH4 oxidation (a CH4 sink). Finally, biochar application often increases plant productivity and hence plant carbon inputs to soil, particularly in highly weathered and acidic soils, averaging about a 10 percent increase in recent meta-analyses (Jeffery et al., 2011; Verhoeven et al., 2017). However, plant responses vary by soil type, biochar attributes, and management systems, and thus further research is needed to better identify conditions under which biochars can best increase plant carbon uptake, reduce non-CO2 GHG emissions, and contribute to net CO2 removals from the atmosphere.
Deep Soil Inversion
Placing organic matter–rich surface soil into subsoil layers, through a one-time deep inversion tillage, may substantially increase the overall SOM residence time. For most soils, microbial activities decrease sharply with depth, because of less aeration, cooler temperatures, and sparser and more dispersed organic matter inputs. In addition, the placement of low organic matter content subsoil at the top of the soil profile where the majority of the root system resides will increase soil carbon sink generated by root system and photosynthesis process. Alcantara et al., 2016 found that deep plowing and burial (to >50 cm depth) of organic matter–rich surface soil horizons, coupled with subsequent enhanced carbon accumulation in surface-exposed and carbon-depleted subsoil horizons, yielded an average carbon accumulation rate of ca. 3.6 tCO2 ha/y over a 40-year time period at several cropland sites in Germany. Testing of deep inversion tillage for carbon storage has recently started in some other locations (e.g., New Zealand; M. Beare personal communication), but not to the committee’s knowledge in the United States. A large portion of U.S. cropland in humid regions, particularly in the Central and North-Central states, might be potential areas for implementation.
High Carbon Input Crop Phenotypes
The most direct way to increase soil carbon storage is to increase the rate of OC addition to soils in the form of plant residues, particularly below ground. Here we discuss two approaches: (1) increasing the relative amount of carbon allocated to roots for the annual crops that dominate current agricultural food production systems and (2) developing new perennial food crop species, which have inherently higher
below-ground carbon allocation and could mimic the high soil carbon storage capacity found in perennial pasture grasses.
Breeding for Annual Crops with Increased Carbon Input
A main reason that annual crops are less effective at increasing soil OC stocks compared to perennial grasses is the smaller amount of dry matter allocation below ground to roots and rhizosphere deposition. Most soil OC is derived from roots (as exudates and through root death and turnover; Rasse et al., 2005), and thus an option to increase soil OC stocks would be to develop crops that allocate more dry matter below ground and/or have deeper root systems where the decomposing OC compounds would have a longer mean residence time (Kell, 2012). Increasing carbon allocation to roots without reducing above-ground yields will be a key issue for crop breeders, but is possible for several reasons. Many root characteristics (e.g., architecture, depth distribution, size) are strongly controlled by genetic traits that can be selected for (Hochholdinger et al., 2004; York et al., 2015). Where available nutrients such as phosphorus limit yields (common in tropical soils), enhanced root growth can increase nutrient acquisition and increase total assimilation and yield (Lynch, 1995). Finally, sink size inhibition often limits total plant carbon assimilation and thus plant breeding because an increased root carbon sink does not necessarily reduce above-ground productivity and yields (Jansson et al., 2010). Preliminary estimates suggest that widespread adoption of annual crops with enhanced root-phenotypes could increase carbon stocks in U.S. soils by 500-800 Mt/y CO2e over several decades (see Table 3.7).
Perennializing Grain and Oilseed Crops
The dominant commodity crops in the world are grains (e.g., maize, wheat, rice, sorghum, millet) and oilseeds (e.g., soybean, pulses, sunflower), which must be replanted each year and have been bred to maximize dry matter allocation to the harvested seed. Compared to perennial grasses and forbs, current varieties for these annual crops allocate much less carbon to roots and unharvested residues and are less effective at building and maintaining high soil carbon stocks. Thus, developing seed crops with a perennial growth habit and greater below-ground carbon partitioning could radically increase the potential for soil carbon gains, if perennialized crops could substitute for a substantial portion of current production from annual crops.
Research to develop perennial analogs for major annual crops commenced only within the past few decades (Glover et al., 2010) and only at a limited scale.
TABLE 3.7 Published Estimates of Global and U.S. Soil Carbon Sequestration Potential on Managed Nonforested Lands
|Global Estimates||Gt/y CO2e|
|Paustian et al., 1998||1.5-3.3||Improved cropland management, set-aside, restoration of degraded land|
|Lal and Bruce, 1999||1.7-2.2||Improved cropland management, restoration of degraded landa|
|IPCC, 2000||3||Improved cropland and grassland management, set-aside, agroforestry, restored peat soils|
|Lal, 2004||1.5-4.4||Improved cropland and grassland management, set-aside, agroforestry, restored degraded lands|
|Smith et al., 2008||5-5.4||Improved cropland and grassland management, set-aside, agroforestry, restored degraded lands, restored peat soilsb|
|Sommer and Bossio, 2014||2.5-5.1||Improved cropland and grassland management, set-aside, agroforestry, restored degraded lands|
|Paustian et al., 2016b||2-5||Improved cropland and grassland management, set-aside, agroforestry, restored degraded lands, restored peat soils|
|Paustian et al., 2016b||4-8||Potential from the practices above, plus unconventional technologies including high root carbon input crop phenotypes and biochar amendments|
|Griscom et al., 2017||3-5||Conservation agriculture, agroforestry, improved grazing, avoided grassland conversion, biochar|
|Fuss et al., 2018||2.3-5.3||Improved cropland and grassland management, set-aside, agroforestry, restored degraded lands, restored peat soils|
|U.S. Estimates||Mt/y CO2e|
|Lal et al., 1998||275-639||Land conversion and set-asides, restoration of degraded land, improved management on cropland|
|Sperow et al., 2003||305||Improved cropland management, set-aside of marginal (highly erodible) cropland to grassland|
|Sperow, 2016||240||Improved cropland management, set-aside of marginal (highly erodible) cropland to grassland|
|U.S. Estimates||Mt/y CO2e|
|Chambers et al., 2016||250||Improved cropland and grassland management, set-aside of marginal (highly erodible) cropland to grasslandc|
|Paustian et al., 2016a||500-800||Deployment of enhanced root phenotypes for major annual crops (assumes 2X root carbon input and downward shift in root distribution equivalent to native prairie grasses)d|
a An additional 1-1.5 Gt CO2e emission reduction was projected from biofuel CO2 offsets.
b This study also included an estimate of “economic potential:” about 2.5 Gt/yCO2e was achievable for < $50 tCO2.
c Based on estimate for widespread adoption of USDA National Resources Conservation Service’s conservation practices on all private lands.
d Excluded nonirrigated semi-arid cropland with major water limitation on production.
