How to Harness Soils’ Carbon-Capturing Power

Each season, in addition to producing the crops growers carefully tend, every field also acts as a huge carbon sink as plants pull carbon dioxide out of the atmosphere. For growers, there are significant benefits to this natural process and with just a little bit of effort, plants’ passive carbon capturing action can not only improve crop and soil health, but also provide growers with another revenue stream. 

Written by Rachel Sim, with expertise provided by Anne Kakouridis, Research Scientist, and Zoe Thorsland, Sustainability Manager. 

What is Carbon Sequestration?

Carbon sequestration is the process of capturing carbon dioxide (CO2) from the atmosphere and storing it. While there are artificial processes for capturing carbon, biological carbon sequestration takes advantage of the natural ability of ecosystems and other life to store carbon. Globally, soils are the second largest carbon pool on the planet, holding approximately 1,600 gigatonnes of carbon. In fact, soils store more carbon than plants and the atmosphere combined. 

When we look at all the options, soils are really the best place to put carbon,” says Anne Kakouridis, Research Scientist at Sound. For example, if carbon gets into the ocean, it causes acidification that impacts marine life, whereas in the soil, carbon has a positive effect — more soil carbon makes the soil healthier.”

Soil organic matter provides significant benefits to growers, and carbon makes up about half of all soil organic matter. High levels of soil organic matter are associated with improved water infiltration, holding capacity and drought resistance, a decline in runoff, local sediment and nutrient loads, improved ground and surface water quality, and more stable soil structure, reducing erosion and compaction and making it easier for roots to travel through the soil. 

Soil organic matter is a spongy material that holds both water and nutrients,” says Anne. The more carbon you have, the more water and nutrients you’re going to be able to store.”

Carbon in the Soil

As part of the carbon cycle, plants and animals, both aquatic and terrestrial, release carbon through respiration and, eventually, decomposition. The ocean and the atmosphere also exchange some carbon dioxide, but the primary way carbon returns to earth from the atmosphere is via photosynthesis.

Through photosynthesis, carbon is removed from the atmosphere and becomes plant biomass,” explains Anne. Although most of that carbon will leave the field at the end of the season when the plants are harvested, some of it sticks around in the soil. 

Soil carbon comes not only from decomposing plants and animals, but also from living plant roots themselves. Crops release exudates through their roots that signal, attract and feed soil microbes as part of a symbiotic relationship. In exchange for the carbohydrates in root exudates, microbes provide nutrients like nitrogen, phosphorus and other micronutrients in forms the crop can use. 

Arbuscular mycorrhizal fungi (AMF) are key players in this underground exchange. Because AMF are unable to produce their own food, they rely on plant roots for the carbohydrates the plants fix, and in exchange they extend the reach of the plants roots, helping them access water and a variety of plant-available nutrients. Their most well-known role is as the intermediary between plants and microbes, passing some of the carbohydrates they obtain to the microbes and returning fixed nitrogen or solubilized phosphorus to the crop. 

Depending on how much photosynthesis is happening — whether it’s a sunny day or not, for example — plants will allocate between 5 and 20 percent of their carbon to AMF,” says Anne. 

A good deal of those carbohydrates go to building out their network of thin, branching hyphae, making the fungi body essentially a carbon repository. When they die, some of those hyphae will decompose and release carbon into the atmosphere and some will stay in the soil, keeping their carbon with them. 

Soils are the second largest carbon pool on the planet, holding approximately 1,600 gigatonnes of carbon

Carbon does cycle through the soil system, but at different speeds. Some carbon leaves the soil quickly through decomposition and the respiration of soil microorganisms, while other forms stick around longer. For example, occluded carbon is carbon that’s physically protected within the soil, making it more stable. Microbe intermediaries help carbon from root exudates form strong bonds with soil minerals, creating mineral-associated organic matter, which persists in the soil even longer. 

The range for how long carbon stays in the soil is really broad — it could be days or it could be thousands of years,” says Anne. Some of the carbon will be mineral-associated or in aggregates, but there’s still some turnover happening; these more stable forms are just statistically more likely to stay in the soil longer.”

There are things growers can do to help keep soil carbon levels high. Soil microbes play a key role in soil health by bringing carbon into the soil and storing it in their bodies. Practices like cover cropping and conservation tillage benefit overall soil health and the beneficial microbes in the soil. In addition to protecting soil from erosion, cover crops pull carbon out of the atmosphere to build plant biomass and keep living roots in the soil, feeding microbes and AMF. Tilling physically damages AMF mycelial networks and gives soil carbon an opportunity to volatilize, so conservation tillage can both help keep carbon in the soil and support a beneficial environment for soil microbes.

What About Carbon Markets? 

