Agriculture in the Driver's Seat: Climate Mitigation and Adaptation Strategies for Policy Makers
by Erin D. Matson
Global warming is a threat...
...to the current and future lives and livelihoods of billions of people around the globe. Reducing the risks of the most adverse outcomes of increased temperature rise will require concerted action from all nations to mitigate humanity’s impact on the natural systems that regulate the planet. As of 2012, net global greenhouse gas (GHG) emissions totaled 52 GtCO2eq (gigatons carbon dioxide-equivalent), a sum which must be lowered to 15-31 GtCO2eq by 2050 in order have a reasonable chance of keeping warming below 2°C (IPCC 2014). As a major contributor to anthropogenic GHG emissions and a critical nexus of human-climate interaction, agriculture as an economic sector stands to suffer significant disruption in the face of rising temperatures and extreme weather.
Recent proposals on the world stage seek to position agriculture as part of the solution to climate change through reducing emissions as well as through realizing agriculture’s carbon negative potential with soil carbon sequestration. The 4 per 1000 (4‰) Initiative introduced at COP21 in Paris challenges world governments and organizations to implement locally- appropriate soil management strategies to increase soil organic carbon (SOC) by 0.4% per year to offset anthropogenic CO2 accumulation in the atmosphere (4 pour 1000 2015). The Initiative posits one objective with three synergistic outcomes: ensuring food security through increasing soil fertility; adapting agriculture to climate change by improving soil resilience; and mitigating greenhouse gas emissions through offsetting CO2 accumulation (Ministère de l'Agriculture 2015). Multiple nations have included similar goals in their (Intended) Nationally Determined Contributions to the Paris Agreement - over 90% position agricultural reform as a mitigation and/or adaptation strategy (CGIAR 2016).
The United States acknowledges the critical role of land management in climate mitigation in its NDC as well as in the White House’s recently-released plan for 80% economic decarbonization by 2050, the United States Mid-Century Strategy for Deep Decarbonization. Though U.S. lands have already acted as a net carbon sink over the last three decades (MCS 2016), U.S. policy can do more to encourage soil-based mitigation strategies above and beyond policies and already in place. Future policy development should supplement the traditional conservation focus of soil management with innovative technologies and techniques like biochar for energy production and carbon sequestration and polycropping for bio-intensification, with co- benefits beyond climate that include improved yields, lower costs, and increased profits.
Policy-makers in the realm of agriculture and environmental protection need to hold the U.S. government accountable for its international commitments by encouraging the U.S build on its history of global leadership and innovation. U.S. agricultural policy should be designed for extreme practicality as well as for highest aspiration. We should implement readily available options as quickly and efficiently as possible, while investing in research and development to signal faith in our capacity to meet future challenges and to move assertively toward the future we wish to build.
Agriculture’s Impact and Mitigation Potential:
Agriculture and other land uses account for approximately 21% of annual anthropogenic greenhouse gas emissions, or 2.7–3.3 GtCeq per year (Tubiello, et al. 2014; Smith et al. 2014). Those emissions include carbon dioxide released by deforestation and land use change, including conversions to agriculture, as well as methane and nitrous oxide (Lal 2004b). In total, approximately 140 GtC have been released to the atmosphere since the industrial revolution due to land use change and cultivation; of that total, 80GtC has specifically come from a depletion of SOC (Lal 2004b).
The idea of soil carbon sequestration to lock atmospheric carbon back into the world's soils has long been posited as a potential avenue for climate change mitigation. According to models by Sommer and Bossio (2014) for cropland and pasture soil-carbon sequestration potential, between 25 and 50 Gt of carbon could be sequestered in farmland by the end of the century, thereby serving as a short-term climate stabilization wedge (Lassaletta and Aguilera 2015).
While the forest regrowth and management have made U.S. lands a net carbon sink over the last three decades, agriculture itself has been a net source of CO2 emissions in the same period (MCS 2016). Over 55% of U.S. agricultural GHG emissions are a result of soil management practices that do not prioritize carbon sequestration, leading to a loss of over 18 million tC from America's valuable agricultural lands (EPA 2016a; EPA 2016b).
Overall U.S. lands sequestered approximately 0.2 GtC in 2014, due mainly to increased forest cover, counteracting about 11% of the entire U.S. economy’s emissions (EPA 2016a; MCS 2016). Cropland sequestration is not included in the Mid-Century Strategy’s defined roadmaps to 2050 because of imprecision in accounting methods for soil carbon and the reversibility of soil carbon retention. However, the report estimates that U.S. croplands could supplement robust forest sequestration with an additional 74 million tC/year if carbon-smart soil management were expanded to over 70% of current farmland. That additional sequestration would be on top of the Strategy’s estimates that U.S. lands could sequester 23-45% of 2050 U.S. emissions.