NOTE: All values reflect technical or “biophysical” potential estimates that are not constrained by carbon price or policy design.
Approaches include creating perennial hybrids between related annual and perennial species (e.g., rice [Zhang et al., 2017a]; wheat [Hayes et al., 2012]; sorghum [Cox et al., 2018]) and domesticating naturally large-seeded wild perennials to further increase seed yield and quality (e.g., intermediate wheatgrass [Culman et al., 2013]; perennial sunflower [Vilela et al., 2018]). The major challenge is developing perennial grains with sufficiently high yields to be viable from an economic and food security perspective, with the most promising results to date shown for perennial rice (Zhang et al., 2017a). It has been argued that evolutionary tradeoffs between annual and perennial life histories (i.e., high allocation of resources to the seed in annuals vs to below-ground structures in perennials) inherently limit the potential for perennial crop replacement of annuals (Smaje, 2015). However, the extended vegetated duration of perennials (hence greater carbon assimilation) and positive feedbacks between greater root development and nutrient acquisition in some soils do not preclude the potential for high-yielding grain production from perennial crops (Crews and Dehaan, 2015; Jansson et al., 2010).
Carbon Dioxide Removal Capacity
For all managed terrestrial carbon sinks, potential carbon removal rates are conditioned on the biological/ecological capacity of the technologies employed as well as economic and social acceptance factors. The majority of published estimates of carbon sequestration rates can be considered technical potentials in which achievable rates per unit area are multiplied by estimates of the maximum feasible land areas for implementation. Such rates and capacities represent an upper limit that does not fully reflect economic constraints, including land availability for competing uses, or other social or policy constraints. A lower economic potential for carbon sequestration reflects the fact that adopting more carbon-friendly land-use practices likely requires an economic incentive (although some amount of carbon stock gains may be achievable at negative costs). Combined ecological and economic analyses have been used to estimate marginal supply curves for carbon storage (e.g., McCarl and Schneider, 2001; Murray et al., 2005; Smith et al., 2008), where each additional increment of storage comes at a higher cost per unit carbon stored. However, relatively few formal analyses have considered the economic potentials, given the complexity and high degree of uncertainty in future market conditions. Finally, the actual potential storage achievable would factor in additional constraints related to social acceptance and policy implementation. Estimates for the major land uses in the United States and globally are presented below.
Conventional Forest Practices
For the United States, comprehensive analyses have shown that the potential carbon removal from afforestation/reforestation ranges from 0.0 to 0.45 Gt CO2 annually, over a period of approximately 50 to 100 years (Table 3.8). The soil component of this increase in carbon removal may be significant according to a recent study that estimated a range of 0.05 to 0.08 Gt/y CO2 (Nave et al. 2018).
At the global scale, the range of published estimates for afforestation/reforestation is large, from 2.7 to 17.9 Gt CO2 annually based on global analyses using top-down approaches for the high end of the range, and bottom-up approaches for the low end (Table 3.8). The wide range reported in studies represents different assumptions and modeling approaches, and variable prices or incentives for implementing activities.
TABLE 3.8 Total Global and U.S. Estimates of Carbon Removal Potential for Forestry Activities
|Study/Citation||Estimate Gt/y CO2e||Scope|
|Nabuurs et al., 2007||4.0||Afforestation/Reforestation|
|Griscom et al., 2017||2.7-17.9||Afforestation/Reforestation|
|Smith et al., 2016||4.0-12.1||Afforestation/Reforestation|
|Nabuurs et al., 2007||5.8||Improved forest management|
|Griscom et al., 2017||1.1-9.2||Improved forest management|
|Nabuurs et al., 2007||0.445||Afforestation/Reforestation|
|McKinley et al., 2011||0.001-0.225||Afforestation/Reforestation|
|Jackson and Baker, 2010||0.15-0.4||Afforestation/Reforestation|
|Nabuurs et al., 2007||1.6||Improved forest management|
|McKinley et al., 2011||0.029-0.105||Improved forest management|
At the low end of the range, the assumed price of carbon is low and secondary impacts are few—for example, sufficient marginal agricultural land is available and landowners are willing to participate in mitigation. At the high end, the price of carbon would be as high as $100 tCO2, and tens of millions of hectares would be incentivized to convert from crop or grass production to forest. Food production would be significantly impacted, so that the total land area that could be reforested or afforested is constrained by the higher value of alternative land uses (i.e., food price impacts from cropland conversion). The net CO2 reduction may be reduced over time by leakage (i.e., timber harvest elsewhere to meet continued economic demand for forest products), and concerns about permanence (subsequent clearing or natural disturbances).
Improved Forest Management
The potential of improved forest management ranges from 0.03 to 1.6 Gt/y CO2 for the United States and from 1.1 to 9.2 Gt/y CO2 for the world based on published studies (Table 3.8). Improving forest management on existing forestland faces limited to no
competition with other land uses compared with reforestation/afforestation, because this activity involves no change in land use. However, the cost may be higher because of the large area that must be treated to achieve similar results, and forest management would need to involve a much larger percentage of forest landowners than has ever participated in any incentive program. As with afforestation/reforestation, both leakage and risk of reversal from disturbances could reduce potential carbon removal gains over time.
Near-term forest carbon removal potentials
For a recent and practical reference point, summing the forestry activities proposed by countries in their intended nationally determined contributions (INDCs) to the Paris agreement yields a total expected global carbon removal benefit from afforestation/reforestation and improved forest management (excluding avoided deforestation) of approximately 1.0 Gt CO2. This is comparable to the low end of the estimates reported above, because the INDCs were purposely modest, representing what different governments considered achievable toward a target of limiting climate change to 2°C.
A modest program of afforestation/reforestation near the lower end of published ranges could be 0.15 Gt/y CO2 for the United States and 1.0 Gt/y CO2 for the world, taking into consideration availability of land and secondary impacts (described later in this chapter). Considering land constraints, a practical upper limit for carbon removal from afforestation/reforestation would be about 0.4 Gt/y CO2 for the United States and likely no more than 6.0 Gt/y CO2 for the world (Box 3.1). Similarly, the near-term carbon removal from improved forest management is likely to be near the lower end of these ranges, about 0.1 Gt/y CO2 for the United States and 1.5 Gt/y CO2 for the world. The upper limit of a more aggressive program to improve forest management could achieve a rate of carbon removal of 0.2 Gt/y CO2 for the United States and 3.0 Gt/y CO2 for the world.
