UNDERSTANDING THE SOIL CARBON CYCLE

Caley Gasch – Assistant Professor of Soil Health Research, NDSU

You’ve probably heard that soils have the capacity to capture carbon from the atmosphere and store it for a long time, and that conservation-based farming practices can help this process. This concept is part of current carbon market programs incentivizing farmers to increase carbon content of their soils. We know that increasing soil carbon provides many soil health benefits, but capturing carbon is not a simple or fast process-the soil carbon cycle is complicated!

Let’s begin with carbon dioxide (CO2), which is a gas in the atmosphere, and which is “fixed” into solid form by photosynthesizers, or primary producers like plants and algae (Figure 1). Plants use CO2 to build biomass and to perform their metabolism. Some of the CO2 that plants consume from the atmosphere is returned to the atmosphere as a result of cell functions (such as respiration), but most of the carbon captured by plants is converted into carbohydrates to create plant biomass.

Carbon fixed in plant material is then cycled to consumers. Unlike primary producers, consumers (like herbivores and decomposers) cannot make their own biomass from the air, soil, and water, so they need to get all their nutrients (including carbon) from their food. As the carbon is consumed, some of it goes into building consumer biomass, and some of it returns to the atmosphere as CO , a byproduct of respiration. The amount of carbon retained in consumer biomass can range between 20-60%, which means that a good portion of consumed carbon is returned to the atmosphere at this step in the cycle.

So, how does carbon get into the soil? When primary producers and consumers die, their biomass becomes carbon-rich organic matter. In the soil and on the soil surface, decomposers consume the material, respire CO , build biomass, and die, and this turnover can proceed for many generations, each time sending CO2 back into the atmosphere (Figure 2). Over time, the organic matter and the carbon within it is reduced. The carbon within fresh organic matter only resides in the soil for a short amount of time (maybe a few years) before it is decomposed and reduced to CO2. We refer to this material as “active carbon” because it is actively feeding decomposers and moving through the cycle. Often, this material is still recognizable as plant residue, or it is soluble in water as simple sugars and small carbohydrates. However, during this active cycling, a small amount of carbon is trapped in the soil and stored in a stable form for a long time.

Unlike the actively cycling carbon in fresh organic matter, stable soil carbon is chemically and physically different. As soil particles come together to form soil aggregates (little granular units that give soil a crumbly structure), bits of organic matter can get trapped and bound-up in the aggregates. The carbon is then physically protected from decomposers by soil particles, so it sticks around until the aggregate is broken open (by disturbance) and releases that organic matter. Another route for carbon to become stable is by forming a strong chemical bond with the surfaces of soil particles, especially clay particles. Carbon coatings on clay particles gives soil its dark color, and this carbon is also not readily available to decomposers as a food source, so it sticks around for a long time. Locking carbon into these stable pools is the goal of soil carbon sequestration.

The soil’s natural tendency is to cycle carbon in this way, with some carbon continuously  feeding those stable pools for a net carbon gain. However, we can disrupt and slow this cycle through changes in management, and we can lose carbon from the soil through the action of erosion. There are two practices that promote carbon cycling and storage in soil: (1) maximize primary productivity, and (2) minimize disturbance. To maximize plant growth, we can consider management options  that fill fallow spaces and periods-bare soil is not feeding carbon into the cycle, and this is why cover crops are · promoted for carbon storage. Minimizing disturbance prevents the release of carbon that is locked up into stable  aggregated  forms, and this is why reduced tillage is promoted for carbon storage. If the soil is occupied by rooted plants, and the surface is protected by residues, we’re doing everything that we can to limit erosion from wind and water, which is an added benefit and another way to promote carbon storage.

How do we know if carbon is being stored in our soils?

We can use soil tests to monitor carbon content. Most soil tests provide a measurement  of soil organic matter, which we can use to roughly estimate carbon content. Approximately 58% of organic matter is made of carbon, so you can do a simple conversion to estimate your carbon content in a soil sample(soil organic matter% x 0.58 = soil carbon%). Many commercial soil testing labs also offer direct soil carbon analysis. Keep in mind that the carbon analysis is only as good as your sampling effort, and to truly estimate carbon stocks in a field is a little more complicated. Most soil carbon storage programs have their own protocols for sampling and monitoring carbon in soils. If you decide to keep tabs on your carbon content on your own, here are some recommendations:

Minimize residue in the sample. Since we’re interested in tracking carbon that will remain in the soil a long time, including plant residues (which decompose quickly) will inflate carbon estimates.

Beware of carbonates. Many soils have accumulations of calcium carbonate in the soil, which is an inorganic form of carbon. Be aware that your carbon reading may include this pool of carbon, especially at deeper depths. You can ask your soil testing lab to measure this pool separately and you can subtract the value from your total carbon reading to estimate organic carbon.

Sample as intensively as you can afford. The more samples we have across a field, and across multiple depths, the better the estimate.

Geo-reference sample locations. This is always a good idea so that you can return to the sample location to track carbon change after a few years.

The best way to make decisions about carbon management and incentive programs on your farm is to understand the processes that control carbon cycling in soil. I hope that this overview gives  you  a  little insight into how carbon moves through the soil and how management influences that cycle.