Soil Degradation and Carbon Sequestration
Soil degradation is detrimental to the functioning of a healthy agricultural ecosystem, even without the likely compounding effects of a warming climate. Soils provide humankind with essential ecosystem services that depend in large part on adequate SOC content. These functions include supports like soil fertility, water retention, and biodiversity; regulating services like flood control; and provisioning services through the production of food, fiber and fuels (Victoria et al. 2012). As each of these services is compromised through the loss of SOC, it threatens not only humanity's ability to maintain current levels of agricultural production, but also our potential to adapt to the effects of climate change.
In the face of the dual trend of a warming earth and depleting soils, soil carbon sequestration provides the promise of adaptive and resilience benefits for global agriculture that speak to its utility even without its carbon-negative potential. Rising temperatures and increased incidences of droughts and flooding due to climate change are expected to decrease overall agricultural yields by 17% by the year 2050 (Nelson et al. 2014). Returning carbon to agricultural soils would serve to rehabilitate degraded land and to improve soil health, a critical component of ensuring future food security. In principle, cropland carbon sequestration is possible; however, there are real limits to the mitigation potential of sequestration that should be considered when designing policy (Powlson et al. 2016; Spokas et al. 2012). First, the soil’s capacity to reabsorb carbon is finite over a defined time period, as SOC levels reach a new equilibrium. Second, sequestration is reversible if land managers do not maintain the required management practices indefinitely. Finally, carbon returned to the soil must be in stable forms that are resistant to microbial mineralization to be considered truly sequestered (Powlson et al. 2016). Different management practices result in various ratios of stable versus labile (easily mineralized) carbon (Spokas et al. 2012), indicating the need for comparative assessment and targeted policies that support the most efficient practices for mitigating climate change.
Current Soil-Carbon Practices
Given the scale of U.S. agriculture and its global impact, it is imperative that domestic policies be evolved to encourage aggressive action on soil carbon sequestration. Effective mitigating soil management strategies are not merely theoretical; on the contrary, a number of techniques for carbon-smart farming are ready for immediate implementation. In this section, current policy and techniques are presented and critiqued, while avenues for future innovation are proposed. No-till agriculture is among the most widely adopted soil conservation strategies in the United States. Recent studies, however, express skepticism on no-till’s carbon sequestration potential and permanence. For this reason, the complementary approaches of polycropping and biochar are presented, offering other concrete opportunities for the agricultural sector to engage in climate change mitigation while enhancing productivity and resiliency.
Mainstream soil policy in the United States has, since the catastrophic erosion events of the 1930s, focused primarily on soil conservation (NRCS 2016a). The standard practices of the conservation approach center on erosion-control, seeking to limit annual soil loss to a "tolerable" level (NRCS 2016a). Considering that about 24% of agricultural land is currently degraded (Dumanski and Peiretti 2013), a defensive approach to soil management, while necessary, is no longer sufficient to ensure continued productivity. Rehabilitation of degraded farmland is compatible with a range of goals beyond climate mitigation, including resilience and adaptation, maintaining environmental quality, and ensuring food security as global population increases.
Conservation agriculture, with its three main practices of reduced/no tillage, year-round soil cover through cover-cropping or retaining crop residues, and diverse crop rotations, focuses on preserving and rebuilding the valuable and vulnerable top layer of soil (Dumanski and Peiretti 2013; NRCS 2016b; Powlson et al. 2016). As of the 2012 USDA Census of Agriculture, 62% of eligible U.S. cropland was under either no-till or conservation-tillage management, increasing at an average rate of 2.3% over the last 40 years (Dobberstein 2014). This increasingly popular approach to soil management has received attention for its carbon sequestration capacity as well as the reduction in fossil fuel use from reducing field passes.
The cornerstone of conservation agriculture is no tillage (“no-till”), a method where seeds are drilled through unincorporated crop residue directly into the soil below. Theoretically, soil organic carbon is preserved and increased by reducing oxidation from tillage, maintaining soil structure and a healthy soil microbiome, and reducing erosion. Though a number of studies (Lal 2004a; Lal et al. 2011; Neufeldt, Kissinger, and Alcamo 2015) support the notion of no-till as a climate change solution, Powlson et al. (2014) offer a critique of soil carbon accounting methods and conclude that most studies overestimate sequestration. A later review of the literature reports that most carbon increases are in labile, rather than stable, forms (Powlson et al. 2016). Other concerns with no-till’s climate impact include the effect on fertilizer uptake of transitioning between soil management systems; nitrous oxide emissions persist at increased levels for up to a decade after the transition to no-till (Six et al. 2004). Finally, no-till’s gains are easily reversible. Tilling the soil even once can undo years of carbon accumulation, and estimates show that fewer than 10% of American farmers who practice no-till do so continuously (VandenBygaart 2016).
