The truth about soil’s ability to sequester carbon

Much fanfare accompanies programs that pay farmers to sequester green-house gases such as carbon dioxide, methane, and nitrous oxide in their soils. Yet, questions linger as research casts doubt on whether the promise equals reality.

“Just about everywhere we look, there are claims about carbon sequestration being akin to the holy grail for agriculture and, by extension, the environment,” says Ben Palen, a fifth-generation Kansas farmer and manager at Ag Management Partners. “There is nothing wrong with modest improvement, but often the promise and the reality do not match.”

“I worry that we are selling ourselves a pie-in-the-sky dream we might not realize, and that it could come back to bite farmers and ultimately not get us any further down the road toward reducing greenhouse gases in the atmosphere,” says Gregg Sanford, senior scientist, Department of Agronomy at the University of Wisconsin-Madison.

Reining in the hype

About two decades ago, Sanford began reading scientific literature that suggested certain farming systems could help combat climate change by sequestering carbon in the soil.

“We love to support farmers and incentivize them to do things that benefit society, but we also put a ridiculous amount of pressure on farmers to save us from every-thing,” Sanford says. “I don’t feel the science is there to say that if you do these practices (e.g., cover crops, no-till, etc.), you’re going to sequester carbon across the board. In some cases, we will, but in many cases, we won’t.”

Findings from his doctoral work and a decade of subsequent research by Sanford and colleagues bear that out. The data challenges the climate impact of reducing soil tillage and questions how much car-bon cover crops can sequester in some circumstances.

“Even with best management practices like no-till and cover crops, we’re losing carbon in the upper portion of the soil that we'd expect to be able to improve with management,” Sanford says. “The data also shows that carbon is being lost in soil horizons, as much as a meter deep. Those losses are likely due to a combination of historic vegetation, current management, and a changing climate and are much harder to change via farming practices like cover crops or no-till.”

In 2009, Sanford began analyzing archived soil samples from the Wisconsin alfalfa, he says. “Across the board, we have seen declines in soil carbon in our grain systems regardless of management (e.g., conventional, organic, tilled, no-till, cover crops). Our dairy-forage rotations have also incurred losses, although at a slower rate, likely because of the reduction in tillage, application of manure, and deep-rooted perennials.”

It is only in their grassland systems, whether it’s rotation-ally grazed pasture, CRP, or prairie, where they can document carbon sequestration in the surface soils. However, Sanford says that in many cases they are still observing losses of carbon at depth.

Results may vary

One of the caveats with soil carbon sequestration, Sanford says, is there are places in the United States where certain types of ag management will be able to build soil carbon resources, but results may vary.

“There are going to be areas where, despite our best efforts, we continue to lose carbon simply because the amount being respired as carbon dioxide from the soil due to the microbial turnover of organic matter is going to outpace the residue carbon being returned,” he says. “And that gets tricky because the climate is getting warmer, speeding up the process of soil organic matter turnover.”

What’s relevant for farmers in terms of maximizing carbon, Anna Cates says, is whether they can maximize how carbon helps with other functions.

“Can they maximize how much organic matter is building soil structure? Can they maximize how much organic matter is delivering organic nutrients for the crop?” says Cates, assistant Extension professor, University of Minnesota.

Transformational change needed

The main way to build soil organic matter is by adding plant biomass while simultaneously keeping as much of the carbon currently in the soil from being respired as CO2 by limiting soil disturbance. Sanford believes that to make a big impact on soil carbon and use it as a tactic to mitigate climate change, a transformational change must occur in agriculture.

“The area in the U.S. we now call the Corn Belt was once almost entirely covered by tallgrass prairie with deep-rooted perennial plants, grazing animals, and regular fires. Over thousands of years, the prairies created the fertile, carbon-rich soils we now farm for annual grain commodities like corn and soybeans. It took a long time for those deep-rooted, diverse, and perennial prairies to accumulate that soil carbon” he says. “The annual systems we have replaced the prairies with invest less in root biomass because they're focused on producing grain. During any year, there are also long stretches when there is nothing covering the soil, even if it is no-tilled.”

If the prairies built these soils, should we consider emulating what they can do in our production systems? It’s a question Sanford and others are asking.

“To get soils in the Midwest back as close as we can to those tallgrass prairies to stabilize or accrue carbon, we need to switch from farming systems dominated by annual crops to systems dominated by or exclusively built around perennials, perennial forages, or grasses,” Sanford says. “That is our best hope for building carbon in this region, but it’s a big change and a hard pitch to make. Maybe there are things we can do to our dominant systems today to make them ‘behave’ more like perennial systems.”

