Soil Carbon Storage
The world’s soils hold a significant amount of carbon – more than double the amount in the atmosphere. The Intergovernmental Panel on Climate Change’s most recent report finds that storing, or “sequestering,” carbon will be essential in lowering atmospheric carbon levels. Agriculture, forestry and other land use practices that store carbon in the ground offer an opportunity to mitigate climate change. Healthy soils with more organic matter can store carbon while providing agricultural and environmental benefits.
Potential for soil carbon sequestration
Land in the US already helps to store carbon from the atmosphere. In 2014, forests, croplands, trees in urban areas and organic material in landfills offset 11.5% of US carbon emissions. However, US cropland currently only sequesters 8.4 million metric tons of CO2-equivalent (MT CO2e) annually, compared to the estimated 100 million MT CO2e potential. Farmers can help soils reach their carbon storage potential by making sure crop residue and animal manure re-enters the soil. The amount of carbon that soil can hold, or the “carbon budget,” depends on the type of soil. For instance, clay-based soils hold organic carbon for longer than sandy soils. The carbon budget also depends on the regional climate, how often the soil is disturbed and even the characteristics of soil microbe communities.
Soil carbon storage directly benefits farmers by improving soil fertility, reducing erosion and increasing resilience to droughts and floods. For these reasons, many farmers already use conservation practices that keep soils healthy and full of carbon, such as no-till agriculture, agroforestry and planting cover crops.
There are four main pathways toward increasing soil carbon:
- Increased soil stability. Minimizing soil disturbances lowers the rate of organic matter decomposition, in turn reducing the amount of carbon lost back into the atmosphere. No-till, or “conservation” tillage, is a popular way to increase soil stability because it is relatively quick and affordable to implement. However, the carbon benefits are often short-lived and carbon is lost as soon as the soil is plowed again, even after years of accumulation. Agroforestry, in which crop cultivation is intermixed with growing trees and sometimes livestock grazing, has the highest potential to hold carbon, ranging from 1.7 to 2.5 MT CO2e per acre annually, because of long-term storage in increased root growth and soil stability. This process, however, involves converting cropland to woody systems, which may not be feasible in certain climates and can be costly for farmers.
- Increase plant and animal inputs. Applying animal manure to fields not only provides nutrient-rich fertilizer, it also promotes larger and more diverse soil microbe communities that break down organic matter and store it as soil carbon. Biochar is another method of increasing organic inputs, created by burning organic waste in an oxygen-free chamber and then burying the charcoal-like product. Biochar enriches soil and stores carbon in a stable form. While biochar’s carbon-storage potential has been estimated at 12% of global emissions, there are concerns about scaling up biochar production to a level that offers significant carbon offsets.
- Improve diversity and number of soil microbes. Soil carbon storage depends on microbes eating and processing nutrients. Increased plant diversity can increase microbial activity and build more diverse microbe communities. Applying manure also supports more diverse soil communities and increases microbial biomass. According to recent research, microbial diversity is also important because microbial byproducts and dead cells may contribute more to soil organic matter than plants themselves.
- Provide living cover for soils. Having a continuous cover of vegetation, instead of letting fields lie bare after harvest, reduces the vulnerability of soil to carbon loss. Cover cropping is a common method where a “cover” of crops is planted and not intended for harvest. A 2014 meta-analysis estimates that using cover crops on 25% of the world’s farmland could offset 8% of emissions from agriculture. However, there is uncertainty about the impact this would have on factors that drive climate change, such as increased emissions of other greenhouse gases like nitrous oxide. Creating borders of permanent vegetation along the edges of fields is another way to provide continuing living cover for agricultural soils.
Barriers to adoption
Soil carbon storage is supported by federal conservation programs, primarily through the Environmental Quality Incentives Program and the Conservation Technical Assistance Program, which provide funds for soil health practices on 2-5% of croplands. Regional programs, like the Regional Conservation Partnership Program, also support soil carbon storage by providing conservation assistance. The USDA provides other resources for “climate-smart agriculture” and building healthy soils, but the initiatives are all voluntary and have yet to be adopted large-scale.
Farmers face significant barriers to adopting healthy soil practices, and responsibly managing cropland involves trade-offs. For instance, tilling soil is a traditional practice for controlling weeds, and shifting to no-till requires altering farm equipment and using other weed-control methods. Farmers face up-front costs in order to implement conservation practices and also risk lower yields in the short-term or less cropland for production, if acreage is instead used for permanent vegetation.
Another barrier to adoption is that the benefits of healthier, carbon-rich soils only add up in the long-run. Furthermore, while farmers face individual cost and risk, these long-term benefits are spread among entire communities that benefit from cleaner water, less erosion and resilience to a changing climate. Therefore, subsidies are one pathway toward incentivizing farmers to invest in cultivating healthier soils.
Much remains unknown about soil carbon storage, so it is difficult to estimate total benefits and which soil management practices offer the most potential for a given soil type, climate and crop. The Natural Resources Conservation Service (NRCS), a branch of the USDA, provides technical advice and planning tools that can help farmers make informed decisions about soil management. However, the NRCS lacks sufficient staff to make sure practices are implemented correctly over the long-term.
Almost 40% of US farmland is operated by renters, and 80% of agricultural land is owned by a non-farming landlord. This makes it difficult to incentivize investments in soil health, since renting tenants face short-term costs but might not receive the long-term benefits.
Overcoming these barriers requires financial and technical assistance, continued investment in research, developing private partnerships and better aligning incentives for farmers renting land. An analysis by UC Berkeley’s Goldman School of Public Policy recommends that the NRCS’s adjust program implementation work, through:
- Increased partnerships with businesses and investors to attract private capital and expand capacity to provide technical assistance
- Better outreach to target absentee and female landowners
- Additional funding to lower farmer costs and increase technical assistance