While no-till's climate potential may be limited, its agronomic benefits resulting from improved soil quality are valuable for the adaptive capacity they provide. Higher rates of surface-layer SOC can increase yields for more intensive production, help retain moisture in the face of drought, and improve soil structure to resist erosion (Powlson et al. 2016). Additionally, some farmers see higher profits from reduced time and fuel expenditure on tillage (Dumanski and Peiretti 2013). No-till seems to work best when combined with the other critical aspects of con- servation agriculture, cover cropping and crop rotation, rather than being implemented on its own. Studies show that risk of yield loss increases on farms practicing no-till without the other practices (Powlson et al. 2015). When no-till and cover-cropping are practiced in tandem, however, yield risks decline while carbon sequestration rates climb to 0.45 tC/ha/year (Powlson et al. 2016). The synergy of the three-pronged CA approach speaks to the aggregate benefits of applying multiple, simultaneous strategies for soil rehabilitation through soil carbon sequestration. While no-till is not sufficient, on its own, to mitigate climate change and position agriculture as a net carbon sink, it is an important strategy that is easy to implement, enjoys broad-based support among U.S. farmers, and is already incorporated into U.S. soil policy.
The Future: Promising Approaches to Scale Up
Moving beyond no-till agriculture to include alternative strategies for soil carbon management may hold the key for U.S. cropland to fulfill its restorative, carbon-sink potential. The following practices, polycropping and biochar, have demonstrated potential for enabling climate mitigation while improving agriculture’s adaptive capacity.
Polycropping broadly refers to planting more than one type of crop within the same field in the same growing season. It includes such diverse practices as cover cropping, intercropping and companion planting, and agroforestry. The key to polycropping’s benefits is planting complementary crops that fill different “niches” in the field ecosystem, thereby reducing competition and increasing overall utilization of sunlight and soil space (Brooker et al. 2014). Through strategic design and careful implementation, like intercropping rows of nitrogen-fixing legumes adjacent to nitrogen-loving corn, or incorporating carbon-sequestering tree species into agroforestry systems, overall yields can be preserved or improved while increasing total biomass produced per acre (Powlson et al. 2016).
Soil carbon is on average 4% higher in intercropped fields than in monocultures (Cong et al. 2014). This increase stems from the synergy of an average of 23% more root matter in intercropped fields along with enhanced soil microbial activity (Cong et al. 2014; Lange et al. 2014). Cong et al. estimate that the net benefit from intercropped versus monocropped systems is relatively small at about 184kgC/ha/year (2014). However, in concert with other carbon-focused soil management practices, intercropping could contribute to an overall soil-based carbon sink.
Agroforestry, a form of polycropping where annual or perennial crops are grown between stands of trees and which often incorporates the rotational grazing of livestock, offers even stronger potential for increased SOC sequestration and yield gains than simple intercropping. In a meta-analysis of over 115 studies and reports on agroforestry, Lorenz and Lal (2014) outline the potential benefits and research needs for incorporating agroforestry into modern agricultural systems. Because tree roots reach deeply into the soil, and carbon from plant roots is 1.5 to 3 times more likely to be stabilized into the SOC pool than above-ground carbon, cropping systems that integrate trees have the potential to store more stable carbon than traditional cropping systems (Lorenz and Lal 2014). Some additional co-benefits of agroforestry include that tree roots can recover nutrients and water from deep soil horizons, making them available to other, shallow-rooted crops; and tree roots provide erosion control services (Lorenz and Lal 2014). Some studies, however, reported negative yield effects due to root-space competition, indicating a need for further research into optimal tree/crop pairings. Though data on total pools of SOC in agroforestry systems is easier to come by than sequestration rates, overall global potential is estimated at 1.1 to 2.2 GtC/year, which could offset a significant portion of agriculture's total GHG emissions of 2.7 to 3.3 GtC/year (Lorenz and Lal 2014).
When designed well, intercropping can boost soil fertility, decreasing the need for fossil fuel-derived fertilizers, a natural answer to concerns over nitrous oxide emissions from fertilizer. The enhanced microbial activity from intercropping fixes N from the air in partnership with symbiotic plants while reducing N leaching as nitrous oxide (Cong et al. 2014). As a result, intercropped fields were found to have about 11% more plant-available N than monocropped, suggesting a sequestration rate of 45-50 kg N/ha/year (Cong et al. 2014; Powlson et al. 2016). Given average U.S. fertilizer consumption of 140 kg/ha in 2013 (FAO 2016), intercropping can reduce fertilizer use by about 1⁄3, with exponential reductions in nitrous oxide emissions (Shcherbak et al. 2014). These emissions reductions, unlike soil carbon sequestration, are not reversible and have no limit, meaning that the long-term reduction of fertilizer use may be the most significant GHG mitigation effect of improved soil management (Powlson et al. 2016). In addition to reducing expense on fertilizer, polycropping diversifies farm portfolios, adding resilience and economic value in the face of crop failure or price shocks (Powlson et al. 2016).
