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Author: Dr. Jennifer Wells
As the latest Truterra carbon offer is in full swing, it is a great time to discuss factors that affect carbon sequestration in soil. As in most carbon programs, we tend to focus on practice changes such as reduced till, no till, and/or adding a cover crop to your rotation. While these are extremely important practices in helping to identify fields that have recently started sequestering additional carbon for us to measure, the rate in which soils sequester carbon in response to these practice changes can vary greatly depending on several factors surrounding your soil, environment, and vegetation.
Each soil has a maximum amount of organic carbon it can store. The maximum amount is tied very closely to the soil type. Soils that have a higher clay content tend to sequester and store more carbon than sandier soils. Clay soils have a smaller particle size and bind more tightly to organic matter (where carbon is stored), which helps protect it from microbes and other organisms that break down the carbon, thus returning it to the atmosphere. Organic matter that is in sandier soils, where the particle size is larger, is not as protected from microbes, so it decomposes and moves to the atmosphere much more quickly. (1) Another soil attribute that affects carbon sequestration and storage is bulk density. Bulk density is an indicator of soil compaction. Soils with lower bulk densities tend to be more porous, less compact and rich in organic matter and have the ability to sequester and hold more carbon than those with higher bulk densities. Soils with higher bulk densities have less pore space and root penetration and higher compaction with less room to store organic matter. While bulk density is somewhat soil type dependent, other factors such as over-plowing, over-grazing or lack of crop biodiversity can increase bulk density on any soil, causing a reduction in carbon sequestration and storage. (2)
Environmental factors such as moisture and temperature can affect both inputs and losses to organic matter in the soil. Adequate water availability for the crop (factoring in irrigation, rainfall and soil water-holding capacity) is highly correlated with the leaf area index (LAI) of the plant. The higher the LAI, the more photosynthesis can take place, therefore the more carbon sequestered in the soil. Likewise, not having adequate water will affect a plant’s ability for photosynthesis and result in less carbon being sequestered. (3) Higher temperatures generally lead to a higher rate of decomposition of soil organic carbon, thus returning it to the atmosphere. Decomposition of organic matter is slower in cooler climates. The combination of temperature and moisture are important because higher temperatures and adequate water can result in higher plant biomass, thus increasing organic matter and carbon in soils. However, this combination can also support high levels of microbial activity which leads to quicker decomposition, making carbon storage less likely. (4)
In the midst of all of the emerging carbon markets, it is important for growers to think of and manage carbon as their “double crop” scenario. While crop rotation should be based on the right fit for your farm for your main crop, knowing which types of crops sequester the most carbon can still be helpful in the event you have the choice, especially when commodity prices are low and carbon prices are high. More than 10% of all carbon dioxide in the atmosphere passes through the plant and soil relationship each year. Results show that root biomass is the most important factor of vegetation in relation to carbon sequestration and storage. If soil and environmental factors are equal, grasses have the ability to sequester and store the most carbon, followed by cereals, legumes, oil crops, and fiber crops. This also applies to cover crops during the off season, so if increasing carbon in the soil is the field goal, implementing a grass or cereal crop will give you the highest amount of carbon sequestration. (5)
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