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The Science of Carbon Storage in Soils

Dr. Andrew Margenot, assistant professor of soil science at the University of Illinois Urbana-Champaign, discussed carbon sequestration and carbon credits during his presentation at the Chicago Farmers’ February 14, 2022, webinar.

Dr. Margenot stressed that the mission of his work was to use soil science to support the goals of Illinois agriculture. In order to accomplish this, his group studies soil organic matter, nutrient management and the Illinois Nutrient Loss Reduction Strategy. He said they are able to do this through relationships they have developed with on-farm research and generous funding support from the state, USDA, and federal government. They also have partnerships with farmers, the Illinois Farm Bureau and conservation districts.

In reviewing how carbon gets into the soil, Dr. Margenot noted that plants removed the carbon from the atmosphere through photosynthesis. The addition of plant biomass and thus carbon into the soil leads to some of it being stored. Carbon is put back into the atmosphere as soil microbes respire the carbon in soils.

The basis of carbon markets is that farmers and landowners are earning credits for carbon stored on their property and corporations purchase credits to offset their emissions, said Dr. Margenot. “My focus is what is going on below the ground,” he related.

He went on to say that the nature of organic carbon is quite dynamic. He said there are other parameters that are sometimes overlooked, such as erosion, climate, the necessity of nitrogen inputs to enable sequestration, and the variability of carbon.

There is a dynamic equilibrium to consider regarding carbon, it is moving and can be altered by either increasing inputs or decreasing output. For example, more carbon can be added to the soil through cover cropping and reducing tillage. Both help carbon stay in the soil. “We have losses and inputs throughout the growing season,” Dr. Margenot noted.

Dr. Margenot said that he believes soil saturation is not sufficiently discussed in relation to carbon storage potential. He noted that it is possible to saturate out how much carbon a soil can hold so landowners can’t expect to keep storing additional carbon indefinitely. The capacity of soil to hold carbon is dependent on the soil type or texture. In general, the more clay content a soil has, the greater the potential to hold more carbon. “Soil types in Illinois have largely silty clay loam textures, with some patches of sand from alluvial or glacial outwash areas; if you are not on these sand patches, you have pretty good soil texture that will benefit the storage of carbon,” he related.

Tillage and no tillage affect carbon storage with no tillage being the most beneficial. “You will generally have a higher storage of carbon in the soil if you go to no till,” said Dr. Margenot.

Climate, too, affects a soil’s ability to store carbon. A hot and dry climate means there will be less saturation possibilities. A cool, wet climate provides more saturation possibilities.  He said there were inherent constraints on how much carbon we can store in soils. As one goes farther south in the U.S. toward hotter, drier climates, there is less carbon because the microbes are more active and burning up carbon. In northern areas where it is cold and very wet, there is more carbon and a greater potential for more carbon sequestration. “In Illinois we have decent potential for carbon sequestration due to the moderately high clay content of our soils, and our climate,” said Dr. Margenot.

He also noted, “You can put on as much carbon as you want, but over time there will be diminishing returns on how much you are able to hold in the soil. Carbon credit payments promised indefinitely might not happen because the promise is predicated falsely on the belief that soils are an unquenchable sink for carbon.”

Regarding carbon deficits, Dr. Margenot said the greatest historical (10,000 year) deficits are found in northern Europe, the U.S. Corn Belt, China, and eastern Asia because these are the most productive lands in the world and have been intensively tilled. This means that the land has a greater opportunity to store more carbon.

He pointed out that erosion is not always a detriment. The loss of soil in one area means a net gain somewhere else. “Soil is redistributed across the landscape. Erosion is not necessarily a net loss of carbon across the system,” remarked Dr. Margenot. “Whoever is on the receiving end of the soil might actually be gaining carbon along with nutrients.”

Carbon requires nitrogen to be stored as soil organic matter, which means that nitrogen fertilizer is not necessarily bad for soil carbon sequestration, if it is used in the right amounts. He remarked that landowners should be considering the nitrogen cost in sequestering carbon.

“You don’t necessarily lose carbon with more nitrogen,” he said. “There will be a trade-off. If adding insufficient nitrogen, then carbon is lost because microbes are going to work more to get more nitrogen. If too much nitrogen is added, it might ‘prime’ the microbes to decompose carbon.”

In determining how much carbon a soil holds, Dr. Margenot said it was important to not only measure across a field, but to measure vertically (downward) also.

In closing thoughts, Dr. Margenot pointed out:

• It is difficult to know the real value of carbon stock (tons of carbon per acre) in soil, it is an accuracy challenge, but that does not mean that precise monitoring of the changes in the soil should not be ongoing.
• Depth must be defined; digging deeper can change the stock value, which is measured on a per acre basis. “Always ask what the depth is.”
• Should agriculture prioritize food production before fuel and fiber to ensure that America has enough food supply?
• Is there a role for inorganic carbon? “There is discussion about could we sequester inorganic carbon, such as limestone. Keep a heads up on the two kinds of carbon.”

Regarding concerns about what a landowner should take into account when storing carbon, Dr. Margenot said it is hard to have too much carbon, as long as the landowner accompanies it with the right amount of nitrogen, which is a 10:1 ratio. To determine the amount of carbon and nitrogen in the soil, measurements should be done vertically to a depth of three feet.

Regarding economic incentives, Dr. Margenot said that if society sees reducing carbon emission as a benefit, then he believed it was important to support it. “However, it is unfair to make farmers do something that is costly,” he said. “We need to do more soil conservation. I am uncomfortable with incentives if they become regulations. It should be voluntary.”