Biochar refers to a type of charcoal made from biomass through low-oxygen exposure to extremely high temperatures. It is characterized by extremely high levels of stable carbon that resists microbial mineralization, pointing to its potential for long-term C storage in agricultural soils (Spokas et al. 2012). Two main processes for creating biochar, pyrolysis and gasification, each also produce syngas, which is useful for power generation with about half the energy potential of natural gas. While pyrolysis produces more biochar than gasification, the resultant carbon is in a less-stable form due to differences in temperature and oxygen levels during production (Spokas et al. 2012; Hansen et al. 2015).
Biochar can be produced from any available biomass, though the greatest synergy with agricultural production would be to use crop residues in on-farm cookstoves, local commercial- scale pyrolyzers, or gasification plants for regional power generation (Wheeling 2016; Hansen et al. 2015). Incorporating biochar into croplands results in significantly more carbon being retained in the long-term when compared to the incorporation of untreated crop residue (Hansen et al. 2015). Indeed, various studies have confirmed that the mean residence times (MRT) of biochar carbon in agricultural soils range from just over 100 to over 1,000 years, depending on biomass source, production process, and average temperature of the ground (Fang et al. 2014; Vochozka et al. 2016). In aggregate, biochar’s negative emission potential is estimated to be 0.7 GtC/year, not including offsets from fertilizer reduction or fossil fuel displacement (Smith 2016).
The key to biochar’s agronomic benefits is its extremely high surface area, which promotes greater soil conductivity, a key indicator of soil fertility; nutrient and water retention; improved habitat for the soil microbiome; and strengthened soil structure (Vochozka et al. 2016; Hansen et al. 2015). While most studies indicate a neutral or positive yield response to biochar application, current biochar pricing limits farmers’ ability to incorporate the substance into their soil management portfolio (Vochozka et al. 2016). Among biochar’s major benefits is its capacity for synergy with other soil management techniques to localize energy and fertilizer production. Biochar can complement polycropped systems by utilizing increased biomass to power regional syngas electricity production while returning biochar for fertilizer application (Hansen et al. 2015). Advocates for biochar point out its relative logistical superiority to bioenergy with carbon capture and storage (BECCS). While BECCS requires captured carbon to be transferred to a centralized storage location, biochar can be produced and utilized locally, closing the transport loop and improving soil fertility simultaneously (Woolf, Lehmann and Lee 2016). Technology for local and regional biochar and syngas production already exists in implementable forms (Woolf, Lehmann and Lee 2016; Hansen et al. 2015); what remains are the policies and incentives to drive investment and further research into this promising technology.
As the United States moves forward under its commitment to combat climate change via the Nationally Determined Contribution pledge submitted to the Paris Agreement, the following policy recommendations for domestic agriculture will allow the U.S. to lead on climate action while remaining on track for achieving 80% economic decarbonization by 2050. The United States Mid-Century Strategy for Deep Decarbonization (MCS) recognizes the critical role that land sinks play in a net-zero carbon future. In outlining the role of land sinks, however, the MCS does not factor soil management strategies into its decarbonization roadmaps due to measurement uncertainty and sequestration reversibility. This omission presents an opportunity for soil management to exceed the limited expectations laid upon it. Additionally, the co-benefits beyond climate mitigation from improved soil carbon management provides ample incentive for constructing supportive policy. And, unlike other carbon negative technologies like BECCS, methods for carbon sequestration in agricultural soils are ready to deploy immediately at scale, given the proper policy climate to incentivize private landholders. Having made the commitment to fight climate change on the world stage, the United States will be held to account by the international community and should set domestic policy that aligns with the global interest in halting and reversing the negative impacts of climate change.
Payments for on-farm carbon sequestration: Most agricultural and forest land in the U.S. is under private management, meaning that effective “incentive structures” must be implemented to inspire private action for the public good (MCS 2016). Ideally, incentives would compensate farmers proportionately for the total amount of carbon sequestered on their land as well as that added yearly, in recognition of the public service such sequestration represents. However, due to flawed measurement methodologies and the cost of monitoring and tracking, results-based payments would be costly to implement within existing policy structures (MCS 2016). Therefore, practice-based payments will be the best option for immediate deployment. Agricultural policy incentivizing voluntary conservation practices has a long and established history in the U.S., dating back to the 1930s and the establishment of the Soil Conservation Service. The key to practice-based incentives is that while the government determines which practices to encourage, participation by private landowners is entirely voluntary (Richards, Sampson, and Brown 2006). Voluntary participation is critical because the Fifth Amendment's "takings" clause restricts the government’s ability to impose regulations (such as limits on carbon emissions or mandating certain practices) that might diminish land value (Richards, Sampson, and Brown 2006).