Frontier Forestry Practices
Increased Use and Preservation of Harvested Wood Products
If most (up to 80 percent) of discarded wood products and associated wood wastes from current harvest and manufacturing were placed in a landfill designed for slow decomposition, then this would create an additional sink of 0.1 to 0.3 Gt/y CO2 in the United States and 0.2 to 0.8 Gt/y CO2 globally, which could be extended indefinitely as long as construction of such landfills continued. Preservation of currently harvested
wood plus increased harvesting of secondary forests to use all available growth (sustainable harvest) has the capacity to remove 0.1 to 0.7 Gt/y CO2 in the United States and 0.8 to 9.3 Gt/y CO2 globally. This could be accomplished without involving protected or intact forests, or affecting food supply or biological diversity.
Zeng et al. (2013) estimated that the global potential of green-tree burial was between 1.0 and 3.0 Gt/y CO2, with the lower end of this range roughly doubling the current global harvest and affecting about 800,000 ha of forestland. They excluded agricultural land, protected areas, inaccessible forests, and wood used for other
purposes such as timber and paper. To date, this proposed approach has not been tested, although the technology is simple and easily applied.
Conventional Cropland and Grassland Practices
During the past 20 years, several estimates of the soil carbon sequestration potential for the United States and the world have been released (Table 3.7). Nearly all represent a technical or “biophysical potential” assuming nearly complete practice adoption. As such, they represent upper-bound estimates of the carbon sequestration potential for the practices considered. However, the estimates do account for limitations relating to land availability, such as the limited land-use conversions to high carbon storage practices (e.g., permanent grassland set-aside that could occur without compromising food and fiber production). Most published estimates use aggregate data stratified by broadly defined land-use and climate categories (in some cases soil type), together with representative per hectare rates for different land-use/management practices determined from long-term field experiments or other measurements (e.g. chronosequence).
Frontier Cropland and Grassland Practices
Detailed analyses of the global sink potential of the frontier practices described earlier (i.e., high carbon input phenotypes, deep soil inversion) have not been published, with the exception of biochar amendment. Recent estimates of the global carbon sink capacity of large-scale deployment of biochar application to soil suggest gigaton per year potential (e.g., 6.6 Gt/y CO2e by Woolf et al. , 2.6 Gt/y CO2e from Smith , and 0.5-2.6 Gt/y CO2e by Fuss et al. ). Because biochar applications mainly target existing current cropland where large addition rates (up to 50 tC/ha; Smith, 2016) are agronomically feasible, there is no land constraint for application, although there is for biomass feedstock sourcing (see Land Requirements section below). Key uncertainties remain, including net life cycle GHG impacts, long-term impacts on crop productivity and economic feasibility of large-scale biochar deployment.
Other Radiative Impacts
The impact on climate from reducing CO2 by afforestation/reforestation may be partially offset by albedo changes (the proportion of radiation reflected by the land surface). Generally, an increase in forest cover reduces surface reflectivity causing more surface warming, whereas a harvest increases surface reflectivity causing a cooling effect, implying that the effect of increasing harvest or forestland clearing would have a less negative impact on climate than indicated by only accounting for GHG changes. Albedo effects are typically larger in the years immediately following harvest, last longer for land use changes, and are most significant in response to changes in conifer coverage above the snow line (Cherubini et al., 2012; Holtsmark, 2015). These effects on climate forcing are usually less influential than the effects of changes in GHGs but may be highly variable (Anderson-Teixeira et al., 2012). In general, afforestation/reforestation in boreal zones will have a warming effect that exceeds the cooling effect of reducing GHGs, and the opposite effects in the tropics. In temperate zones, the effects are highly variable in space, depending on vegetation type, timing of snow cover slope, aspect, and other factors. Recent work has extended the analysis of radiative effects to include nonradiative processes such as air turbulence, which may have larger and locally more variable effects than albedo changes (Bright et al., 2017).
In designing and implementing CO2 removal strategies involving agricultural land management, impacts on other GHGs, particularly N2O and CH4, are of paramount importance. Agricultural soils are the single largest source of N2O, driven by the large additions of nitrogen to promote plant growth. Flooded cropland soils (rice) are a major CH4 source. Soil-based CO2 removal strategies should avoid requiring additional nitrogen fertilizer, which has large embodied fossil carbon emissions for its manufacture, in addition to impacts on N2O emissions (van Groenigen et al., 2017). Thus, any additional nitrogen requirements to support increased plant production and carbon inputs should be sourced from more efficient use of nitrogen already applied to agricultural soils as well as from biological nitrogen fixation. Management options that offer synergisms with both increased soil carbon stocks and reduced N2O (and CH4) emissions should be prioritized.
Agricultural practices to increase CO2 removal likely have minimal impact on other (e.g., aerosols, albedo) radiative forcing factors. For example, conversion of
winter-fallow bare soils to lighter surface of senescent winter vegetated cover, as well as reduced tillage systems that retain surface residues, increases albedo in most cases, providing a small cooling effect (Davin et al., 2014). Most other agricultural carbon removal practices will have negligible impacts on albedo.
Current Commercial Status
The current commercial status of carbon removal via terrestrial land-use options remains limited but is growing. Only a few compliance cap-and-trade markets for GHG reductions are in operation (e.g., European Union, California), and for most of those, land use–based offsets (i.e., allowable emission reduction from noncapped entities) play a minor role, if included at all. Hence, the demand for, and thus the monetary value of, terrestrial carbon storage remains still low. More direct financing of carbon sequestration projects involving agriculture and/or forestry exists in the voluntary carbon/emission reduction markets. In 2016 the total volume of forestry and land use project carbon reductions was 13.1 Mt CO2e, at an average price of $5.10 t/CO2, for an aggregate value of $67 million. Greater than 95 percent of the CO2 reduction activities focused on forest biomass carbon, of which REDD+ (Reduced Deforestation and Forest Degradation Plus) projects dominated with 75 percent of total CO2 reductions (Hamrick and Gallant, 2017). In the United States, an indirect measure of the commercial value, to the landowner, are the subsidies paid by government for implementation of conservation practices that also promote carbon sequestration. In many cases, these payments address multiple environmental outcomes and often take the form of cost shares or loans, as opposed to direct payments for tonnes of carbon sequestered as in voluntary or cap-and-trade markets. Chambers et al. (2016) estimated that conservation payments that promoted soil carbon storage averaged around $60M/y from 2005 to 2014, with an estimate carbon storage increase of 13 to 43 Mt C on U.S. cropland, which equates to a carbon price of $5-17/t CO2e.
Land Requirements and Competition for Land among Alternative NETs
Although scientific uncertainty about the amount of carbon sequestered locally from improved land management practices is low, uncertainty about far-field impacts caused by the long-term effects of climate change and the economic feedback is high. An important consideration is that land requirements and competition for land cuts across several of the NETs in this report—particularly, afforestation/reforestation and BECCS. Soil carbon sequestration and biochar use land, but do not compete for land, which can still be used for the same purpose. Each of these technologies were
evaluated separately, but their maximum potential cannot be realized simultaneously. Here we examine the land requirements for these different activities and compare them with current land uses and potentially available land for additional carbon removal. Table 3.9 summarizes the practically achievable ranges of carbon removal from forestry and BECCS, including an estimate of the land area required for each.
