Written by WILL FULWIDER, Diane Mayerfeld and Kevin Shelley

Right now, there are at least a dozen carbon credit companies willing to pay farmers a range of prices to add conservation practices, like cover crops and no-till planting, to their annual crop production systems. Although these companies are paying farmers to implement the conservation practice, what they are really investing in is the additional carbon they expect to be stored (sequestered) in the soil through implementing the practice. Once enough carbon has accumulated within a soil/field, the carbon credit company will then sell the stored carbon to other businesses looking to offset their greenhouse gas emissions in the form of carbon credits.

Why care about soil carbon science? 

Companies paying farmers for carbon credits are using a number of different approaches to determine and verify the amount of carbon being stored in the soil, including modeling, field sampling, and soil testing, and various hybrid models of the two. Currently, the industry lacks consensus on which approach, model, and/or sampling scheme is most appropriate and most accurate for measuring changes in soil carbon over time.^, With no current governmental oversight, carbon verification methods are determined company-to-company and may not accurately reflect in-field carbon accumulations. 

Currently, the industry “standard” for sampling agricultural soils for carbon storage is to a depth of 30 cm (about 12 inches). Some long-term trials evaluating carbon concentrations to a depth of one meter (about 39 inches) following the implementation of a conservation practice have shown carbon may be added in the upper foot of soil, but carbon is lost further down in the soil profile resulting in a net loss of carbon. So, by understanding the state of the science of soil related to carbon sequestration resulting from conservation practices, farmers and landowners can better evaluate the risk of engaging with the carbon credit market. Besides, soil carbon has benefits to soil health beyond the potential for carbon credits.

Soil organic matter vs. soil carbon

Agriculture is all about growing plants and the animals that eat those plants. During photosynthesis, plants take in carbon dioxide (CO₂) from the atmosphere and use it to build their tissue. Soil organic matter is composed of this tissue, living, dead, and decaying, from plants, animals, and microbes. Total organic matter is difficult to measure, but we know that on average 50% of soil organic matter is carbon, and labs can calculate the carbon fraction of a soil sample relatively easily by burning. Since it is carbon that reunites with oxygen to form the greenhouse gas CO₂, it is soil carbon that we are concerned with in regard to climate change. In simple terms, carbon crediting is based on the idea that if we can sequester, or store, carbon in the soil, then it isn’t free in the atmosphere to bond with oxygen and create CO₂. And because building soil carbon relies on growing plants, agriculture has a big role to play in drawing down atmospheric CO₂.

So, when we’re talking about carbon in this article we are referring to soil carbon. Finally, the terms store and sequester are mentioned often when talking about soil carbon. Both are referring to a long-lasting increase in the amount of soil carbon found in the soil, or in the lingo of carbon credits, permanence. At the same time, it is important to remember that soil carbon is dynamic and changes in management can release carbon that was once stored or sequestered. Stocks of soil carbon should never, therefore, be considered permanently sequestered. 

Carbon cycle

Soil carbon is one component of the larger carbon cycle that, along with the nitrogen and water cycles, is foundational for life on Earth. Plants take in CO2 and water through photosynthesis to create sugars for plant growth and release oxygen. The plant uses these sugars for energy in both the roots and aboveground plant matter in a process called respiration, which releases CO2. Carbon is a key building block of sugars and the primary component of plant biomass. It is also exuded through plant roots in the form of sugars to feed soil microbes and fungi that help transfer nutrients to the plant. As the plant sheds material and/or dies, the plant’s carbon is added to the soil and broken down by microbes. The breakdown of this plant material is conducted by soil organisms, which also respire (breathe), resulting in a process called soil respiration, which also releases CO2 to the atmosphere. As these microbes die off, their dead matter is further added to the pool of soil organic carbon.

Back on top of the soil, livestock consume the carbon in plant matter and expel it through respiration as CO2 and CH4 and in their manure, which is further processed by soil microbes. All of this processing by soil microbes uses carbon, expels it as CO2, and renders it into more stable forms. In agriculture, practices like tillage introduce more oxygen into the soil and create more surface area for the microbes to process the organic matter, releasing nutrients to soil, while also drawing down soil organic matter.