There are two main advantages for promoting carbon sequestration through practice-based policies. First, the knowledge required for carbon-focused soil management can be conveyed through existing networks like farm extension services; and second, cost and effort does not have to be expended on measuring and tracking changes in SOC (Richards, Sampson and Brown 2006). The Mid-Century Strategy report suggests that continued improvement of existing pro- grams like crop insurance is the best avenue for encouraging carbon sequestration practices while achieving conservation and water quality goals. Considering the above review of the limited efficacy of no-till agriculture, payments should be structured to incentivize cover cropping and crop rotation as part of a no-till strategy. In addition, the crop insurance program should expand to include special consideration for innovative experimentation with promising techniques like polycropping and the use of biochar. Though the 2014 Farm Bill expanded crop insurance to allow whole-farm coverage of specialty crop growers (USDA 2014), increasing targeted support for crops like vegetables, nuts and fruits could drastically increase the sequestration potential of America’s farmlands by encouraging intensive interplanting and agroforestry systems. Similarly, payments for the direct application of biochar to farm soils would be one of the most reliable ways to increase overall SOC content in the long-term, considering the mean residence times for biochar-derived carbon.
As methodologies for measuring and tracking SOC improve, transitioning from practice- based to results-based payments will lead to cheaper, more effective carbon sequestration in the agricultural sector in the long run (MCS 2016). Targeting payments based on the amount of carbon sequestered by farmers, rather than on the implementation of specific practices, will encourage innovation and ultimately achieve a lower cost with better results (Richards, Sampson, and Brown 2006). Existing programs like subsidies for crop insurance should be reserved or increased for farms demonstrating year-over-year increases in SOC levels. Simultaneously, long- term sequestration should be incentivized to reduce relative incentives for carbon-releasing prac- tices like returning to conventional tillage. First, carbon-based payments could include a payback clause similar to that in the Environmental Quality Incentives Program, wherein farmers would be required to return all federal payments received for sequestration upon use of a banned tech- nique within a defined period (Richards, Sampson, and Brown 2006). Second, subsidy payments could increase over time conditional upon continuous implementation of carbon-farming techniques, with payments in proportion to the cumulative pool of SOC stored on-farm. Payments designed to account for the total storage of carbon on a farm rather than merely on yearly increases will avoid penalizing early adopters who begin carbon management while payments are practice-based (Richards, Sampson, and Brown 2006).
Investment in research and development: The full implementation of the most efficient carbon sequestration policies, including results-based payments for carbon sequestration and financial supports for alternative strategies like agroforestry, currently face hurdles due to the lack of standardized carbon accounting methodologies and sound estimates of carbon sequestration potentials. For this reason, the 2018 U.S. Farm Bill should have an increased budget for research and development on carbon accounting protocols and quantifying sequestration capacities for a range of soil management practices (MCS 2016). Such protocols are critical for ensuring targeted payment systems that result in the most carbon storage, and for guiding best practices for overall farm health in conjunction with climate mitigation goals.
The public sector should also invest in research, development, and implementation of demonstration-scale biochar-based energy and soil amendment production on a regional basis. Because of the lack of a market-wide, standardized carbon price, biochar has thus far failed to breakthrough as a cost-effective means of agricultural soil sequestration. Investing in technology research and development for biochar can compensate for this market failure (Smith et al. 2014). Finally, in concert with the Mid-Century Strategy’s stated goal of exploring new avenues for reducing nitrogen fertilizer application to reduce nitrous oxide emissions, further research money should be devoted to the quantification of the fertilizer offsetting potential of companion planting and strategic intercropping.
Promoting yield increases to spare other land: Actively-managed forests can sequester more carbon than farmland, so meeting food demand on less acreage will reduce land competition and increase carbon sequestration, both through increased total forest cover and through greater biomass concentration on existing farmland (Lamb et al. 2016). The Mid-Century Strategy advocates targeting policies to promote research on further increasing yields to reduce competition for land. Evidence shows that expanding niche utilization through polycropping (Brooker et al. 2014) and improved soil fertility from biochar (Vochozka et al. 2016) achieve the dual goals of increasing yield intensity while allowing land with marginal productivity to be reserved for more carbon-focused perennial plantings.
The Conservation Reserve Program, which provides payments to farmers in exchange for retiring and converting marginal land, has historically provided significant carbon storage benefits (Richards, Sampson, and Brown 2006). However, recent Farm Bills (in 2004, 2008 and 2014) reduced the number of eligible acres from 39.2 million to 24 million currently (Richard, Sampson, and Brown 2006; Stubbs 2014). Program retention has also declined due to high commodity prices encouraging farmers to put reserved acres back into production (Stubbs 2014). Federal payments for reserved land must be designed to be competitive with market rental rates and adjusted for food prices to incentivize continued enrollment in the program (Lamb et al. 2016). As the government invests more resources in improving cropland productivity, it can design more meaningful yet cost-effective incentives for CRP enrollment and should, therefore, reduce the trend of lowering acreage caps.
