During photosynthesis, plants absorb CO2 from the atmosphere and use it to make organic compounds that are crucial for plant growth. When the plant dies or sheds the leaves, soil organisms like bacteria, fungi, and earthworms break down the decomposing plant material and transform it into organic matter – known as soil organic carbon. Soil organic carbon in the soil stores carbon, preventing it from re-entering the atmosphere.
Factors that Affect Soil Organic Carbon
Soil carbon sequestration depends on the temperature, vegetation, tillage, crop rotation, soil texture, and microbial activity. Soil is one of the best storehouses of carbon and carbon dioxide. Tropical areas have lower soil organic materials (SOM), while the colder soils store higher SOM. Tropical carbon is stored above the ground, while the colder climates store carbon in the soil because of the limited temperature and moisture.
With every 100F rise in temperature, microbial activity is doubled, releasing carbon and carbon dioxide into the atmosphere. Prairie soils have higher SOM because 50% of the roots turn over per year, developing carbon and SOM deeper and quicker. Prairie grasses and climate have a significant impact on soil carbon sequestration and soil carbon storage.
Tilling also reduces soil organic carbon. Tillage incorporates soil oxygen, and it speeds up the inefficient microbial decomposition of the organic residues. The soil texture is also crucial because sandy residues are more permeable, and they are able to hold less carbon than clay soils. Corn and wheat will raise the SOM levels, while the soybean crop will reduce the SOM levels.
The Best Ways to Increase Soil Organic Carbon
The fastest and best way to enhance soil carbon sequestration is converting long-term no-till, adding high-carbon crops, and adding cover crop mixtures high in carbon (mostly grasses but also legumes to stabilize the soil carbon). The addition of root diversity or cover crop mixes allows a variety of roots to populate the soil throughout the year to increase soil organic carbon through root exudates. Developing soil carbon is a slow process because almost 60-80% of carbon in the plant residue, especially the surface residue, will be lost to the atmosphere annually. Since the roots are much more protected, the carbon from the roots makes up a significant portion of the soil carbon.
Grasses are excellent in building SOM. Planting sorghum or Sudan species for forage or as a cover crop will increase the root mass 5-10 times the crop is cut or harvested. Annual ryegrass has 50% more roots than barley, oats, and cereal rye. Other grasses include pearl millet, spelt, and wheat. Every 1% of SOM needs 1000 pounds of N to stabilize the SOM. Therefore, adding the legumes and clovers to the cover crop mixture can build SOM faster.
To build SOM, it is crucial to have healthy soils with live plants and roots to add soil carbon. Tilled soils are dominated by bacteria that are 20-30% more efficient at converting organic residues into soil carbon, while fungi are 40-55% more efficient in soil carbon sequestration. No-till fields with cover crops have a balance of bacteria and fungi that increases soil carbon sequestration. There are 2000X more microbes associated with live plants and roots than in tilled soil. Soil microbes are “soluble fertilizer bags,” so the healthy soils are much more fertile and build carbon faster. The long-term soil humus includes dead microbial bodies, so it takes a long time to develop SOM and soil carbon stocks.
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