Changing the conversation around soil carbon
Bradford, Mark A., et al. “Soil carbon science for policy and practice.” Nature Sustainability 2 (November 2019): 1070-1072. DOI: https://doi.org/10.1038/s41893-019-0431-y
Tautges, Nicole E., et al. “Deep soil inventories reveal that impacts of cover crops and compost on soil carbon sequestration differ in surface and subsurface soils.” Global Change Biology 25.11 (July 2019): 3753-3766. DOI: https://doi.org/10.1111/gcb.14762
Oldfield, Emily E., Bradford, Mark A., & Wood, Stephen A. “Global meta-analysis of the relationship between soil organic matter and crop yields.” SOIL 5 (2019): 15-32. DOI: https://doi.org/10.5194/soil-5-15-2019
A broad consensus of scientists agree that keeping global climate change in check will require humans to both reduce greenhouse gas emissions and remove considerable amounts of carbon dioxide from the atmosphere – a process known as carbon sequestration. Regenerative agricultural practices have the potential to help meet this goal by capturing and storing carbon in the soil – an idea that is gaining steam among landowners, policy makers, and entrepreneurs. Start-ups like Nori and Indigo Ag are launching voluntary carbon markets where corporations can “offset” their own emissions by paying farmers to adopt carbon-capturing land management techniques. New government programs from Australia to California are incorporating soil carbon offsets into their climate change mitigation strategies.
However, measuring the amount of carbon sequestered in soils at the scale and accuracy necessary for these programs to succeed is a challenging task. Much more research is needed to understand how useful these initiatives will be in achieving overall climate goals. Three recent articles provide insights into the challenges and opportunities related to soil carbon and present new ideas for farmers, investment companies, and policy makers to consider when structuring agricultural soil initiatives. Together, studies like these make the case for a fundamental reframing of the public discourse on soil carbon away from a narrow focus on offsets and sequestration and towards a holistic environmental framework that may better fit what these strategies can achieve.
Most nature-based climate solutions, such as tree planting and forest restoration, focus on capturing carbon in living biomass. The dynamics of carbon capture within soils, however, are less understood. All plants draw in carbon dioxide from the air and incorporate it into their tissues through the process of photosynthesis. This carbon can then be released to soil microorganisms through the roots or incorporated into the soil when the plant dies and decomposes. This process, on a global scale, has stored an enormous amount of carbon in the soil – more than three times the amount of carbon currently in the atmosphere.
Soil scientists agree that soil carbon is crucial for soil health and fertility, as Yale’s Dr. Mark Bradford outlined in a recent Nature Sustainability article. Soil carbon improves agricultural yield size and stability by preventing erosion and retaining moisture and nutrients. Modern industrial agricultural practices like monocropping, overgrazing, and intensive tillage are known to deplete soil carbon, resulting in land degradation, reduced yields, and increased reliance on external inputs like synthetic fertilizers and pesticides. These inputs pose high costs for farmers while also generating greenhouse gas emissions and pollution in surrounding habitats and waterways. Scientists estimate that about 133 billion tons of carbon have been lost from global soils – equivalent to about a quarter of all human emissions. Rebuilding this lost carbon would restore fertility, provide economic security to farmers, and maintain global food security – while also removing large amounts of carbon dioxide from the atmosphere.
Regenerative practices, like cover cropping, no-till and low-till farming, and agroforestry, can protect and rebuild soil carbon, improving soil health, and overall farm sustainability. Less is known, however, about their long-term ability to mitigate climate change. Bradford and colleagues from the Science for Nature and People Partnership highlighted uncertainties in how much carbon these practices can sequester. The rate of carbon accumulation varies widely among different climates, soil types, and moisture levels, and can even vary within the same agricultural field. Regenerative practices may only increase soil carbon up to a certain point, after which soils become saturated and cease removing carbon from the atmosphere. Furthermore, there remains little knowledge of how long sequestered carbon will remain in the soil, especially if the farmer halts regenerative management. If the sequestered carbon does not stay in the soil long enough to permanently reduce atmospheric concentrations of carbon dioxide, this strategy’s climate change mitigation potential will be limited.
