Carbon Cashflow

Cycling more carbon through the soil should be your top priority

Carbon is a hot topic in agriculture these days. You’ve likely heard about carbon sequestration programs. But let’s set that aside. Getting more carbon to cycle through the soil should be your top priority. Carbon is the currency of life. The cyclic flow of carbon through plants, animals and soil provides the energy source (food) for soil microbes. This carbon-powered microbial activity drives everything from nutrient cycling to decomposition to water purification. It also creates the glues that build soil aggregates, which are the building blocks for healthy, well-functioning soil. 

Soil organic matter is about 50 percent carbon. It serves as a critical buffer for physical, chemical and biological stability in the soil. Generally, the more organic matter a soil has, the greater its capacity to store and supply nutrients and water. Low-organic-matter soils are less productive and usually less profitable to farm. 

Carbon is the unit of exchange that enables the biological community in the soil to function. Root exudates are liquid carbon that plants pump into the soil to feed microbes in exchange for nutrients. Think of organic matter as a savings account, whereas root exudates are like the cashflow to fund a checking account. If there is extra cashflow, some of it can be moved to savings for a rainy day. If the checking account runs dry, microbes will tap into the savings account to get their food, and organic matter levels will decline as they eat through their cash reserves. The biological community in the soil starves without a continual influx of carbon, just like your community would go hungry if the grocery store shelves were running empty. 

When the flow of carbon through the soil is interrupted, farmers and ranchers eat through their own cash reserves by buying expensive inputs to replace the lost soil function and productivity. Carbon as the cash cow that drives farm and ranch profit. If you invest in cycling more carbon through your soil, you will be paid many dividends in the long run. 

Figure 1 is a basic “stocks and flows” diagram showing how carbon cycles through a corn field that is yielding around 200 bu/acre. It’s a simplistic but useful way to conceptualize this invisible process. The box represents the “storage tank” of carbon in your soil. The size of the tank varies depending on organic matter percentage and depth of measurement. Typical soils will have 50,000 to 100,000 pounds of carbon per acre in the top four feet of soil.

The arrows represent flows into and out of the tank. The inflow starts with the plant pulling in carbon (CO₂) from the atmosphere during photosynthesis. An average corn crop in Iowa is pulling in around 30,000 pounds of carbon per acre annually. That’s a big number! Where does it all go? Carbon in the plant ends up as crop residue or roots that decay after harvest. Plants are around 40 to 45 percent carbon on a dry basis. Some carbon is pumped into the soil as root exudates, and some is used for root respiration.

Now let’s look at the outflows. A portion of the carbon is given off by the plant aboveground as CO₂, and some is removed when the grain is harvested. Microbes respire a large amount of CO₂ as they go about their work in the soil, and plant roots also release CO₂. Much of it ends up back in the atmosphere. The diagram shows that soil breathes, with roots and microbes taking in oxygen and giving off CO₂ just like we do. One of the reasons soil aggregation is so important is that it allows for better gas exchange. 

There are also some smaller outflows — carbon is lost by soil erosion or dissolved carbon leaching. The actual flows and interactions are much more complex, but this is the basic concept for a process that is happening everywhere.

The remaining arrow is the tiny input from fertilizer and seed. We typically add very little carbon unless we are adding a lot of manure or compost. That is OK, because plants can do it for us, but we need to remember that to increase soil organic matter, we need the inflows to exceed the outflows. Unfortunately, historical farming and ranching activities made the numbers net negative, leading to soil organic matter loss worldwide. We need to reverse this process. And it’s simple to do, at least in theory — do less of what makes the outflows bigger and more of what makes the inflows bigger. 

Let’s tackle outflows first. Soil disturbance increases erosion and accelerates organic matter decay. The rapid addition of oxygen during tillage causes a huge spike in microbial activity. The microbes eat through organic matter and respire CO₂ in the process. Minimize disturbance and maximize surface armor to prevent these losses. A larger flow of CO₂ from soil microbes is a general indicator of a healthy system, as long as it isn’t due to excessive disturbance. Too little flow of carbon into the soil to feed microbes is one of the most common problems we encounter on farms and ranches. These can both be assessed with a Haney soil test.

