The case for cover crops goes beyond climate benefits

“Nature is very resilient but little by little you can degrade it — and also you can’t fix it overnight once it is degraded,” says Martens. “Cover crops are a management change that at least slows down that rate of [soil] degradation and potentially could reverse it.”

Cover crops — primarily used to cover soil rather than be harvested — are well known for improving soil health. In addition, they provide an impressive array of benefits, from reducing soil erosion to attracting pollinators. However, now they’re in the spotlight for something new: climate change mitigation. Since these plants sequester carbon dioxide, they offer an easy way to reduce agriculture’s sizeable carbon footprint. Consequently, an increasing number of groups — from federal and state agencies, NGOs to private organizations — are promoting cover crops.

Despite the P.R., cover crops may not live up to their climate change promise. For instance, they don’t always stockpile carbon in the soil, and under some circumstances, they could actually increase greenhouse gas emissions. Yet, with environmental benefits extending beyond carbon capture, cover crops do a lot for us.  Like many farming practices, some of these perks depend on the conditions, while others are still being tested out. How do cover crops shake out as a boon for the environment and the climate?

Cover crops 101

Although cover crops are currently taking center stage, they’re nothing new.

“Cover crops were a key part of traditional European agriculture…” says Martens. Speaking about his parents, he adds, “Both of them were peasant farmers, and there was a rotation that goes back to the agricultural revolution [cycle] they would always follow — their small grains would be interseeded with a medium red clover. And that was the nitrogen source for the farm and also an important source of livestock feed. But it was also a cover crop that kept the land covered and protected it from erosion.”

While some cover crops produce nitrogen, most also help keep existing nitrogen, calcium, potassium and phosphorus from leaching out of the soil, making them more available to plants, explained Jay Brandt, the owner of a regenerative farm growing corn, soybeans and heritage grains in Ohio. They do this by locking nutrients in plant matter rather than letting them leach out of the soil. Then, when these plants die and decompose, the nutrients return to the soil. This added fertility directly benefits Brandt’s farm.

“It allows us to reduce at least 50 percent in some years and up to 80 percent of [fertilizer] applications compared to our conventional neighbors,” says Brandt. But that’s just the tip of the iceberg. “What we saw as we started adopting it across more acres was an increase in soil organic matter and the ability to absorb significant rainfall events,” says Brandt.

Cover crops also provide substantial erosion control — reducing soil loss up to 100 percent compared to fields without these beneficial plants — although their impact varies with how the farm is managed. “Cover crops aren’t as important for erosion control if you do no-till, because you already have pretty good control,” says Kenneth Staver, an Associate Research Scientist at the University of Maryland’s Wye Research and Education Center.

The crops also mitigate another common farm problem: pests. “Now when we get done harvesting wheat, we immediately plant buckwheat,” says Martens. “The buckwheat kills certain weeds. It destroys some of our disease problems, and it kills some insects. And it improves the soil structure.”

Buckwheat attracts the natural enemies of pests, while another cover crop planted by Martens, yellow mustard, also suppresses pest populations.  Once it’s incorporated into the soil, mustard gives off a gas that’s toxic to certain organisms.

In turn, this reduces the need for pesticide use — saving money and sparing workers the myriad negative impacts on their health.

In addition, they harvest buckwheat as a second crop — blurring the lines between cash and cover crops. “It’s a triple purpose there,” says Martens. “It’s a cover crop, but it’s also a cash crop, and it provides aesthetic benefits.”

Further cover crops also improve soil structure, reduce compaction and increase water infiltration. Besides the boost to soil health, cover crops aid in another critical area: water quality. By absorbing nutrients, cover crops reduce rates of leaching from the soil, protecting waterways from pollution. For instance, according to an Ohio State University study, Brandt’s farm had lower levels of nitrates and phosphates (or forms of nitrogen and phosphorus available to plants) leaching from the soil than a neighbor’s property that had been tilled and lacked cover crops.

One downside of cover crops is that they need to be killed before planting cash crops. If cold weather doesn’t do the job, farmers apply herbicides, till crops under, or cut and crimp the aboveground vegetation. That leaves hard choices for farmers — since herbicides damage the environment, tillage disturbs the soil and increases erosion, and the equipment used for cutting and crimping can be expensive.