Even relatively low amounts of carbon removal and carbon sequestration from afforestation/reforestation (0.15 Gt/y CO2) in the United States would require converting 3-4 Mha of nonforested land to never-harvested forest. Improved forest management would require changes on 11-19 Mha of existing forest for a modest increase in carbon removal of 0.1 Gt/y CO2. Global land requirements for afforestation/reforestation to achieve 1.0 Gt/y CO2, and improved forest management to achieve 1.5 Gt/y CO2, are 70-90 Mha and more than 1,000 Mha of existing forestland, respectively. These levels of activity should be achievable at low carbon prices (i.e. $10-50/Mg CO2), but safeguards would be necessary to reduce negative impacts and ensure permanence. In addition, implementation would require a very high participation rate by landowners.
TABLE 3.9 Range of Practical Annual CO2 Removal Fluxes and Associated Land Requirements for Estimates Presented in This Report
|Activity category||Carbon removal-low (Gt/y CO2)||Carbon removal-high (Gt/y CO2)||Area-low (Mha)||Area-high (Mha)|
aBased on Jackson and Baker, 2010. Would require a CO2 price of $15/t for carbon removal-low and $50/t for carbon removal-high.
bBased on Griscom et al., 2017 and Smith et al., 2016.
cRefer to Chapter 4 for sources of estimates. At the low end, biomass is sourced from existing land uses so no new land dedicated to BECCS is required.
NOTES: See Chapter 4 for the source of estimates for BECCS. Activity categories that do not require land-use shifts (e.g., agriculture, improved forest management) are not included in the table.
Although higher levels of carbon removal are technically achievable from afforestation/reforestation, attaining 0.4 Gt/y CO2 in the United States would require much larger areas of land (up to 20 Mha) and a high carbon price (i.e., >$50/Mg CO2). Globally, the area of land needed to support a 6 Gt/y CO2 carbon removal from afforestation/reforestation would require up to 500 Mha of land. These higher levels of afforestation/reforestation would have significant negative impacts, including reduced food production and increased food prices.
Higher levels of improved forest management in the United States to achieve 0.2 Gt/y CO2 would require 22-38 Mha of existing forestland. Worldwide, the levels to achieve 3.0 Gt/y CO2 would require more than 2,500 Mha of existing forestland. These management changes could result in leakage by shifting some harvesting elsewhere and would require participation by most private landowners, which would be difficult to achieve.
The majority of management practices considered for carbon removal in agricultural soils do not involve changes in land use and hence would largely be operative on the existing land base devoted to agricultural production. To the extent that many practices improve soil health and productivity, carbon removal activities could contribute toward meeting increased food and fiber demands without increasing the area of land under agriculture. The main activity that could contribute to displaced agricultural production and potential land-use conversions elsewhere would be setting aside cropland into perennial grassland (or forest, see above), for conservation purposes. To the extent that degraded or otherwise marginal agricultural lands are targeted for set-asides, leakage effects (i.e., displacement of agricultural production resulting in land-use conversion and soil carbon losses elsewhere) would be minimal. Currently, roughly 9.8 Mha of cropland is in set-asides under the U.S. Conservation Reserve Program (CRP). At its height during the late 1990s, the CRP area exceeded 13 Mha (Mercier, 2011), which resulted in estimated leakage of around 20 percent of the sequestered carbon (Wu, 2000), although potentially larger leakage has been suggested (Murray et al., 2007).
As discussed in Chapter, 4 the committee calculated the low-end estimate for this approach assuming that sources of biomass from existing land uses (e.g., wood and other unutilized organic waste) are sufficient and no land-use changes are necessary.
Dedicated land requirements for the high-end estimate for the United States are based on reported economically feasible energy crops that could produce up to 0.65 Gt CO2e annually (U.S. DOE, 2016), Assuming an average productivity of 18 t CO2e per ha, this level of production would require 36 Mha of land. Scaling this estimate up to our high end of 1.4 Gt CO2e indicates a requirement of 78 Mha, equivalent to almost 20 percent of the current agricultural land base (Table 3.9). Smith et al., 2016 estimate the global land area required to deliver 12 Gt/y CO2e at approximately 380-700 Mha in 2100 for high productivity energy crops, such as willow, poplar short-rotation coppice, and miscanthus. Therefore, the large-scale implementation of BECCS is expected to compete with terrestrial carbon capture and storage initiatives, as well as with food production (e.g., Smith et al., 2010) or the delivery of other ecosystem services (e.g., Bustamante et al., 2014).
Aggregated Carbon Removal Land Requirements and Availability Of “Marginal Land”
The total area of land managed for agriculture and forestry is 791 Mha in the United States and 7,130 Mha globally (Table 3.10). Some of this land may be considered marginal by landowners, which could serve as an indicator of the amount of land that could be shifted to another use without significant effects on production of necessary services, particularly food. There is no generally accepted definition of marginal land (it is an economic decision that varies over time or is based on subsistence needs), but an indicator in the United States is the ca. 10 Mha of farmland currently enrolled in the CRP. A similar global estimate is more difficult to determine, but one recent study estimated that the global pool of marginal land is about 1,300 Mha. This land supports about one-third of the world’s population and therefore only a fraction would be available for afforestation/reforestation and BECCS. Another global analysis estimated
TABLE 3.10 Managed Land Area by Selected Land-Use Categories in 2015 (Mha)a
a U.S. definitions according to EPA, 2017. World definitions according to FAOSTAT database.
c Data from FAOSTAT database, accessed March 11, 2018.
that 760 Mha of land are available and suitable for afforestation in IPCC non-Annex 1 countries, which are mostly in tropical and subtropical biomes (Zorner et al., 2008).
Because new land dedicated to BECCS would likely be planted with fast-growing species such as miscanthus (on more productive sites), the area required per unit of carbon removal would be much less than that required for afforestation/reforestation. Humpenöder et al. (2014) highlighted the trajectories and relative land requirements for afforestation and BECCS (Figure 3.3) using a land-based modeling approach that simulated a similar level of carbon removal as the high-end calculation in this report. They suggest that the area required for BECCS is much lower than that required for afforestation (Table 3.9) based on strongly increasing yields per hectare for herbaceous biofuel, up to 25-30 t/(ha y) C, compared with afforestation yields that are estimated at a much lower rate for natural regeneration of forests, about 2-6 t/(ha y) C (Table 3.1). However, as noted in the BECCS chapter, the temporal nature of bioenergy production should also be considered.
