Posted 25 June 2007. Crop Management.
Farming for Carbon
University of Minnesota. www.cfans.umn.edu
St. Paul, Minnesota (June 11, 2007) - Carbon, especially carbon dioxide, has received a lot of attention from policy makers and the press lately. But what is carbon and how does crop management affect it?
Carbon doesn't exist alone. It is usually attached to other elements, such as oxygen or hydrogen.
Carbon has a cycle. When carbon is in the atmosphere, it is in the carbon dioxide form. Plants can photosynthesize the carbon dioxide for sugar formation and other plant processes. As this happens, the carbon is transported to the plant roots, stems, leaves and grain. For food crops such as corn, wheat, and soybeans, a small percentage of the carbon is leaked out into the soil via the roots, some carbon is removed by harvesting the grain, and the remainder exists in the roots and crop residue. Crop residue is about 40 percent carbon.
Another important piece of this cycle is soil organic matter. Organic matter is approximately 50 percent carbon. This carbon can be tied up in the stable fraction called humus, or in the short-term pool called the active fraction. The carbon in the active fraction feeds soil microbes, which run the soil life cycle.
In the carbon cycle, the carbon from the residue and decaying roots helps build the carbon in the organic matter. Sounds simple enough, just till the residue under into the soil and it will replenish the organic matter, right?
Well, here is where common sense fails us. We are actually feeding the soil microbes, not replenishing the organic matter.
Soil microbes enjoy a meal consisting of carbon and then respire it off as carbon dioxide. Since carbon dioxide is a gas, it can escape back into the atmosphere. When it escapes, the carbon is no longer available for building organic matter.
This process is accelerated when we till our soils. Substantial research has shown the deeper and more aggressive the tillage, the more carbon dioxide is released from the soil. Research by Don Reicosky and others, with the USDA Agricultural Research Service in Morris, has shown that tilling with an 11-inch moldboard plow can release more than 1,600 pounds of carbon dioxide per acre in a 24-hour period. In comparison, no-till released 87 pounds per acre in the same time period.
So how do we "feed" the soil and not just the microbes? By reducing our tillage and keeping as much residue on the soil surface as possible. When the residue is left on or near the soil surface, it can be slowly broken down by microbes, incorporated into soil aggregates, and finally into organic matter. This part of the cycle takes atmospheric carbon and sequesters it into soil organic matter.
This is why to qualify for carbon credits, a producer must reduce or eliminate tillage. It's also how producers can increase their soil organic matter, which has many benefits for the soil, the crop and the producer.
Jodi DeJong Hughes is crops educator with University of Minnesota Extension.