Soil carbon and soil health

The connection between soil carbon and organic matter ties soil carbon directly to soil health. Higher levels of soil carbon–and therefore larger percent soil organic matter–generally increase surfaces onto which positively charged nutrients such as calcium, magnesium, sodium, and potassium cling, making them more readily available for plant uptake. Soil carbon is often stored in and contributes to the development of soil aggregates, which improve water infiltration and soil structure, facilitating root growth and mycorrhizal activity. The increased fungal activity in turn contributes to more aggregation, creating a positive feedback loop. While research and farmer experiences tell us conservation practices do advance soil health by improving water infiltration and soil aggregation, calculating the actual amount of soil carbon sequestered from conservation practices is less certain. 

Practices that affect soil carbon in annual cropping systems

The ability of cropland soils to store carbon depends upon a host of factors. Soil is a living ecosystem with complex interdependent relationships that science is still learning about on a daily basis. Pair this complexity with the influence of the local climate, and you have a situation in which one practice in one region may store carbon, while that same practice in another region may have little effect or may even reduce carbon stores. Further complicating matters is that, in some instances, while carbon might be stored in the upper portion of the soil under conservation practices, carbon stores are being broken down by microbes in the lower portion of the soil profile, leading to a possible net loss of soil carbon in that location. This leads to seemingly contradictory findings among scientists investigating the fate of carbon in our soils. So, when we talk about carbon storage, we need to consider the whole soil profile and think locally: taking into account the potential influence of the soil type, the management, and the climate. 

Despite local variability, there are general trends. A study that combined the results of 417 peer-reviewed studies found that across all soil types, climates, and study durations: 

• Soils with the highest capacity to increase soil carbon are somewhat coarse-textured with low initial levels of soil carbon, low crop productivity, and have received low rates of recycled residues and manure;
• No-till practices increased net soil carbon content by an average of 8% (example: an increase from 2% to 2.16% soil carbon) when evaluated across the soil profile up to 120 cm; 
• The increase in soil carbon for no- and reduced till was greatest (22%) in the top 10 cm of the soil;
• No-till actually led to an average loss (14%) of soil carbon from depths between 10 and 50 cm;
• Cover crops increased soil carbon content by an average of 6% (example: an increase from 2% to 2.12% soil; carbon) when evaluated at depths down to 70 cm. 

Scientists generally agree that conservation/soil health practices like cover crops and no-till store or leave more carbon in the upper inches of the soil profile than conventional tillage and bare soils over the winter. However, there is less consensus about whether conservation practices increase soil carbon deeper in the soil profile, and some practices like no-till may even speed up carbon loss at those depths, as described above. 

The fate of soil carbon in long-term cropping trials is site specific

Using samples from the long-term Wisconsin Integrated Cropping Systems Trials (WICST), a study investigating the fate of soil carbon across six different crop rotations and associated management practices at the Arlington Agricultural Research Station in South Central, Wisconsin (medium textured, high organic matter, well-drained, highly productive loess soils) found all crop rotations lost soil carbon. However, those fields under soil conservation practices like no and reduced tillage, perennial forages, and cool-season grass pastures lost less carbon than those fields under conventional tillage practices. The combination of a cool, humid climate in Wisconsin and the already high soil carbon soils in southern Wisconsin place cropping systems in this study at a low potential for increasing soil carbon. While that may be true for studies conducted around Arlington, WI, just across Lake Michigan at Michigan State’s Kellogg Research Station (coarse textured, lower organic matter, well- to poorly-drained, moderately productive soils) studies have shown an increase in carbon in the upper soil profile in no-till situations without losing carbon at depth. 

The climate also has an effect on the amount of soil carbon that can be sequestered in your soils, regardless of soil type. Warm and dry regions show the highest potential increase in sequestration from no-till and cover crops. The warmth stimulates more crop growth than in cool areas, which is returned to the soil, and the relative lack of rain means that soil carbon breaks down more slowly than in wet climates. While our temperate and humid climate here in Wisconsin lowers our carbon storage potential, soil health practices will still positively impact the fate of soil carbon.

Organic amendments and soil carbon

While reducing tillage and planting cover crops generally help soils retain carbon, the largest impact on soil carbon is through the addition of organic soil amendments such as manure. This makes sense. Conservation practices that maintain crop residue and minimize the breakdown of soil carbon may add some soil carbon. However, crop growth is part of the carbon cycle that both adds carbon to the soil and extracts carbon from the soil through associated microbial activity and root respiration. Applying compost, manure, or other soil amendments adds significant quantities of carbon to the soil. That being said, organic amendments are more or less a reshuffling of carbon. If they are not derived from the land that initially turned CO2 into plant biomass they result in additions in one location but losses in another.