With the adverse effects of climate change already evident...
...across the globe in the form of extreme weather, rising sea levels, and desertification, the time for decisive action to halt anthropogenic contribution to atmospheric greenhouse gasses is now. Agriculture, as humanity’s most direct point of interaction and exchange with world soils, provides an opportunity for diffuse, immediate, and impactful action to rebuild the earth’s fertility while combating climate change.
Land management in the United States is a net carbon sink, but its agricultural lands still contribute to GHG accumulation in the atmosphere. Traditional, conservation-based soil management has so far been insufficient to realize the potential of U.S. farmland to be net carbon-negative. Therefore, new and innovative strategies like polycropping and biochar should be introduced in tandem with existing soil conservation methods to maximize carbon sequestration potential while diversifying agricultural output, increasing soil fertility, and ensuring long-term resiliency to the adverse effects of climate change.
Domestic agricultural policy must be designed to encourage immediate implementation of carbon-sequestering practices across all U.S. cultivable land. Simultaneously, investments in research and development of reliable, science-based carbon accounting methods will allow a transition from practice-based incentive structures toward more cost-effective payments for results. As the United States invests in and implements new aggressive strategies for cropland carbon sequestration, it will inspire action from the rest of the global community while meeting its obligation to contribute to concerted international action on climate change.
4 per 1000 Initiative. 2015. “Understand the ‘4 per 1000’ Initiative.” 4 POUR 1000. http://4p1000.org/understand.
Brooker, Rob W., Alison E. Bennett, Wen-Feng Cong, Tim J. Daniell, Timothy S. George, Paul D. Hallett, Cathy Hawes, et al. 2015. “Improving Intercropping: A Synthesis of Research in Agronomy, Plant Physiology and Ecology.” New Phytologist 206 (1): 107–17. doi:10.1111/nph.13132.
CGIAR. 2016. “Agriculture’s Prominence in the INDCs: Data and Maps | CCAFS: CGIAR Research Program on Climate Change, Agriculture and Food Security.” April 8. https://ccafs.cgiar.org/agricultures-prominence-indcs-data-and-maps.
Cong, Wen-Feng, Ellis Hoffland, Long Li, Johan Six, Jian-Hao Sun, Xing-Guo Bao, Fu-Suo Zhang, and Wopke Van Der Werf. 2014. “Intercropping Enhances Soil Carbon and Nitrogen.” Global Change Biology 21 (4): 1715–26. doi:10.1111/gcb.12738.
Dobberstein, John. 2014. “No-Till Movement in U.S. Continues to Grow.” No-Till Farmer. August 1. https://www.no-tillfarmer.com/articles/489-no-till-movement-in-us-continues-to- grow.
Dumanski, J., and R. Peiretti. 2013. “Modern Concepts of Soil Conservation.” International Soil and Water Conservation Research 1 (1): 19–23. doi:10.1016/S2095-6339(15)30046-0.
EPA. 2016a. “Greenhouse Gas Inventory Data Explorer | US EPA. https://www3.epa.gov /climatechange/ghgemissions/inventoryexplorer/#iagriculture/allgas/source/current.
EPA 2016b. “Understanding Global Warming Potential.” Retrieved from Greenhouse Gas Emissions: https://www.epa.gov/ghgemissions/understanding-Global-Warming-Potentials
Fang, Y., B. Singh, B. P. Singh, and E. Krull. 2014. “Biochar Carbon Stability in Four Contrasting Soils: Biochar Carbon Stability in Soils.” European Journal of Soil Science 65 (1): 60–71. doi:10.1111/ejss.12094.
FAO. 2014. “The State of the World’s Land and Water Resources for Food and Agriculture (SOLAW) – Managing Systems at Risk.” Food and Agriculture Organization of the United Nations, Rome and Earthscan, London. http://www.fao.org/docrep/017/i1688e/i1688e00.htm.
FAO. 2016. “Fertilizers.” FAOSTAT. http://www.fao.org/faostat/en/#home.
Hansen, Veronika, Dorette Müller-Stöver, Jesper Ahrenfeldt, Jens Kai Holm, Ulrik Birk Henriksen, and Henrik Hauggaard-Nielsen. 2015. “Gasification Biochar as a Valuable by- Product for Carbon Sequestration and Soil Amendment.” Biomass and Bioenergy 72 (January): 300–308. doi:10.1016/j.biombioe.2014.10.013.
IPCC. 2014. “Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)].” Geneva, Switzerland: IPCC. https://www.ipcc.ch/report/ar5/syr/.
