Building Soil Carbon

There is renewed interest in paying farmers to sequester soil carbon by building soil organic matter (SOM) levels. Building soil carbon is dependent upon temperature, moisture, vegetation, tillage, soil texture, crop rotation, and microbial activity. Soil is a major storehouse for carbon and carbon dioxide. Ohio soils originally had 5-6% soil organic matter (50-60 tons decomposed SOM) in the top furrow slice (6.7 inches) of soil. Most Ohio soils today only have about 2-3% SOM, so an additional 2-4% SOM could be added. 

Temperature, moisture, and vegetation controlled most carbon and SOM storage historically. Tropical areas have lower SOM while colder soils store more carbon in SOM. Tropical carbon is stored above ground while colder climates store carbon in the soil due to limited temperature and moisture. Every 100F temperature increase will double microbial activity and releases carbon as carbon dioxide into the atmosphere. The type of vegetation impacts carbon sequestration. Hardwood trees in the Eastern USA have low root turnover, so carbon sequestration is slower and accumulates closer to the surface. Prairie soils have high SOM because 50% of the roots turn over every year, building carbon and SOM deeper and quicker. Vegetation (prairie grasses) and climate have a huge impact on SOM and soil carbon storage. 

Tilling soil results in the immediate loss of soil carbon as carbon dioxide. Tillage incorporates soil oxygen, speeding up inefficient microbial decomposition of organic residues. Soil texture is important, with sandy soils being more permeable and able to hold less carbon than silty clay or clay soils. Corn and wheat generally raise your SOM levels while a soybean crop reduces SOM levels. Soybeans have less total biomass and a lower carbon to nitrogen (C:N) ratio which makes it easier and faster to decompose, which results in a loss of soil carbon.

The fastest way to gain soil carbon is to convert to long-term no-till, adding high carbon crops (corn and wheat), and adding cover crop mixtures high in carbon (grasses primarily but also legumes to stabilize soil carbon). Adding root diversity or cover crop mixes allows a variety of roots (tap roots from broadleaves and legumes and fibrous roots from grasses) to populate the soil year-round to increase soil carbon through root exudates (sugars and carbohydrates). Building soil carbon is a slow process because almost 60-80% of carbon in the plant residue, especially surface residue, is lost to the atmosphere annually. Since roots are more protected, the carbon from roots makes up the majority (80-90%) of soil carbon.

Here is a method to estimate how fast you can build soil carbon. It takes 10 tons of decomposed organic matter residue to equal 1% SOM. If you start with 40 tons organic matter residue and lose 75% or keep 25% after the microbes decompose the residue, 10 tons decomposed SOM or 1% SOM is added to your soil. Cereal rye accumulates 4-6 tons of crop residue and loses 75% or gains 25% which equals 1-1.5 tons decomposed SOM or 0.1 to 0.15% (1/10 or 1.5/10) SOM gained annually. SOM is about 58% carbon (we often assume 50%) so you gain roughly .05 to .075 tons of actual carbon per year by using cereal rye with long-term no-till.

Grasses are the most efficient way to build SOM. Planting sorghum or Sudan species for forage or as a cover crop increases root mass 5-10X each time the crop is cut or harvested. Annual ryegrass has 50% more roots than cereal rye, barley and oats. Other grasses include pearl millet, wheat, and spelt. Every 1% SOM requires 1000 pounds of N to stabilize the SOM, so adding legumes and clovers to a cover crop mixture builds SOM faster.

Building SOM requires healthy soils with live plants and roots to add soil carbon. Tilled soils are dominated by bacteria which are 20-30% efficient at converting organic residues into soil carbon while fungi are 40-55% efficient. No-till fields with cover crops have a balance of bacteria and fungi which increases and improves carbon sequestration. There are 1,000-2,000X more microbes associated with live plants and roots than in a tilled soil. Soil microbes are “soluble bags of fertilizer” so healthy soils are more fertile and build carbon faster. The long-term soil humus is mainly composed of dead microbial bodies, so it takes a long time to build SOM and soil carbon stocks. Building soil carbon is good for the environment and improves farming profitability.