A pillar of the climate-smart agriculture movement is on shaky ground

Cover crops have gained elite status as a way for farmers to fight climate change. But a closer look at the growing body of research raises questions about their ability to lower greenhouse gas emissions.

• Gabriel Popkin

It’s one thing the Biden administration, agribusiness leaders, soil scientists and environmentalists all agree on: farmers across the country should plant cover crops. The U.S. Department of Agriculture and food giants such as Land O’Lakes, Corteva, Bayer, and Cargill are paying farmers millions of dollars to sow rye, clover, radishes or other crops after, or even before, they harvest their corn and soybeans.

These off-season plantings have long been used to keep soil and nutrients in place and prevent runoff that fouls waterways. But that’s not why they’ve become a linchpin of the red-hot climate-smart and regenerative agriculture movements. With support from influential international bodies like the IPCC and leaders like former vice president Al Gore and U.S. Secretary of Agriculture Tom Vilsack, cover crops are being asked to do something new and high-stakes: draw atmospheric carbon into the soil to help fight climate change.

Cover crops, proponents argue, can soak up carbon dioxide, via photosynthesis, when fields are normally bare. When the plants are later killed and their roots break down, some of that carbon can remain in the soil. 

A major pandemic program reduced crop insurance premiums for farmers planting cover crops, and USDA has boosted its cover crop cost-share program. Newly announced climate-smart commodities partnerships are expected to supercharge these efforts. And there’s a new and potentially powerful player in the game: private carbon markets that are starting to pay farmers for the carbon they sequester.

But as the hype for cover crops mounts, so does the scrutiny. New research suggests that cover crops may struggle to make a significant dent in agriculture’s greenhouse gas emissions. A review from earlier this year found that only a third of published studies in which researchers compared fields that were cover-cropped with those that weren’t reported significant gains in soil carbon. And a study published last month illustrated one major reason why farmers may be reluctant to plant cover crops. Researchers, using satellite data, found that cash crop yields in the corn belt dropped significantly—on average 5.5 percent for corn and 3.5 percent for soybeans—on fields that were cover-cropped, compared to fields that were not. 

Such losses could dissuade farmers from planting cover crops, no matter the financial incentives. And they raise the risk of additional acres being plowed up to compensate for the lower yields. “Five percent in agriculture is a big deal, especially in a very high-production region,” says the study’s author David Lobell, a food security researcher at Stanford University. “That’s many millions of tons of grain.”

Even if cover crops can, in theory, add carbon to soil, it’s far from clear that the way they’re being deployed in practice is benefiting the climate. Many scientists want stronger evidence before endorsing cover crops as a climate solution.

“We need to be cautious based on some disappointing data we get from research experiments,” says Humberto Blanco-Canqui, a soil scientist at the University of Nebraska-Lincoln who conducted the review. “Planting cover crops is better than not doing anything, obviously, but expecting large changes in carbon may not happen.”

One thing that’s not in doubt: Agriculture is responsible for around 10 percent of U.S. emissions and a quarter of emissions globally. An estimated 133 billion tons of carbon, roughly a fourth of all carbon emitted by humans since the Industrial Revolution, has been lost from soils globally. 

The growing recognition of agriculture’s climate problem has birthed a regenerative agriculture movement backed by scientists, private companies, the UN, and celebrities like Woody Harrelson and Rosario Dawson. All promote the idea that farmers could replenish their soils with that lost carbon, building soil health and a better climate at the same time. Also on board is President Biden, who in a 2021 speech to Congress extolled the idea of “farmers planting cover crops, so they can reduce carbon dioxide in the air and get paid for doing it.”

But the experience with another climate-smart practice — no-till or reduced-tillage farming — which many hoped could draw down large amounts of carbon dioxide, offers a cautionary tale. Those hopes were largely dashed when several studies in the late 2000s and early 2010s found that no-till often doesn’t add carbon to the soil; instead, it seems to mostly concentrate existing carbon near the soil surface. While reducing tillage can protect topsoil from erosion and improve soil health, scientists have grown less optimistic about its potential as a climate solution.

Growing trees and other perennial plants, by contrast, reliably adds carbon to the soil over time. But modern production agriculture is largely designed around high-yielding annual crops such as corn, soybeans and wheat, and a rapid, large-scale switch to perennials is not realistic. That has left cover crops as essentially the last major idea standing for turning the roughly 400 million acres that U.S. farmers grow annual crops on into a carbon sink. While the idea has been slow to take off — a 2017 USDA survey estimated that around 5 percent of cropland acres were cover-cropped, a number that has increased modestly since — the public and private money now flowing toward farmers is intended to rapidly boost adoption.

Without cover crops, “you have this whole period during the year when you’re missing the opportunity to fix carbon through photosynthesis,” says Hanna Poffenbarger, a soil scientist at the University of Kentucky. “With cover crops, all of that fixed carbon is going back to the soil; none of it’s getting removed. It makes sense.”