Lal, R. 2004a. “Soil Carbon Sequestration Impacts on Global Climate Change and Food Security.” Science 304 (5677): 1623–27. doi:10.1126/science.1097396.
———. 2004b. “Soil Carbon Sequestration to Mitigate Climate Change.” Geoderma 123 (1–2): 1–22. doi:10.1016/j.geoderma.2004.01.032.
Lal, R., J. A. Delgado, P. M. Groffman, N. Millar, C. Dell, and A. Rotz. 2011. “Management to Mitigate and Adapt to Climate Change.” Journal of Soil and Water Conservation 66 (4): 276–85. doi:10.2489/jswc.66.4.276.
Lamb, Anthony, Rhys Green, Ian Bateman, Mark Broadmeadow, Toby Bruce, Jennifer Burney, Pete Carey, et al. 2016. “The Potential for Land Sparing to Offset Greenhouse Gas Emissions from Agriculture.” Nature Climate Change, January. doi:10.1038/nclimate2910.
Lange, Markus, Nico Eisenhauer, Carlos A. Sierra, Holger Bessler, Christoph Engels, Robert I. Griffiths, Perla G. Mellado-Vázquez, et al. 2015. “Plant Diversity Increases Soil Microbial Activity and Soil Carbon Storage.” Nature Communications 6 (April): 6707. doi:10.1038/ncomms7707.
Lassaletta, Luis, and Eduardo Aguilera. 2015. “Soil Carbon Sequestration Is a Climate Stabilization Wedge: Comments on Sommer and Bossio (2014).” Journal of Environmental Management 153 (April): 48–49. doi:10.1016/j.jenvman.2015.01.038.
Lorenz, Klaus, and Rattan Lal. 2014. “Soil Organic Carbon Sequestration in Agroforestry Systems. A Review.” Agronomy for Sustainable Development 34 (2): 443–54. doi:10.1007/s13593-014-0212-y.
MCS. 2016. “United States Mid-Century Strategy for Deep Decarbonization.” Washington, D.C.: The White House. https://www.whitehouse.gov/sites/default/files/docs/mid_century_strategy_report-final.pdf.
Ministère de l’Agriculture, de l’Agroalimentaire et de la Forêt. 2015. “Join the 4‰ Initiative: Soils for Food Security and Climate.” Paris: Ministry of Agriculture, Agrifood and Forestry. http://agriculture.gouv.fr/sites/minagri/files/4pour1000-gb_nov2015.pdf.
Nelson, Gerald C., Dominique Mensbrugghe, Helal Ahammad, Elodie Blanc, Katherine Calvin, Tomoko Hasegawa, Petr Havlik, et al. 2014. “Agriculture and Climate Change in Global Scenarios: Why Don’t the Models Agree.” Agricultural Economics 45 (1): 85–101.
Neufeldt, Henry, Gabrielle Kissinger, and Joseph Alcamo. 2015. “No-till Agriculture and Climate Change Mitigation.” Nature Climate Change 5 (6): 488–89. doi:10.1038/nclimate2653.
NRCS. 2016a. “Manage for Soil Carbon | NRCS Soils.” https://www.nrcs.usda.gov/wps/portal/nrcs/detailfull/soils/health/mgnt/?cid=stelprdb12375 84.
———. 2016b. “Role of Soil Organic Matter | NRCS Soils.” https://www.nrcs.usda.gov/wps/portal/nrcs/detailfull/soils/health/mgnt/?cid=nrcs142p2_05 3859.
Powlson, David S., Clare M. Stirling, M. L. Jat, Bruno G. Gerard, Cheryl A. Palm, Pedro A. Sanchez, and Kenneth G. Cassman. 2014. “Limited Potential of No-till Agriculture for Climate Change Mitigation.” Nature Climate Change 4 (8): 678–83.
———. 2015. “Reply to ‘No-till Agriculture and Climate Change Mitigation.’” Nature Clim. Change 5 (6): 489–489.
Powlson, David S., Clare M. Stirling, Christian Thierfelder, Rodger P. White, and M. L. Jat. 2016. “Does Conservation Agriculture Deliver Climate Change Mitigation through Soil Carbon Sequestration in Tropical Agro-Ecosystems?” Agriculture, Ecosystems & Environment 220: 164–74.
Kenneth R. Richards, R. Neil Sampson, and Sandra Brown. 2006. “Agricultural and Forestlands: U.S. Carbon Policy Strategies.” Pew Center on Global Climate Change. http://www.c2es.org/publications/agricultural-and-forestlands-us-carbon-policy.
Shcherbak, Iurii, Neville Millar, and G. Philip Robertson. 2014. “Global Metaanalysis of the Nonlinear Response of Soil Nitrous Oxide (N2O) Emissions to Fertilizer Nitrogen.” Proceedings of the National Academy of Sciences 111 (25): 9199–9204. doi:10.1073/pnas.1322434111.