To test that hypothesis, researchers created nested experiments in some of the large plots at WICST. At one end of the spectrum, they are just reducing tillage or adding cover crops. At the other end, they are reducing tillage, adding cover crops, adding manure, and trying to diversify the rotation. As they head into their fourth season, the impact on the soil is being tracked, and they are just now starting to dig into the data.

An approach that Palen believes holds promise is a closed-loop system involving crops and hogs. A project he is currently working on replaces commercial fertilizer with hog manure. "Hopefully, we will achieve carbon-neutral grain, which will be fed to the hogs, and then they can be sold as car-bon-neutral pork, creating a closed-loop system," Palen explains. "There's a lot of opportunity there for carbon credits by replacing commercial fertilizer but also by scrubbing the methane out of the hog manure."

Healthy skepticism

As with any new concept, healthy skepticism about soil’s ability to sequester carbon is a good thing.

“In the Upper Midwest, we have some of the highest organic matter soils in the world, so it’s going to be difficult to make them better,” says Jodi DeJong-Hughes, University of Minnesota Extension soil and water quality educator. “What I tell farmers is that it's a slow process to build organic matter, and while I don’t know if the carbon markets are for you, reducing tillage and adding cover crops are great for the soil.”

DeJong-Hughes also believes we are missing an opportunity when it comes to fossil fuels. “We know how much carbon is emitted from diesel fuel. It’s harder to quantify carbon sequestered in the soil,” she says. “The programs pay to sequester carbon, but carbon is also not being emitted through diesel fuel because a farmer makes less tillage passes across the field.”

Palen also notes the potential for generating carbon credits by reducing commercial fertilizer rates via the use of certain biostimulants. For instance, he suggests bench-marks for nitrogen use could be established on a local or regional level, and farmers could earn carbon credits by demonstrating a reduction in use based on a reference point or benchmark. “That sort of approach might provide more transparency and legitimacy for both buyers and sellers of carbon credits,” he says.

Initially, Kevin Bahr was among the carbon program skeptics. As he learned more about the Truterra carbon program, Bahr says it seemed like something worth exploring and joined the program in 2021. The Kansas farmer also sits on the program’s advisory board where he and other farmers can provide feedback.

Like most central Kansas farms, Bahr’s land is diversified, handling alfalfa, corn, milo, soybeans, wheat, and a cow-calf herd. His soils range from good black dirt to sandy loam.

“While we may be talking about carbon, this program is more about soil health, which is a journey that doesn't hap-pen overnight,” he says. “I’m employing certain practices like no-till and cover crops because they are what is best for my soil; the carbon credit money is like gravy on top of my mashed potatoes. It’s a reward for taking care of the land the way I should.”

Payment is based on how much carbon has been stored, which is quantified through a combination of modeling and soil sampling. Growers are paid $30 an acre for every ton of carbon sequestered, which can vary year to year based on crop rotation.

To get a feel for the program, Bahr enrolled only a portion of the acres he covers. “In some of my fields, I don’t think it quite fits like I want it to, but there are other areas where it fits well,” he says. “We’ll continue to evaluate where we’re at, and if I feel good about it, I’ll consider adding more land.”

Gauging attitudes toward carbon farming

Palen recently surveyed 50 farmers to learn about their attitudes toward carbon farming. He says the most revealing question asked farmers to rate carbon markets, on a scale of 1 to 10 (with 10 as an opportunity that provided good financial and environmental benefits). The average reply was slightly over three.

“Comments that led to that low rating included (a) no recognition of good practices that I’ve already done for years; (b) no clearly defined rules, and vaguely written contracts; (c) too much paperwork; and (d) practicalities of changing some practices because of site-specific conditions,” Palen says.

What’s the takeaway?

In all these discussions about carbon markets, Anna Cates, assistant Extension professor, University of Minnesota, says farmers not only need to pay attention to their soil functions, but also check the contracts to make sure they’re not stuck with the bill if soil carbon doesn’t rise.

“If you want to join a carbon market, especially in the Upper Midwest, make sure you understand what happens if the sample shows you have not sequestered carbon, which could be through no fault of your own,” she says. “You could have grown a beautiful cover crop. You could have successfully grown no-till corn and soybeans and still not see your organic matter number move.”

The higher the organic matter is, the harder it is to improve it. Jodi DeJong-Hughes, University of Minnesota Extension soil and water quality educator, says if your organic matter is over 4%, it’s going to take a very long time to build more soil carbon. She also says she would like to see farmers paid by the practice rather than be on the hook for a certain carbon number by the end of the contract.

“Ultimately, no-till, cover crops, and integrating livestock are fantastic practices that have so many benefits to the farm that should be the focus. And carbon is this fantastic thing that may come along with it,” says Gregg Sanford, senior scientist, Department of Agronomy at the University of Wisconsin-Madison.