The authors assert that these uncertainties are mainly due to a lack of long-term and large-scale studies and the immense difficulty of measuring soil carbon, which is still largely done through manual sampling. The process of collecting and processing enough soil cores to generate accurate data on soil carbon content is costly and time-consuming for both farmers and carbon offset organizations in voluntary and public carbon markets. This expense may deter adoption and compromise policy effectiveness. Given these challenges, critics fear that the flurry of attention surrounding soil carbon will accomplish little besides diverting resources from the imperative to reduce emissions from energy, transportation, and industry.
One example of the uncertainties outlined by Bradford’s article is illustrated in a study recently published by Nicole E. Tautges and colleagues in Global Change Biology. This research was part of The Century Experiment, an ongoing long-term trial of sustainable cropping systems at the University of California, Davis. In a 19-year trial of rotational cropping systems, the addition of winter cover crops increased carbon stocks in the top 30 cm of the soil. However, when measured to a depth of two meters, there was an overall loss of carbon across the entire soil profile. Since 30 cm is the standard depth of measurement commonly used in carbon offsetting schemes, such a program would have overestimated the cover crops’ climate benefit. Conversely, in a system with both cover crops and compost, measuring only to 30 cm underestimated total sequestration. These findings highlight the importance of accurate, long-term measurement and the vulnerability of soil carbon capture schemes to misrepresentations of their effectiveness. Offset markets and government agencies that fail to account for the full soil profile risk having their entire system’s sequestration estimates called into question.
Despite the current challenges in verifying their climate change mitigation potential, evidence shows that the effect of regenerative practices on soil carbon generates other crucial benefits for the environment and global food systems. In a study in the journal SOIL, Dr. Emily Oldfield and other Yale researchers examined the effect of increased soil carbon on agricultural yield. The researchers found that maize and wheat yields increased with soil carbon concentrations and that increasing soil carbon on degraded agricultural lands could raise global maize and wheat production. Improved fertility resulting from increased soil carbon would also reduce reliance on synthetic nitrogen fertilizers, thus lowering farmers’ operating costs, greenhouse gas emissions, and pollution of nearby waterways.
Oldfield’s study confirms long-held assumptions about the benefits of soil carbon for crop yields, food security, and ecosystem health. Furthermore, even if the carbon sequestered through regenerative agriculture does not stay in the soil long enough to permanently reduce atmospheric carbon dioxide concentrations, these practices still contribute to climate change mitigation in other ways. By enhancing productivity per hectare, regenerative practices reduce the need for more land conversion to feed a growing population. By enhancing fertility, they reduce emissions from applying and manufacturing energy-intensive inputs like fertilizers and pesticides. And by stabilizing crop yields, they can safeguard global food security and boost resilience to extreme weather and climatic shifts.
Given these benefits, scientists like Dr. Bradford argue for a fundamental re-framing of the conversation around soil carbon. Ongoing research is still needed to better understand soil carbon dynamics under different agricultural practices and to develop better technologies and standards for measurement. In the coming decades, advanced computer models built from long-term experimental data, remote sensing, and crowd-sourced land-use information may provide robust soil carbon sequestration estimates that can help guide management decisions and efficient offset pricing.
Until then, however, scientists and policymakers should highlight and promote carbon capturing agricultural practices for their broad environmental and emissions-reducing potential, but without relying on them to “offset” energy, transportation, and industrial emissions. Policy incentive structures should center around the objectives of soil health and whole-system agricultural sustainability – rather than unproven estimates of permanent carbon sequestration. This strategy will better accelerate the adoption of ecologically and economically appropriate agricultural practices, at least until more accurate measurement and modeling techniques can be brought to scale.
Agricultural soil carbon sequestration is a complicated field, fraught with scientific uncertainties that interact with broader economic and political dynamics. Decision makers must go forward with efforts to promote agricultural sustainability while being honest about what soil carbon sequestration efforts may or may not achieve.