Shindell, Drew, Johan CI Kuylenstierna, Elisabetta Vignati, Rita van Dingenen, Markus Amann, Zbigniew Klimont, Susan C. Anenberg, et al. 2012. “Simultaneously Mitigating near-Term Climate Change and Improving Human Health and Food Security.” Science 335 (6065): 183–89.
Six, Johan, Stephen M. Ogle, F. Jay breidt, Rich T. Conant, Arvin R. Mosier, and Keith Paustian. 2004. “The Potential to Mitigate Global Warming with No-Tillage Management Is Only Realized When Practised in the Long Term.” Global Change Biology 10 (2): 155–60. doi:10.1111/j.1529-8817.2003.00730.x.
Smith P., M. Bustamante, H. Ahammad, H. Clark, H. Dong, E.A. Elsiddig, H. Haberl, R. Harper, J. House, M. Jafari, O. Masera, and C. Mbow, N.H. Ravindranath, C.W. Rice, C. Robledo Abad, A. Romanovskaya, F. Sperling, and F. Tubiello. 2014. “Agriculture, Forestry and Other Land Use (AFOLU).” In: Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Edenhofer, O., R. Pichs-Madruga, Y. Sokona, E. Farahani, S. Kadner, K. Seyboth, A. Adler, I. Baum, S. Brunner, P. Eickemeier, B. Kriemann, J. Savolainen, S. Schlömer, C. von Stechow, T. Zwickel and J.C. Minx (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
Smith, Pete. 2016. “Soil Carbon Sequestration and Biochar as Negative Emission Technologies.” Global Change Biology 22 (3): 1315–24. doi:10.1111/gcb.13178.
Sommer, Rolf, and Deborah Bossio. 2014. “Dynamics and Climate Change Mitigation Potential of Soil Organic Carbon Sequestration.” Journal of Environmental Management 144 (November): 83–87. doi:10.1016/j.jenvman.2014.05.017.
Spokas, Kurt A., Keri B. Cantrell, Jeffrey M Novak, David W. Archer, James A. Ippolito, Harold P. Collins, Akwasi A. Boateng, et al. 2012. “Biochar: A Synthesis of Its Agronomic Impact beyond Carbon Sequestration.” Journal of Environmental Quality 41 (4): 973–89.
Stubbs, Megan. 2014. “Conservation Reserve Program (CRP): Status and Issues.” Congressional Research Service. http://nationalaglawcenter.org/wp- content/uploads/assets/crs/R42783.pdf.
Tubiello, F. N., M. Salvatore, R. D. Cóndor Golec, A. Ferrara, S. Rossi, R. Biancalani, S. Federici, H. Jacobs, and A. Flammini. 2014. “Agriculture, Forestry and Other Land Use Emissions by Sources and Removals by Sinks.” Statistics Division, Food and Agriculture Organization, Rome. http://www.uncclearn.org/sites/default/files/inventory/fao198.pdf.
USDA. 2014. “New Pilot Program Offers Coverage for Fruits and Vegetables, Organic and Diversified Farms | USDA Newsroom.” May 21. http://www.usda.gov/wps/portal/usda/usdahome?contentidonly=true&contentid=2014/05/0 100.xml.
VandenBygaart, A. J. 2016. “The Myth That No-till Can Mitigate Global Climate Change.” Agriculture, Ecosystems & Environment 216 (January): 98–99. doi:10.1016/j.agee.2015.09.013.
Victoria, Reynaldo, Steven Banwart, Helaina Black, John Ingram, Hans Joosten, Eleanor Milne, Elke Noellemeyer, and Yvonne Baskin. 2012. “The Benefits of Soil Carbon: Managing Soils for Multiple Economic, Societal and Environmental Benefits.” UNEP Year Book 2012. UN Environmental Programme. http://www.unep.org/yearbook/2012/pdfs/UYB_2012_CH_2.pdf.
Vochozka, Marek, Anna Maroušková, Jan Váchal, and Jarmila Straková. 2016. “Biochar Pricing Hampers Biochar Farming.” Clean Technologies and Environmental Policy 18 (4): 1225– 31. doi:10.1007/s10098-016-1113-3.
Wheeling, Kate. 2016. “Biochar at COP22: Fighting Climate Change From the Ground Up.” Pacific Standard. November 8. https://psmag.com/biochar-at-cop22-fighting-climate- change-from-the-ground-up-c3ab90875da7.
Woolf, Dominic, Johannes Lehmann, and David R. Lee. 2016. “Optimal Bioenergy Power Generation for Climate Change Mitigation with or without Carbon Sequestration.” Nature Communications 7 (October): 13160. doi:10.1038/ncomms13160.