Negative Emissions Technologies and Reliable Sequestration A Research Agenda (2019) / Chapter Skim
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3 Terrestrial Carbon Removal and Sequestration
3 Terrestrial Carbon Removal and Sequestration
Pages 87-136
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From page 87... ...
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)
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From page 88... ...
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.
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From page 89... ...
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.
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From page 90... ...
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)
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From page 91... ...
while the effects on below-ground biomass and soil carbon remain largely unknown (Noormets et al., 2015)
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From page 92... ...
. Estimates of the potential carbon sink from afforestation/ reforestation include changes in biomass and soil carbon without consideration of i mpacts of future harvesting or natural disturbances.
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From page 93... ...
. 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 condi tions, which includes both increasing carbon stocks by returning a forest to its original vegetation type that is better adapted to site and climate, and reduc ing 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)
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From page 94... ...
. These estimates include changes in biomass and soil carbon but exclude changes in the stock of harvested wood products.
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From page 95... ...
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 carbonaccruing management practices are adopted. Furthermore, because of the dynamics of mineral-organic matter interactions that largely control the residence time of 95
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From page 96... ...
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.
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From page 97... ...
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.
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From page 98... ...
b These studies reported annual changes in SOC stock (0-30 cm depth) as percentage changes relative to baseline soil carbon stocks.
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From page 99... ...
Perennial grasses purposed as dedicated energy crops (see Chapter 4) and established on former cropland typically increase soil carbon stocks, 99
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From page 100... ...
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)
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From page 101... ...
. 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)
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From page 102... ...
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 Δ SOC (SE)
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From page 103... ...
. 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)
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From page 104... ...
. 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.
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From page 105... ...
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.
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From page 106... ...
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.
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From page 107... ...
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)
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From page 108... ...
cropland to grasslandc Paustian et al., 500-800 Deployment of enhanced root phenotypes for major annual 2016a 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.
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From page 109... ...
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)
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From page 110... ...
Estimates Nabuurs et al., 2007 0.445 Afforestation/Reforestation McKinley et al., 2011 0.001-0.225 Afforestation/Reforestation Jackson and Baker, 0.15-0.4 Afforestation/Reforestation 2010 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.
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From page 111... ...
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)
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From page 112... ...
The estimates include additional carbon removal capacity from frontier technologies that are not constrained by land availability (e.g., carbon burial approaches, substitution of high carbon input phenotypes within current land-use area distributions)
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From page 113... ...
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)
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Management options that offer synergisms with both increased soil carbon stocks and reduced N2O (and CH4) emissions should be prioritized.
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From page 115... ...
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.
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From page 116... ...
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, r espectively. These levels of activity should be achievable at low carbon prices (i.e.
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From page 117... ...
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.
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From page 118... ...
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)
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From page 119... ...
(2014) highlighted the trajectories and relative land requirements for afforestation and BECCS (Figure 3.3)
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From page 120... ...
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.
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(2008) estimated economically-achievable GHG reductions (>90 percent of which were from soil carbon sequestration)
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N E G AT I V E E M I S S I O N S T E C H N O LO G I E S A N D R E L I A B L E S E Q U E S T R AT I O N FIGURE 3.5 Mitigation potential of land-based NETs in the United States for three carbon price scenarios ($15, $30, and $50 per ton CO2e)
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From page 123... ...
. 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.
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From page 124... ...
Thus proposed funding is a 4-5X increase over this level Soil dynamics at depth 3-4 5 Funding to initiate 4-6 projects Basic Research per year Harvested wood 2.4 3 Funding to initiate 3 multiyear preservation 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 Monitoring of forest >5 ≥3 System development of $1M/y stock enhancement for 3 years projects Continuous operation -$4M/y Development and Measurement/Monitoring 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 5 Ongoing Augmentation of USDA's existing monitoring system NRI system Data-model platform 5 5 Initial development focuses on for predicting systems integration, including of and quantifying existing data sources and models agricultural soil carbon removal and storage 124
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From page 125... ...
Experimental network 6-9 ≥12 10-15 sites at a cost of $600K/y improving agricultural per site soil carbon processes Social sciences 1 3 Extension and outreach research on improving educational programs for landowner responses transferring research findings to incentives and and technologies to farmers equity among and practitioners. Funding for landowner classes initiation of 3 multiyear projects Deployment Research on GHG 1 3 Funding for initiation of 1 multi and social impacts of year project reducing traditional uses of biomass for fuel Scaling up agriculture 2 3 Support for initiation of 4-5 sequestering activities regional projects per year to identify solutions to overcome barriers to adoption 125
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From page 126... ...
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.
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From page 127... ...
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.
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From page 128... ...
. This system would provide an ongoing data stream to improve national-scale soil carbon inventory systems and reduce uncertainties.
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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.
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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.
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From page 131... ...
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. Deployment 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.
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From page 132... ...
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 132
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From page 133... ...
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)
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From page 134... ...
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.
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From page 135... ...
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)
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