In summary, at the low end of the committee’s estimates for carbon flux and capacity for afforestation/reforestation and BECCS, any competition for land between these two NETs would not be significant (because BECCS biomass could be sourced entirely from existing land uses), nor would there be competition for land currently required for food production. Land requirements for afforestation/reforestation both in the United States and globally could be met by tapping into land considered “marginal” for food production, although globally, some of the required marginal land is used to support population needs for food and fiber. At the high end shown in Table 3.9, the total area required for afforestation/reforestation and BECCS combined would likely exceed the availability of marginal land. Considering the additional impact on land required for food production, this level of carbon removal is not likely to be realized.
ESTIMATED COSTS OF IMPLEMENTING TERRESTRIAL CARBON SEQUESTRATION AT SCALE
The direct costs of establishing new forests and performing management activities in different regions are well known based on experience, and several studies have revealed how landowners would respond to various carbon price levels. However, scaling direct costs to high levels of activity is a challenge. Estimates of indirect costs and impacts on carbon associated with collateral effects such as offsetting land-use change and commodity production are available, but knowledge of these effects is limited by the ability of macro-economic models to simulate complex responses.
The costs of different forestry practices are highly variable by practice and region, and their estimates when scaling to a high level of carbon removal differ wildly depending on the method used to estimate feasibility (Alig, 2010; Figure 3.4). Engineering methods to estimate costs result in higher costs at the low end of total tons sequestered, and lower costs at higher levels of carbon removal, compared with econometric or optimization approaches that account for market adjustments.
Cost estimates for implementing conventional soil carbon sequestering practices (e.g., planting cover crops, changing tillage practices and crop rotations) are readily available as state- and regionally-specific farm budgets.1 However, more general information on future costs and projected willingness to adopt carbon sequestering practices as a function of carbon prices are mainly available from academic studies. While varying for different geographic areas and farming systems and practices (Alexander et al., 2015), many estimates suggest significant adoption rates of improved cropping
1 e.g., https://www.ers.usda.gov/data-products/commodity-costs-and-returns.
practices could occur at values <$50 tCO2e (Tang et al., 2016). In a global analysis, Smith et al. (2008) estimated economically-achievable GHG reductions (>90 percent of which were from soil carbon sequestration) of 1.5, 2.2, and 2.6 Gt/y CO2e at carbon prices of 0-20, 0-50, and 0-100 USD/tCO2e, respectively. Economic feasibility and marginal cost curves for nonconventional (frontier) carbon sequestering technologies described earlier are not known.
An economic optimization model used to estimate forestry, agriculture, and bioenergy supply functions for emissions reductions in the United States (Figure 3.5) indicated that at low carbon prices, forestry and agriculture opportunities were about equal in terms of economic potential carbon removal and higher than bioenergy. At higher prices, agriculture had the largest impact on carbon removal followed by afforestation and forest management.
The impacts of changes in land management on biodiversity, water, and other land attributes may be positive or negative depending on types of changes in land cover and site-specific land characteristics. For example, timber harvesting changes forest
structure, composition, and productivity, which in turn affects many other properties and services of forests such as wildlife habitat, biodiversity, and runoff (Venier et al., 2014). These impacts may be profound and long-lasting by establishing spatial patterns that broadly affect forest and landscape ecology, similar to the effects of many natural disturbances. Some studies show that managing for carbon in particular forests can decrease biodiversity, because habitat is reduced for species that depend on disturbance to create habitat (Lawler et al., 2014; Martin et al., 2015). Impacts of harvesting roundwood on forest ecology are significant and should be considered in formulating policies as well as assessed through observations, although additional studies are needed to determine how to assess such impacts at different scales. Turner (2010) highlighted key concepts of how disturbances and recovery have profound and long-lasting impacts at stand and landscape scale as forests recover. Fundamental ecosystem processes and functions are affected. For example, productivity and mortality are strongly related to time since disturbance, and disturbances create spatial heterogeneity that can be essential for some wildlife species.
Co-Benefits of Soil Carbon Sequestration (SCS) Practices
In addition to CO2 removal, increasing soil carbon contents provides benefits for soil health and other ecosystem services, including enhanced carbon and nutrient pools, water storage, improved soil structure, aggregation, and water and air infiltration, as well as reduced soil erosion and enhanced soil biodiversity (Al-Kaisi et al., 2014; Lefèvre et al., 2017). A study of ecosystem services in conservation planning for targeted benefits vs co-benefits found that inclusion of ecosystem services within a conservation plan may be most cost-effective when they are represented as substitutable co-benefits/costs, rather than as targeted benefits (Chan et al., 2011). Well-managed soils that enhance soil carbon also promote soil biodiversity, which in turn enhances the function and metabolic capacity of soils and plays a crucial role in increasing food production and soil resilience to climate change. Increasing soil organic matter contents will facilitate (1) nutrient storage in SOM; (2) nutrient recycling from organic to plant-available mineral forms; and (3) physical and chemical processes that control nutrient sorption and availability. The dynamic nature of managed soils is what makes soils function and supply ecosystem services (Lefèvre et al., 2017).
Basis for Research Budget Estimates
The proposed budget for terrestrial basic and applied research components for a national research agenda aligns with the current budget for various soil and plant research established by the National Institute of Food Agriculture (NIFA) and the USDA Agriculture and Food Research Initiative (AFRI). The AFRI research program for various components allocates $500K to $750K/y per research project for a period of 3 to 5 years for research programs such as Plant Health and Production, Bioenergy, and Animal Health. Generally, in the estimates quoted for different research activities, the committee allocated $300K per scientist year for postdoctoral-level research, plus funds for administrative and technician support, equipment, travel, publications costs, and other expenses specific to the research described. The cost of each component of the research agenda is summarized in Table 3.11.