It’s a simple idea in theory, but the reality is more complicated, because much of the fixed carbon is ultimately digested by soil microbes and respired back to the atmosphere. Several decades’ worth of efforts to assess how much carbon remains captured have yielded a murky picture. Dozens of field trials have compared fields with cover crops to ones without, and numerous meta-analyses and review papers have attempted to summarize those trials. 

Among the most-cited is a 2015 meta-analysis of cover crop trials around the globe, which found that soils of fields in which cover crops were grown accumulated, on average, roughly an eighth of a ton of carbon per acre more than fields without cover crops. The Nature Conservancy, Project Drawdown and the National Academies of Sciences, Engineering and Medicine have all based optimistic claims of cover crops’ potential benefits largely on this study.

‘Planting cover crops is better than not doing anything,’ says soil scientist Humberto Blanco-Canqui, ‘but expecting large changes in carbon may not happen.’

But the study has drawbacks. Only 10 of the surveyed trials took place in the U.S., which Kim Novick, an environmental scientist at Indiana University, says is far too few for such a large and varied landscape. Some trials involved cover crops grown without a subsequent cash crop, a context very different from the typical scenario, Canadian crop scientist Terry Daynard pointed out in a blog post analyzing cover crop studies. Daynard concluded that cover crops often increase soil carbon content in trials, but questioned the authors’ estimate of how much carbon could be sequestered annually. 

Other meta-analyses have reported similar or even larger average carbon accumulation rates from cover crops. But those findings have problems, too, Daynard noted. Most trials lasted only one or a few years — too short a period to determine whether gains persist long enough to have an impact on the climate. Many sampled only a few inches below the soil surface, meaning they didn’t assess whether carbon could have migrated from deeper soil levels rather than from the atmosphere (the finding that undercut no-till as a climate solution). And few consistently measured a property called bulk density — the weight of a soil divided by its volume — which is essential to accurately quantify stored carbon. Still others claimed that cover crops increase carbon sequestration but actually found no statistically significant effect. And researchers were more likely to find large carbon gains in warmer regions with longer growing seasons, such as the U.S. Southeast; gains in the corn belt — the nation’s most important crop region — were much more modest.

In an email, Christopher Poeplau, the lead author of the 2015 paper, acknowledged another issue: The studies he surveyed didn’t account for nitrous oxide, a major planet-warming greenhouse gas that you’ve probably heard little about. Nitrous oxide, or N₂O, comes mostly from agriculture, emitted as soil bacteria digest nitrogen-containing compounds. This can happen when farmers apply too much fertilizer — or when nitrogen enters the soil from decaying plant matter, including cover crops. Each N₂O molecule warms the Earth 300 times as much as a molecule of CO₂, and once it’s in the atmosphere, it’s not going anywhere for a long time. While other studies have found that, on average, N₂O emissions from cover crops don’t fully offset carbon sequestration benefits, they can in some cases.

Newer papers have further dimmed cover crops’ climate prospects. In his review, published this February, Humberto Blanco-Canqui found that 55 of 77 published trials comparing fields with and without cover crops found no carbon sequestration benefit from cover crops. His conclusion: Cover crops “capture a bit of carbon, but that’s not going to make a huge, huge dent.” 

Given all this, some scientists are raising alarms that billions of dollars are flowing toward supposed solutions that may do little or nothing for the climate, potentially misleading consumers and the public while delaying real climate action.

Indiana University’s Novick says that, even if measurements do find more carbon in fields after cover-cropping, it’s not clear that carbon came from the atmosphere. It could simply mean the cover crop prevented carbon from being leached from soils into runoff — a good thing, but not necessarily a climate benefit. Novick and colleagues are launching a new study that will use large towers to measure the flows of greenhouse gases into and out of fields that are cover-cropped and those that aren’t, which should provide a clearer picture of how much carbon soils can absorb from the atmosphere.

Then there’s the new satellite study, which estimated cover crop plantings and crop yields based on the greenness of fields at different times of year, as measured by optical satellites. 

Some scientists are raising alarms that billions of dollars are flowing toward supposed solutions that may do little or nothing for the climate.

The University of Kentucky’s Poffenbarger says the study’s results track with previous findings that cereal cover crops such as rye — by far the most commonly planted in the corn belt, in part due to its ability to withstand harsh winters — can deplete soil of nitrogen, sulfur and water, hampering the growth of the following cash crop, especially corn. “The cover crop does take some resources from the cash crop,” she says. 

If yields decline, that could drive farmers to plant crops on other land, including acres set aside for the USDA’s Conservation Reserve Program. Carbon released by this additional planting could offset some or all climate benefits of the cover crops.

“This is a cautionary piece of evidence,” Stanford’s David Lobell says of his study. “It’s not to say the whole idea is terrible, but we have some work to do” to ensure cover crops provide the benefits they’re supposed to.