The application of manure was found to increase soil carbon by a third when averaged across 101 different studies. Consistent with the findings from the first meta-analysis, soil carbon increased the most in soils with low initial levels of soil carbon. In this study, the fine-textured, especially clay soils saw higher total changes to soil carbon than coarse-textured soils. It is important to note that 80% of these studies only sampled soil carbon down to one foot. In contrast, even with additions of manure, soils during WICST continued to lose soil carbon when the first meter of soil was evaluated. Although soil carbon was still lost in WICST with manure additions, it did help to minimize that loss.

Perennial systems and soil carbon 

Compared to annual cropping systems, perennial forages, and grasslands as well as woody perennials like fruit trees have enhanced potential for increasing soil carbon. Perennial crops are more deeply rooted, pumping carbon deeper into soil; creating more biomass which itself acts as a carbon store; and photosynthesize for a longer portion of the year, drawing more CO₂ from the atmosphere. In WICST, even the manure amendments to organically managed annual croplands could not outpace the soil carbon maintenance from managed perennial pastures, which saw a 15%-28% greater increase in carbon stocks compared to every other management system.^, In a global comparison of soil carbon over a 20-year period, a change from annual to perennial crops led to an average increase in soil carbon of 20% down to 30 cm, and a 10% increase when averaged down to 100 cm.

Soil carbon is not the only climate factor

Focusing solely on soil carbon does not take into account the full picture. An in-depth life cycle analysis would be needed to determine the net climate impact of these conservation practices. Each of these practices, from cover crops to manure composting and amendments, comes with a carbon cost: the amount of greenhouse gasses, including carbon dioxide, released through the implementation of this practice. 

For example, cover crops can have a major net benefit or even result in losses depending on how they are managed. Biomass is the name of the game for maximizing carbon inputs from cover crops. Practices like planting green into an overwintered cover crop like cereal rye take advantage of accelerating cover crop growth in the spring; rye biomass can more than double over a two-week period from early to mid-May, increasing carbon additions to the soil. In contrast, if winter-killed cover crops are flown on an airplane, the emissions generated from the airplane’s fuel consumption and the nitrous oxide emissions generated by the cover crops decaying in the waterlogged soils of spring could exceed the carbon added to the soil. 

Manure is beneficial to soil carbon stores, but if that manure is trucked halfway across the state, the emissions from that fuel consumption may also outweigh any climate benefit from added soil carbon. On the other hand, practices like no-till have knock-on effects: lower horsepower tractors needed for fieldwork consume less fuel as do fewer passes on the field. In any case, in order for the true carbon cost or benefit of conservation practice to be determined, a full life cycle analysis is needed.

So, what’s to be done?

Even with a lack of full carbon accounting and variability across soils and climate, conservation practices still benefit soil carbon by reducing losses or contributing to sequestration. Organic amendments and particularly perennial pastures and crops have the greatest capacity to increase soil carbon.^, Manure’s capacity is further increased after stabilizing it through composting. Even though we are in the dairy state, transporting manure to all fields is not practicable, and conversion to more perennial systems of agriculture is unlikely to take place quickly. With these constraints, the most logical and lowest cost management strategies that store carbon, or at least minimize carbon losses, in annual cropping systems are soil health practices like no-till and cover crops. Given the variability in the relationship between conservation practices and carbon sequestration in agricultural soils shown by the still-developing body of science, we may need to proceed cautiously when participating in markets with carbon credits in annual cropping systems. Nonetheless, conservation practices are certainly beneficial for climate change resiliency, combating soil erosion, water quality, and agricultural productivity.

Take-home points

Soil carbon sequestration is a complex process that is dependent on soil type, and climate, and varies with depth. Current scientific understanding is inconclusive on the degree to which specific practices result in carbon sequestration. Sampling methods, particularly the depth of sampling, have an important impact on results.

The most effective ways to increase soil carbon include: 

• Incorporating pasture and perennial crops onto the landscape, and composting manure
• In annual cropping systems, incorporating no-till and maximizing cover crop biomass will provide the greatest return to soil carbon