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's usually attached to other elements, such as oxygen or hydrogen.
Carbon has a cycle. When carbon is in the atmosphere, it's 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 corn, wheat and soybeans, a small percentage of the carbon leaks out into the soil via the roots. Some carbon is removed by harvesting grain, and the remainder exists in the roots and crop residue. Crop residue is about 40% carbon.
Another important piece of this cycle is soil organic matter. Organic matter is approximately 50% carbon. This carbon can be tied up in the stable fraction called humus, or in the short-term pool called the active fraction.
Humus is dark in color and is the stable building blocks of organic matter. Humus does not release nutrients for plant and microorganism use and can be greater than 400 years old. The carbon in the active fraction feeds soil microbes, releases nutrients for plant use and is essential to the soil life cycle.
In the carbon cycle, the carbon from residue and decaying roots helps build carbon in organic matter. This sounds simple enough, just till the residue 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 of carbon and then respire it off as carbon dioxide. Carbon dioxide escapes as a gas into the atmosphere and 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, MN, has shown that tilling with an 11-in. moldboard plow can release more than 1,600 lbs. of carbon dioxide/acre in a 24-hour period. In comparison, no-till releases 87 lbs./acre in the same time period.
So how do we “feed” the soil and not just the microbes? We need a balance in the soil. It is essential to have a diversity of microbes in the soil and they need carbon as a food source. Microbes may respire off carbon as carbon dioxide. However, they can also incorporate carbon into soil aggregates where it's slowly incorporated into organic matter. Here is where our management affects this balance.
When we till soil, residue is broken into smaller pieces, the soil is oxygenated and the bacterial populations explode. They devour the carbon out of the residue. If we reduce our tillage and keep as much residue on the soil surface as possible, fewer microbes can live in that environment. Fungi will come onto the scene, slowly breaking down the residue. Their hyphae — long threadlike filaments — bind the soil together into aggregates, protecting the carbon inside from predation or use. This process sequesters atmospheric carbon into the soil and ultimately into soil organic matter.
To maintain the levels of organic matter we have today, we need to leave approximately 2.5 tons of residue/acre on the soil surface with minimum tillage. Some crops do not produce this amount of dry matter. In that case, producers need to look at growing a crop rotation that maintains 25 tons over a 10-year period.
This is why to qualify for carbon sequestration 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.
Editor's Note: Jodi DeJong-Hughes is an Extension educator at the University of Minnesota, based in Marshall, MN.