TABLE 3.11 Costs and Components of a Terrestrial NET Research Agenda
|Recommended Research||Estimated Research Budget ($M/y)||Timeframe (y)||Justification|
|Basic Research||High carbon input crop phenotypes||40-50||20||The DOE/ARPA-E ROOTS program is currently funded for $35M total, allocated to 10 multiyear projects. Thus proposed funding is a 4-5X increase over this level|
|Soil dynamics at depth||3-4||5||Funding to initiate 4-6 projects per year|
|Harvested wood preservation||2.4||3||Funding to initiate 3 multiyear projects at $800K each, involving representative locations|
|Biochar studies||3||5-10||Funding for 3-5 projects per year to assess biochar amendment impacts for different management systems and soil types|
|Development and Measurement/Monitoring||Monitoring of forest stock enhancement projects||>5||≥3||System development of $1M/y for 3 years Continuous operation -$4M/y to staff a small office to analyze data, coordinate field checks, develop reports Improving international forest monitoring and reporting—10 to 20 times the amount needed in the United States|
|National on-farm monitoring system||5||Ongoing||Augmentation of USDA’s existing NRI system|
|Data-model platform for predicting and quantifying agricultural soil carbon removal and storage||5||5||Initial development focuses on systems integration, including of existing data sources and models|
|Recommended Research||Estimated Research Budget ($M/y)||Timeframe (y)||Justification|
|Demonstration||Forest demonstration projects: increasing collection, disposal, and preservation of harvested wood; and forest restoration||4.5||3||Demonstration projects to improve disposal and collection of wood products after use (three 3-year projects at $500K/y each); 3 multiyear projects for preserving harvested wood in different environments (500K/y each); and 3 multiyear projects to demonstrate carbon benefits of forest restoration in different geographic regions ($500K/y each)|
|Experimental network improving agricultural soil carbon processes||6-9||≥12||10-15 sites at a cost of $600K/y per site|
|Deployment||Social sciences research on improving landowner responses to incentives and equity among landowner classes||1||3||Extension and outreach educational programs for transferring research findings and technologies to farmers and practitioners. Funding for initiation of 3 multiyear projects|
|Research on GHG and social impacts of reducing traditional uses of biomass for fuel||1||3||Funding for initiation of 1 multiyear project|
|Scaling up agriculture sequestering activities||2||3||Support for initiation of 4-5 regional projects per year to identify solutions to overcome barriers to adoption|
High Carbon Input Crop Phenotypes
Research is needed to develop crop varieties with altered root morphology and biomass (e.g., more roots, deeper root distributions, more recalcitrant to decomposition) to add and maintain high root carbon inputs to soil, while maintaining high aboveground yields. Specific research areas include crop breeding and selection for annual crops with larger and deeper roots, perennialization of major grain and oilseed crops, advanced root phenotyping technologies, new crop performance trials, and soil and ecosystem responses to new crop varieties. Such a research program can build on the recently initiated ROOTS (Rhizosphere Observations Optimizing Terrestrial Sequestration) program established by ARPA-E.2 The cost for this research is approximately $40-50M/y for an initial 20-year period and could be conducted through USDA, the National Science Foundation (NSF), and/or the Department of Energy (DOE). For comparison, current annual research and development (R&D) funding for conventional crops improvement and genetics in the United States is approximately $1.5 billion from the public sector and $1.8 billion from the private sector (Fuglie and Toole, 2014).
Soil Carbon Dynamics at Depth
Research is still limited on the controls on decomposition and stabilization of SOM in subsoil, that is, below 30 cm. Several promising technologies to increase soil carbon stocks (e.g., prairie restoration, deep soil inversion and carbon burial, enhanced root phenotypes) are based on increasing carbon additions to subsoil layers. Studies are needed to determine how stabilization and residence times of organic residues vary as a function of depth gradients in aeration, microbial community composition, soil physiochemical properties (e.g., soil texture, soil mineralogy), and plant (root) residue composition. The cost for this research is approximately $3-4M/y for 5 years and could be conducted by USDA and NSF.
Harvested Wood Preservation
Research is needed in two areas: The first area is landfill designs for achieving the lowest possible rate of wood decomposition. Reducing wood decomposition has
2 Projects in the ARPA-E ROOTS program seek to develop advanced technologies and crop cultivars that enable a 50 percent increase in soil carbon accumulation while reducing N2O emissions by 50 percent and increasing water productivity by 25 percent. For more information see: https://arpa-e.energy.gov/?q=arpa-e-programs/roots.
not been a stated goal of landfill designs; rather, they have been designed to contain buried waste, collect contaminated precipitation that percolates through the waste (leachate), and collect and control gas emissions. A large body of landfill design research at several representative sites forms a foundation for focused attention on minimizing wood decomposition (U.S. Forest Service [USFS], U.S. Environmental Protection Agency [EPA], and NSF; Cost: $2.4M/y for 3 projects lasting 3 years). The second area is integrated assessment of net greenhouse balance, costs, and required land, including the implications for worldwide consumption of wood products and their life cycle emissions. Any change in consumption of wood products will have widespread effects on emissions related to other sectors of the economy, for example, construction or material transportation. (For the existing research capacity and cost, see the integrated assessment section in the BECCS chapter.)
Biochar Amendment Studies
Although much is already known about residence time in soil for different types of biochar and how biochar characteristics vary as a function of the feedstock and the pyrolysis process used, additional research is needed to assess secondary impacts of different biochars on crop performance, nutrient cycling and retention, and N2O and CH4 emissions from soils, all of which affect the net GHG consequences of biochar amendments. In addition, research to produce full life cycle analyses that consider the alternative fates/uses of feedstocks from which the biochar is produced (Paustian et al., 2016a) are needed for a comprehensive assessment of net carbon removal potentials from biochar amendments.
Development and Measurement and Monitoring
Monitoring of Forest Carbon Stock Enhancement Projects
For private and public forestlands, USFS should develop a plan to monitor recommended carbon stock–enhancing activities, conduct statistical sampling of total ecosystem carbon stored in a subset of projects, and develop local “climate impact factors” that also account for biophysical effects. It is time consuming and expensive to directly measure the effects of many small projects on net GHG emissions; therefore, approaches that could achieve accurate average estimates for aggregates of projects (based on remote sensing and validated expansion factors) are needed to reduce transaction costs. Additional needs are to monitor leakage (which is a global
phenomenon) and to use the monitoring system to attribute observed changes in carbon removal to management activities vs increasing CO2 or climate change. Monitoring leakage could require a new LiDAR satellite dedicated to mapping global forestry activities. Knowledge and monitoring of lateral transfers of carbon from land to inland waters are lacking, and these transfers are not currently detected by remote sensing or operational field inventories. The substantial capacity of current remote sensing and field monitoring could be built on to address the additional needs described here. For example, the USFS Forest Inventory and Analysis (FIA) program is funded at approximately $70M/y, collects continuous field data on status and trends of U.S. forests, and collaborates with NASA to develop methods to integrate remote sensing data with field data. Internationally, the status of monitoring is highly variable, with many countries lacking field measurements and capacity to implement monitoring programs. However, there is a significant international aid effort to improve capacity in forest monitoring at the country scale, as well as to advance research on global monitoring capability using satellites. The research cost for the United States only is approximately $1.0M/y for 3 years for system development, and continuous operation would cost approximately $4.0M/y to staff a small office to analyze data, coordinate field checks, and develop reports. A significant contribution to improving international forest monitoring and reporting, including detection of leakage worldwide, would require about 10 to 20 times the amount needed in the United States.