Crop removal also makes the outflows bigger. We need to eat, so some removal is inevitable, but we can minimize the negative effects with good management. We need to be very careful with schemes that remove biomass for energy production or other uses. In the long run, we need to do a much better job of composting food waste and humanure to complete the cycle. This was a common practice historically, but humanure is often contaminated today. In nature, all “waste” is recycled. The fact that humanure can’t be safely returned to the soil should give us all pause about what we are putting into our bodies and how removed we have become from the cycle of life that sustains us. 

Now let’s look at inflows. Plant photosynthesis is by far the biggest arrow. The corn crop in the diagram reaches maturity in about 120 days. The remaining growing days are lost opportunity that drains your savings account if the soil is bare. Maintaining living cover with cover crops or perennials is the easiest and best way to increase carbon flow. Diverse mixes increase photosynthetic activity even more. Adaptive grazing and no-till cover crop systems often build organic matter because they reduce outflows and increase inflows of carbon. You can also add carbon via humic products or compost, but it’s often cheaper to let plants do the work. 

Fertilizers and animal manures are special cases because the net result can be positive or negative. Adaptive grazing tends to have a net positive impact that can drive rapid organic matter increases. Excessive nutrient applications can create imbalances, increase salt loads, disrupt soil function and limit plant productivity. Adding too much nitrogen can spike microbial activity and drive the conversion of organic matter to CO₂. However, manure adds carbon to the system, so the net effect is hard to predict. 

There is another way to increase the inflow of carbon that is almost always overlooked. Researchers have used the same process shown in the diagram to prove that it is exceedingly difficult to increase organic matter much beyond 0.1 percent per year. But what if one of the underlying assumptions is wrong? Their calculations typically assume that photosynthetic efficiency does not change. But what happens if the plant converts a greater percentage of sunlight, CO₂ and water to biomass and/or root exudates? What if that same corn crop pulls in more carbon, or your adaptive grazing system increases plant productivity? Now we have a much larger flow of carbon from that same ground cover and the potential for some of that extra carbon to stay in the soil. 

Research hints at this, but it’s often disregarded because it is very hard to predict. Root exudation can vary from as little as 5 percent to as much as 95 percent of photosynthate production. That is a huge range and will vary depending on plant species, stage of growth, environmental conditions and soil function. Most organic matter actually comes from microbes consuming root exudates and creating carbon compounds that stick to soil particles. In Figure 1, root exudation is 20 percent of photosynthate production. What if we had more exudates? 

Figure 2 shows a hypothetical scenario with a cover crop yielding 2,000 pounds of unharvested dry matter, and an increase in photosynthetic efficiency pushing root exudation to 30 percent of carbon intake. In this example, proportions of carbon going to other flows remains about the same and grain yield does not change. The increase in exudation would also drive an increase in microbial respiration, but it is more likely there would be extra carbon available to be converted to organic matter. The numbers are difficult to estimate because there are so many interacting variables, but this scenario supplies enough carbon to increase organic matter by about 0.25 percent per year in the top 6 inches of the profile. 

What limits photosynthesis? The inputs are CO₂, water and sunlight, and the process is controlled by mineral nutrition. To improve photosynthetic efficiency, the plant must be able to access balanced mineral nutrition and grow in well-aggregated soil with functioning energy, nutrient and water cycles. You can’t make more sunshine, but you can capture more of it with a diversity of healthy plants. Following the soil health principles to improve aggregation, infiltration, water holding capacity and mycorrhizal fungi colonization will increase water availability. 

What about CO₂? Look at the diagram. Is there a source of CO₂ right where the plant needs it? Yes! You can improve photosynthetic efficiency by increasing microbial activity and CO₂ respiration from the soil. Nurture soil life with minimal disturbance, surface armor, living roots, plant diversity and livestock integration. Remember, it’s a cycle. The plant-soil system is generating and recapturing CO₂ right in the plant canopy and feeding itself in perhaps the most elegant cycle in all of nature.

Learning how to improve photosynthetic efficiency is the next frontier in regenerative land management. Following the soil health principles is the foundation upon which we can build. It is a time-tested way to reverse degradation, improve soil function, increase organic matter and cycle more carbon through the soil. Understanding how the currency of life is exchanged and how to improve the soil/plant carbon cycle will lead to increased soil organic matter and to healthier soil, plants, animals and people — and more profitable and resilient farms and ranches. 
Brian Dougherty is a farmer, agricultural and biosystems engineer, and consultant with Understanding Ag. A version of this article originally appeared on understandingag.com.