Cover crops and climate

Cover crops are being examined for yet another benefit: climate change mitigation. “Anything that grows, if this is when the soil is covered with growing plants, it’s harvesting sunlight and it’s capturing carbon from the atmosphere,” says Martens.

Some of that captured carbon eventually ends up in the soil. Cover crops’ roots and shoots feed bacteria, fungi, earthworms and other soil organisms. These organisms then decompose and transform soil carbon into a stable form that remains in the soil. In addition, they eventually die, adding even more carbon to the soil. For instance, a study by David Clay, a South Dakota Corn Chair and Distinguished Professor at South Dakota State University, found that planting cover crops in all U.S. corn fields could uptake 107 million metric tons of carbon dioxide in the soil annually. That’s equivalent to the carbon sequestered by nearly 125 million acres of forest in a year.

Notwithstanding those promising results, other research showed no impact of cover crops on soil carbon. “They definitely take up carbon dioxide out of the atmosphere,” says Staver. “But the long-term fate depends on how the whole system is managed. So if you use a lot of tillage, most of the carbon, most of the organic matter that the cover crop adds to the soil is mineralized, which means microbes break it down and they release it.”

Ultimately, only a fraction of the carbon captured by cover crops remains in the soil. However, the timeframe also matters, explains Staver. For instance, a no-till system might accumulate soil carbon over five years, but the results could be more variable over a year or six months. In addition, cover crops have more of an impact on soil carbon with higher biomasses and when the soil has a lower amount of organic carbon initially.

Nonetheless, carbon dioxide isn’t the only greenhouse gas. “That cover crop will come up, and it starts changing the soil organisms that are in our soils, and that changes the gases that are being emitted from our soils,” says Clay. “And so what we’re seeing is very large reductions in nitrous oxide emissions from having that cover crop in that soil. And it’s largely driven by changes in soil organisms that are existing in our soils that are being driven by the cover crops.”

Nitrous oxide has a heat-trapping ability 265 times greater than carbon dioxide. Soils and perhaps plants naturally release nitrous oxide, while fertilizer and livestock are the largest anthropogenic sources of this gas. Clay’s research found that planting a cereal rye cover crop reduced nitrous oxide emissions by 66 percent in spring compared to bare soil.

On the other hand, some studies have found cover crops actually increase greenhouse gas emissions from fields. However, Clay points out studies start their measurements at different times of the year. “There’s other projects that have reported increases in nitrous oxide, but we haven’t seen it in our work,” Clay says. “Later on when the cover crop biomass is decomposing, you might see some increases. But when I consider what’s actually happening early in the spring, there’s a net loss.”

Other reasons for these mixed results include increased energy demand to cultivate cover crops and varying methods of calculating greenhouse gas emissions. “With all the mixes and the legumes and the grasses, it is a pretty broad subject,” says Staver. “Everybody likes simple answers, but you know, unfortunately, lots of things just aren’t simple. Lots of things depend on the conditions and the environment.”

Cover crop adoption

Regardless of the carbon pros and cons, the momentum behind cover crops isn’t stopping anytime soon. The U.S. Department of Agriculture (USDA) Partnerships for Climate-Smart Commodities recently funded $3.1 billion worth of projects to reduce greenhouse gas emissions and improve environmental resilience — many including cover crops. In addition, the U.S.D.A. Natural Resources Conservation Service launched a set of initiatives to promote cover crops and soil health.

However, this windfall of funding has fueled concerns. One is that cover crops are hogging resources that would otherwise go to practices with proven climate benefits. In addition, such funding programs should be structured to incentivize the long-term adoption of cover crops with low or no-tillage systems since this stores more carbon in the topsoil. Finally, support of cover crops for climate change mitigation should be targeted at conditions that will maximize their benefit.

Many of these programs may still pass muster since they improve the soil with relatively few downsides. And their adoption may be encouraged since they provide very real benefits for farmers and the environment — regardless of their carbon uptake.

While currently less than five percent of U.S. farm fields are planted with cover crops, the future looks promising. “Right now, what we’re doing is we have a project funded by USDA and they’re providing incentives for people to plant cover crops,” said Clay. “When given that choice, we’ve had more farmers who want to plant their cover crops than we have money to pay them.”

Cover crops provide a range of tangible benefits, from soil health to water quality. Under the right conditions and farm management, climate change mitigation can now be added to the list.