For example, he says, farmers could reduce yield losses by planting cover crops, such as clover, that fix nitrogen from the atmosphere and thus are less likely to deplete nutrients for the following cash crop. But these legume cover crops can create a separate problem: Because their roots harbor bacteria that fix nitrogen, they often increase emissions of nitrous oxide from the soil. They can also be harder to source and grow.

For a dose of on-the-ground reality, I called Trey Hill, a large grain and soybean farmer on Maryland’s eastern shore who has planted cover crops for more than 20 years with the help of state funding. In recent years, he has planted mixes of rye, clover, turnip, and other species and let his cover crops keep growing even into the beginning of cash-crop season, pushing for maximum biomass production and carbon sequestration. Consistent with Lobell’s findings, Hill’s corn yields have dropped modestly, he says, possibly because the green cover crop shades and cools the soil and slows germination and early growth. His soybean yields, however, have held steady.

Hill has sold carbon credits twice to a company called Nori, even though he says he’s “not a super huge believer” in the idea. Nori pays him around $15 an acre — an amount he suspects is too small to entice most farmers to add the necessary new farming and data-management practices to their operations. And he warns that farmers — and funders — who hope that cover crops’ soil-health benefits will quickly justify the extra cost are likely to be disappointed. 

“You’re talking a 10-year commitment before you would even see anything significant” in terms of higher yields or cost savings, he says. “It’s like the 30-year mortgage on your house … you get to year five and you feel like you haven’t even started.”

Cover crops still have supporters. Rob Myers, director of the Center for Regenerative Agriculture at the University of Missouri, has reported potential carbon sequestration rates several times higher than those found in other studies, based on an analysis of fields where cover crops were established successfully. He says that experienced, profit-motivated farmers often grow cover crops more successfully than university researchers, who often plant too late in the year.

“Farmers in general have more success getting the cover crops established well, especially once they have some experience, than we see in some of these university trials,” he says.

Indigo, an ag-tech company based in Boston, announced in June that it had sold 20,000 credits based on carbon accumulated on 175 farms where farmers had started new climate-smart practices between 2018 and 2020. The company uses soil samples and computer models to estimate the greenhouse gases emitted from, and taken up by, participating farmers’ fields. For areas where the science is uncertain, such as how much nitrous oxide is emitted by a practice such as cover-cropping, Indigo uses “conservative” methods that likely overestimate the practice’s emissions to ensure they aren’t selling more credits than are justified, says A.J. Kumar, Indigo’s vice president of sustainability sciences.

Many scientists say, however, that neither soil samples nor current-generation models can accurately estimate greenhouse gas fluxes over a large farm field. 

And farmers do not always plant cover crops well — largely because they often lack incentives to do so, says Ray Weil, a soil scientist at the University of Maryland. Government programs have generally paid farmers for merely planting cover crops, whether or not they germinate or  fix a meaningful amount of carbon. To realize benefits, farmers need to plant in early fall or late summer, ensure a successful stand, and then let plants grow well into spring to accumulate biomass, as Hill does. “Just growing a cover crop is not enough,” Weil says. “You’ve got to include how you manage that cover crop.”

Larger system-level changes may also be needed. Hanna Poffenbarger points to a recent study that concluded that if every corn farmer in America wanted to cover-crop, at least 3 million acres — an area nearly the size of Connecticut — would be needed to grow seeds, a huge land conversion that would likely release a lot of additional carbon. She hopes that breeding better cover crop varieties, optimizing planting and termination dates, and prioritizing planting of cover crops on soils that are depleted in organic matter can help them meet their carbon-sequestering potential while minimizing downsides.

Ultimately, researchers say, they need large-scale, multi-year data from real fields representing many different regional climates and soil types to accurately predict how cover crops and other practices affect greenhouse gases. Deep in the bowels of the Inflation Reduction Act is a $300-million outlay to the USDA’s Natural Resource Conservation Service to set up just such a soil-measurement network. But it will take time to stand up such a program, and many more years before results are in; NRCS is now collecting public comments on what the program should look like. 

For a story in early 2020, I wrote, “Everybody I spoke with agrees that regenerative agriculture is good for soil health and has important environmental benefits that may be worth paying for. Most believe that soils may have a role to play in drawing down carbon. But nearly all scientists also want more certainty before wholeheartedly endorsing fighting climate change using farming practices.”

Nearly three years later, the momentum and money behind cover crops have mushroomed exponentially, while the evidence of their climate benefits has become, if anything, even more uncertain. Unless that changes, relying on them to help avert catastrophic warming remains, in the words of a USDA soil scientist I spoke to in 2020, “risky business.”

CLARIFICATION: The article has been updated to make clear that the higher rates of carbon sequestration found in Rob Myers’ study came from fields where cover crops had been successfully established.

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