Carbon sequestration is just one of a variety of ways to generate carbon credits, which can then be sold on a carbon market. These markets are opportunities for growers to use practices they want to adopt or may even already be doing to generate some additional revenue, but Zoe Thorsland, Sustainability Manager at Sound, says breaking in can be overwhelming. 

Growers are faced with so many different decisions for how they can plug into the carbon market and it can easily lead to decision fatigue,” she says. There are two large buckets for practices that qualify for carbon credits: carbon sequestration and carbon avoidance. Nutrient management or pulling back the amount of fertilizer you put down is carbon avoidance. Cover crops is a classic example of a practice leading to sequestration.”

Environmental assets that growers generate can be sold as either a carbon inset or a carbon offset. Insets are generally sold to a downstream partner, who claims that asset as a carbon reduction within their supply chain. Offsets are more likely to be purchased by companies not in the agriculture industry as an offset” of their own emissions. 

While both are viable options for growers, there’s generally more flexibility around generating insets than offsets, Zoe explains. These large downstream companies are paying growers for the practices they do every year because they’re trying to reduce their impact within their own supply chain.” 

The standards for generating an offset are often higher, and may require more data and that the practice meets additionality” requirements as a change in practice. That’s because big companies outside the industry are purchasing these environmental assets, and without a rigorous process, there is potential for greenwashing. As a result of this increased scrutiny, the payment for growers also tends to be higher, however. 

Both offsets and insets can be good options for growers. The offset route has more demanding requirements and is generally looking for larger emission reduction,” says Zoe. For growers just dipping their toes into carbon markets, insets can be more approachable.”

Growers who have been practicing cover cropping and conservation tillage for years are often excluded from carbon offset programs because of additionality requirements. However, they may be able to receive payment through other ecosystem asset approaches where downstream buyers are trying to quantify and drive impact in their own supply chains, getting paid for practices they already do each year.

When participating in carbon markets, it is often hard for growers to know what stream they’re being funneled into, though, because intermediaries are generally packaging program requirements and registering or selling projects to buyers,” says Zoe. We’re moving towards a system with more choice and visibility for growers, but we’re not quite there yet.

In addition to cover crops, other practices growers can use to generate environmental assets include conservation tillage and nutrient management. Carbon markets aside, each of these practices can provide benefits for growers by building soil health, reducing erosion, improving water-holding capacity, increasing soil carbon content and supporting crop resilience. Carbon markets are opportunities for growers to capitalize on these practices twice — once in the field and once from the market.

Farming is capital intensive and it can be risky,” says Zoe. It takes a lot of trial and error. Carbon markets are finally compensating growers to make changes that benefit everyone.”

SOURCE and Carbon Avoidance

Whether growers are interested in regenerative practices like cover cropping, reduced tillage and nutrient management as a way to generate sellable environmental assets or for their on-farm benefits, there are many tools that can help them confidently adopt these practices and achieve their goals. SOURCE®, Sound’s microbiome activator can support growers to reduce nutrients and qualify for relevant ecosystem asset programs. 

Nutrient management is growing in importance across the industry, and SOURCE is a tool to ensure growers maintain yield when they reduce the amount of synthetic fertilizer applied,” says Zoe. Growers can then get paid for that reduction as a practice through an inset program.”

By mimicking the signals plants send out through their roots, SOURCE activates the existing microbes in a growers’ soil. These microbes then provide the crop with access to in-season, plant-available forms of phosphorus and nitrogen. 

It activates both AMF and other soil microbes, giving growers a healthier microbiome,” says Anne. 

AMF and soil microbes are key intermediaries between plants and the soil, taking carbon from plants and transferring it into forms that are more stable and persistent in the soil.

Producing SOURCE takes 0.2 percent of the CO2e required to produce a unit of synthetic fertilizer

Sound continuously looks for ways to support growers when it comes to carbon markets and other regenerative practices. For growers who want to reduce their nutrient application, Sound helps mitigate the financial risk with programs like the Fertilizer Replacement Guarantee. Nutrient reduction can then give growers access to another income stream through carbon markets. 

What we’re trying to do is design programming to support growers’ profitability,” says Zoe. Then, we want to get them access to these sustainability programs that allow them to generate environmental assets by reducing their fertilizer.”

Still, carbon is just one part of the larger sustainability puzzle. Air quality, water quality and a grower’s operational resilience are just as important.

Carbon gets a lot of the attention, but it isn’t just about how much carbon is being pulled into the ground or avoided,” says Zoe. When you reduce synthetic nitrogen fertilizer, you reduce nitrous oxide emissions, improve water quality and use a form of nutrition that is significantly less carbon intensive to produce; to produce SOURCE takes an estimated 0.2 percent of the CO2e required to produce a unit of synthetic fertilizer.”