A National On-Farm Soil Monitoring System
USDA should fully implement a national on-farm soil monitoring system on existing National Resource Inventory (NRI) points on cropland and grassland. Full buildout to approximately 5,000-7,000 NRI locations with soil sampling and analysis carried out at intervals of 5-7 year (on an annual rotating basis similar to the FIA system) is recommended. Similar systems already exist in many countries, including in the European Union, Australia, New Zealand, and China (van Wesemael et al., 2011). This system would provide an ongoing data stream to improve national-scale soil carbon inventory systems and reduce uncertainties. Such a monitoring system has the potential for broader utility for tracking and assessment of soil health and to support long-term sustainability of soils for the U.S. agricultural economy, which has a gross domestic product of more than $135 billion from farm output alone. The cost for this system is $5M/y as an augmentation of USDA’s existing NRI system.
Model-Data Platform for Quantifying and Forecasting Carbon Removals by Soils
For implementation of soil carbon sequestering practices at Gt/y scales, improved platforms for quantifying soil carbon storage accurately and cost effectively are needed (Figure 3.6). Such systems should integrate data from a repository of existing and new experimental field sites to inform process-based land use/ecosystem models that can be independently validated using on-farm soil monitoring networks. Spatial data layers of model drivers (e.g., weather, soil maps, topography), together with comprehensive remotely sensed activity data on management practices (e.g., crop species, presence/absence of cover crops, tillage system, irrigation, vegetation productivity) provide the main data inputs to the model. Potential crowd-sourcing of farm management information (e.g., nutrient management) from the land users themselves can provide data that cannot be readily obtained from remote sensing. Such a system is scalable to estimate national-scale outcomes of CO2 removals over time as a function of land use policies (e.g. for national GHG inventory reporting) as well as to provide information on field- and local-scale dynamics that can inform policies based on carbon markets and/or sustainable product supply chains supported by agricultural industries. Many component parts of such a system (e.g. field experiment networks, remote sensing data) already exist and could be leveraged. Funding of $5M/y for a 3-year development period followed by annual operating costs of similar magnitude would support system integration, data-model fusion, model development, decision support systems, visualization and communication.
Forest Demonstration Projects
Conventional forestry practices have been implemented worldwide for decades, so demonstrations are readily available. One area requiring new demonstrations is the frontier technology of increasing preservation of harvested wood products, particularly, how to improve the disposal and collection processes of wood products after their useful life. This research is linked to the basic research needed to improve landfill design for for harvested wood product preservation. The cost for USDA and EPA to conduct this research is $1.5M/y for 3 projects for 3 years. Associated with the basic research of improving landfill design to improve preservation of harvested wood, it is necessary to implement several demonstration projects representing different environmental conditions to facilitate scaling up this new technology. The cost for this research done by USFS and EPA is $1.5 M/y for 3 projects for 3 years. Another promising activity requiring demonstrations of potential impacts on carbon removal is forest restoration, which has been described for fire-prone areas as increasing carbon removal in the long term as a result of short-term reduction in carbon stocks of overstocked stands; for areas converted from natural vegetation to unsustainably managed forests that prematurely lose ability to sequester carbon; and for degraded forests that would respond to interventions to improve regeneration and stocking. The cost for USFS and partners to conduct this research is $1.5M/y for 3 projects for 3 years.
Agricultural Systems Field Experiment Network
Field experiments to rigorously evaluate region-specific best management practices for soil carbon sequestration (and net GHG reductions), in comparison to conventional practices, should be established in a coordinated network across the major U.S. agricultural soil types, involving land-grant universities and other research institutions with the relevant expertise in ecosystem carbon and GHG dynamics. Sites selected should complement existing experimental sites in USDA Agricultural Research Service’s (ARS) GraceNET and Long-term Agricultural Experiment networks and relevant sites within NSF’s Long-Term Ecological Research (LTER) and National Ecological Observatory Network (NEON) programs. The new network should have a coordinated set of measurement protocols and methods, including whole system carbon balance (i.e., eddy covariance) and other GHG (e.g., N2O and CH4) flux measurements as well as precision soil carbon stock and stock change measurements. Data sharing and
archiving to support modeling and meta-analysis should be priority and field sites should include significant extension, outreach, capacity building, technology transfer, and demonstration to provide information to producers, extension agents, crop consultants, agency personnel, and other stakeholders. Sites should be designed for the field experiments and demonstrations to run for a minimum of 12 years. This research should be conducted by land-grant universities and USDA at about 10-15 sites at a cost of $600K/y per site for a total of $6-9M/y.
Forest Carbon Project Deployment
Although scientific uncertainty about the amount of carbon sequestered locally from improved forestry practices is low, uncertainty about far-field impacts caused by the long-term effects of climate change and the economic feedback is high. To reduce this uncertainty, the following research is needed:
- Improvement in economic models that estimate leakage (i.e., reduced harvest in one location causing deforestation in another) and the impact of converting cropland to forest on food prices. Biological research is also needed to develop understanding of the impacts of significant impacts of carbon removal on biodiversity and of safeguards to ensure a sustainable biosphere. This research and related costs are discussed in the section on the research agenda for integrated assessment modeling in Chapter 4.
- Improvement in LCA methods to evaluate substitution of wood products for other materials that could expand wood use, especially if these displace structural materials such as steel and cement, which require large carbon emissions to produce. Substituting wood for other material typically results in a net reduction of GHGs; however, tools to quantify the net reductions in GHGs are not readily available or validated. The research needs for LCA and related costs are discussed in Chapter 4.
- Social sciences research on how landowners respond to incentives and the utilization of Extension Service system at the land-grant universities, to reach more landowners and practitioners who work on small parcels of forest to improve the equity of participation in assistance programs that tend to favor larger landholders. Only a small percentage of forest landowners respond to incentive programs or price signals by changing land management practices. Very little research has been conducted on these topics (USDA and NSF; $1.0M/y for 3 projects over 3 years).
- Research on GHG and social impacts of reducing traditional uses of biomass for fuel, which involves households and small entities using wood biomass for heating and cooking. Globally, increasing durable wood products or increasing commercial use of biofuel could reduce consumption of biomass for traditional fuel use, which would have a ripple effect on types of energy supplied (USDA and NSF; $1.0M/y for 3 years).
- Social science research on responses to economic and behavioral incentives to reduce meat consumption and food waste. This research and related costs are included in the research agenda for integrated assessment modeling in Chapter 4 (BECCS).
Scaling Up Agricultural Carbon Sequestering Activities
Current adoption of existing best management practices for soil carbon sequestration are low (e.g., cover crop are used on <5 percent of U.S. cropland; Wade et al., 2015) and barriers to scale-up, including economic and behavioral changes (e.g., value proposition, risk management, motivation), information needs, and technology transfer, are poorly understood. Economic and behavioral research along with continuation and expansion of pilot emission reduction and carbon removal projects, such as through the USDA Natural Resource Conservation Service’s Conservation Innovation Grants, can provide needed empirical knowledge on which barriers are most limiting and on how to design the most effective policies and education programs to promote agricultural carbon removal activities at scale.
