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Jessica Weiss -

How Soil Stores Carbon–and Helps the Atmosphere

The world’s soils are a significant reservoir of carbon. In fact, these “carbon sinks” contain more than double the amount of carbon than the atmosphere. That makes soil an essential—and often overlooked—element in strategies to reduce the emission of greenhouse gases into the atmosphere. It also provides an opportunity for those looking to enhance the health and fertility of their farms and crops.

The Carbon Cycle: Through Soil’s Lens

Carbon flows in a continual cycle from the atmosphere to plants, trees, and other vegetation. To use CO2, plants first naturally convert it from the atmosphere into carbon needed to grow, through the process of photosynthesis. That carbon is stored in the leaves, stems, roots, and other parts of plants.

As plants die and decompose, carbon builds up in the soil. The amount of carbon transferred depends on a number of factors, such as the size and species of vegetation, soil management practices, and conditions of the environment. Soil can slowly absorb CO2 from vegetation as it cycles through life and death.

Carbon-based organic matter is composed of microbes and materials from plants, animals, or microorganisms that are in a phase of decomposition. As the organic components break down, the plant also releases nutrients such as nitrogen, phosphorus, and sulfur. Soil organic matter is about 58-percent carbon. Globally, the first 30 centimeters of soil contains around 680 billion tons of carbon, according to the United Nations, and that carbon can stay protected for a while if it’s undisturbed and not needed by plants or disturbed by practices like tillage.

Soil leaks carbon back into the atmosphere through both natural and manmade processes. Erosion, or when topsoil is degraded due to forces including water and wind, is a leading cause. In agriculture, the processes of tilling and plowing also hasten the release of carbon, as well as emit greenhouse gases into the environment. As much as one-third of the surplus CO2 in the atmosphere comes from agricultural and land management practices.

Citing studies that show that Australian soil carbon levels have dropped to half of pre-agricultural days, the Australian government encourages farmers to take up practices that sequester carbon. Other notable carbon offset programs are in the province of Alberta, Canada, the state of California, and Kenya. The Kyoto protocol, an international climate agreement between 192 countries signed, also supports activities that keep carbon in soil. And the Paris Agreement, signed in 2015, included a commitment to increase soil carbon by 0.4 percent each year.

As much as one-third of the surplus CO2 in the atmosphere comes from agricultural and land management practices.

What Carbon Does for Soil

Carbon sequestration is not just about the atmosphere. As long as carbon inputs and outputs are in balance, soil can maintain its carbon levels. When more carbon is being input through plant growth and biomass decomposition than is being lost, soil grows richer in carbon.

That means soil is healthier, more fertile, and more resilient to droughts and floods, as well as to erosion. Soil with more organic matter has been shown to retain both nutrients and water better. Microorganisms in the soil also use carbon as energy and nutrients, and organic matter contribute to soil’s physical stability and aeration, which, in turn, prevents nutrient leaching.

Farmers’ Role in Carbon Sequestration

Farmers have a role to play in keeping carbon in their soil—but doing this isn’t altruism. Carbon farming, which is a relatively new practice in the United States, seeks to slow the release of carbon back into the atmosphere through a number of techniques. Here’s how some of the common techniques for carbon sequestration are benefiting farms:

Conservation Tilling.

One key way farmers can sequester carbon is to practice conservation tilling (or no-till farming), a system that reduces soil disturbance to lead to a better soil environment. Whereas conventional tilling (in which the soil is turned over) increases erosion and exposes carbon back into the atmosphere through the breakdown of soil aggregates, conservation tillage methods leave crop residue on the soil surface, ultimately reducing soil and water loss. According to the Minnesota Department of Agriculture, conservation tilling reduces soil erosion by as much as 60 to 90 percent, depending on the method.

Compost and cover crops.

Farmers can also use compost and animal manure on the surface of soils and grasses to improve fertility and increase carbon levels; decomposing compost acts as a slow-release fertilizer. And planting cover crops can keep the soil carbon-rich during the winter, while compensating for carbon lost during the growing season. Cover crops can also be broken down and returned to soils for more carbon.

Rotational grazing.

Carbon farmers also rotate livestock, letting paddocks have regular periods of rest and regeneration. This is achieved by dividing pastures into subdivisions and physically moving livestock on a regular basis. The rotational grazing method helps improve pasture quality and fertility, allowing manure to distribute evenly.

Agroforestry.

This land use management system of growing trees and crops together also increases carbon retention, such as through riparian buffers, windbreaks, and silvopasture.

Farmers have a role to play in keeping carbon in their soil—but doing this isn’t altruism.

The results aren’t instantaneous: it takes an average of four years for farmers to create soil organic matter using conservation methods, according to California’s Department of Food and Agriculture, which is spearheading a number of programs to promote healthy soils in the state and is a national leader in carbon farming. Already, nearly half of California’s 58 counties have farmers and ranchers who are developing and implementing carbon farming.

Carbon Farming in Practice

North of San Francisco, the Marin Carbon Project is working to increase carbon in rangelands and farmlands in a first-of-its-kind program that uses compost as a key method. Beginning in 2013, the project chose three sites, ranches totaling some 1,600 acres, to launch the project. Each underwent extensive baseline soil sampling and land assessment. Compost was then applied to about 100 total acres at a rate of 1⁄2 inch or 1⁄4 inch surface covering. About 4,000 cubic yards of compost was applied in total.

Researchers found that applying compost to grasslands was able to sequester carbon at rate of 1 metric ton per hectare over three years. The fields treated with compost had higher grass production and water holding capacity.

Since then, the participating farmers have implemented a number of additional carbon sequestration practices, as laid out in comprehensive and individualized Carbon Farming Plans. At Straus Family Creamery, dairy farmer Albert Straus reports increased yields and more nutritious plants. He sequesters some 320 metric tons of carbon dioxide in the soil each year through carbon farming practices. In addition to compost, Straus plants windbreaks and hedge rows to reduce soil erosion, as well as perennial grasses to increase underground root systems, and practices rotational grazing.

A Slow Transition

But the time required to transition to carbon farming practices, which equals money lost, remains a barrier for many farmers. In addition to California, a handful of states around the country have begun to introduce initiatives to study and support the practice, including Maryland, Massachusetts, and New York. But on the whole, farmers continue to finance the transition to more carbon friendly practices on their own.

Carbon credits—money paid by polluters to a company, organization or project that is offsetting greenhouse gas emissions—are one way governments have thought to incentivize farmers to implement carbon sequestration practices. In the past, credits have gone primarily to initiatives like wind farms or solar panel facilities, but there is growing interest in farm-based carbon credits. The Australian government and city of Alberta, Canada, have carbon farming credit programs, as does the state of California. In the Great Plains region, Duck’s Unlimited has a certified carbon offset program that provides financial compensation to ranchers who voluntarily place their grasslands under a conservation easement that removes the threat of tillage.

Farmers can explore the potential of their lands for improved carbon sequestration using USDA’s free COMET-Farm tool, developed at Colorado State University. Farmers can input different land management scenarios to see which has the greatest carbon sequestering ability, and receive help and guidance.