Regional Life-Cycle Assessments
Any program to offer incentives to landowners for adopting carbon removal and storage should be preceded by a regional assessment of life cycle emissions, costs, co-benefits and negative impacts, including estimates of leakage and the degree of permanence, which can be undertaken by federal land management agencies (e.g., USDA Forest Service and USDA Agricultural Research Service) and/or university researchers, with a review of the findings by an independent scientific board. This research and related costs are discussed in the LCA section of Chapter 4.
Implementation of Terrestrial Carbon Sequestration
Policies that overcome potential barriers to developing robust research and adopting carbon removal and soil carbon sequestration for terrestrial NETs (cropland and
forestland) may include several different mechanisms including (1) government subsidies to landowners to adopt carbon sequestering practices, similar to existing conservation payments in the Farm Bill, (2) carbon offset markets, in which land-based “carbon projects” market emission reductions/carbon sequestration to major GHG emitters participating in either voluntary or mandatory emission reductions (i.e., cap-and-trade), and (3) demand-side programs in which carbon removal activities are undertaken in response to demand for land-based consumer products that have a low carbon footprint. All three approaches currently exist. For government-based incentives (e.g., payments for practice adoption), the United States has a well-developed infrastructure of federal and state land management agencies and extension and outreach specialists to engage in implementation. For market-based systems, the voluntary GHG registries (e.g., VCS, ACR, CAR) and state agencies (e.g., CA Air Resources Board) have experience and governance structures that provide a starting point. New efforts on the part of companies to incentivize producers as part of sustainable or “low carbon” supply chain initiatives are at present much more fragmented, with the potential for double-counting and low transparency in the carbon removal activities actually undertaken. Hence institution building and development of governance structures to better engage the private sector in pursing demand-side carbon removal activities is a future need.
Land use and conservation practices that promote or encourage SCS for agricultural land and forest should be coupled with incentives and levels of regulations that encourage farmers, land managers, and landowners to adopt such practices. Current USDA conservation programs, such as the Conservation Stewardship Program, Environmental Quality Incentives Program, and Conservation Research Program, serve as good examples of cost sharing for implementing such practices. However, a set of incentives should be developed within the Farm Bill by linking conservation adoption and level of compliance to the level of improvement in soil carbon. The permanence of SCS and associated soil and ecosystem services co-benefits can be enhanced via financial incentives through government programs or as price premiums driven by industry seeking to develop more environmentally-friendly supply chains. Ultimately, the widespread adoption and long-term maintenance of working lands that place a high priority on SCS and reduced GHG emissions requires that they are as (or more) profitable to the farmer/rancher/forester as the conventional systems that they replace.
Barriers to the Implementation of the Research Agenda
For the recommended research agenda to achieve its goal and have lasting impact, certain barriers to adoption of carbon sequestering management practices must be overcome. Actions to lower these barriers include capacity building and robust educational programs through extension and outreach and other government and private entities to promote the carbon removal and SCS concept, research findings and technology transfer to end users; economic incentives to compensate for added costs or potential yield loss while transitioning to new management practices; and development of agronomists and specialists that can train and work with end users, to name few. However, a focus on the delivery and promotion of new technology alone will not be sufficient. These efforts should be coupled with social sciences research to better understand the drivers that lead to the fundamental changes in land management practices necessary to achieve terrestrial carbon removal at scale. The research agenda should include an integrated platform for cropland, grassland, and forestry projects such as with the NIFA Coordinated Agricultural Projects (CAP), where research, extension, and education are required components of funded proposals. In particular, the applied components of the research agenda (e.g., on-farm network and demonstration) would benefit from a multidisciplinary approach and collaboration between federal, state, and private agencies to achieve the goals of the research program. Development of relevant K-12 curricula could increase awareness about the practices that enhance carbon removal and soil carbon sequestration, including training opportunities for teachers, students, and the public at large. Such efforts may include hands-on demonstration of proven technologies, such as no-till, cover crop, residue management and agroforestry practices. Universities and USDA scientists can collaborate to develop and facilitate education and technology transfer through existing infrastructure and programs within land-grant universities and other public and private institutions.
Terrestrial ecosystems can play a significant role in carbon dioxide removal and sequestration, through practices that increase the amount of OC stored in living plants, dead plant parts, and the soil. Please note that bioenergy with carbon capture and storage (BECCS), which also involves carbon removal via plant photosynthesis, is covered in Chapter 4 of this report.
Two suites of land management practices involving agriculture and forestlands are sufficiently mature, both scientifically and in practice, to materially increase carbon storage if widely deployed in the United States and globally. Options exist for inexpensive remote monitoring and verification of the practices that lead to carbon capture and storage, which, when coupled with data and model-based quantification of net carbon storage, can reduce the need for expensive on-site direct measurements of carbon on lands that adopt these practices.
Increasing the area of forested land through afforestation/reforestation creates a carbon sink ranging from 1.5 to 6.4 tC/ha/y for a period of 50-100 years or more. Changing forest management practices such as extending timber harvest rotations and improving stocking and productivity by restoring degraded forests could store an additional 0.2 to 2.5 tC/ha/y for several decades. Development of new technologies to increase the use of long-lived wood products as well as carbon burial schemes for harvested wood are options for increasing carbon removal potential from forests.
For cropland and grassland, many long-term experiments document increases of soil carbon in the range of 0.2-0.5 tC/ha/y (over time periods of 2-4 decades) from adopting conservation practices including more diverse crop rotations, use of cover crop, reduced tillage, and improved grazing system. Other changes in land use and management such as agroforestry or reestablishment of wetland and perennial vegetation on marginal lands can achieve increases in SOC stocks of more than 1 tC/ha/y. Additional frontier soil NETs include deep soil inversion and burial of carbon-rich topsoil, biochar amendment, and development of enhanced root carbon input crop phenotypes.
The committee estimates “practically achievable” carbon removal amounts. In other words, existing practices would be implemented at rates that do not require land-use conversion that would jeopardize food security and biodiversity of intact native ecosystems. These estimates are 0.6 Gt/y CO2 from forestland and 0.25 Gt/y CO2 in agricultural soils for the United States, and corresponding estimates of 9 and 3 Gt/y CO2 for the world. Much of this CO2 removal would be achieved for less than $50/t. If frontier NETs prove practical and economical, rates of carbon removal for both forests and agricultural soils could roughly double in size.
Finally, the mix of research needs is broad and includes basic research, measurement and monitoring technology, regionally representative demonstration projects, and barriers to deployment and scale-up.
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