Cover crops may not be solution for both crop yield, carbon sequestration

(Press-News.org) ITHACA, N.Y. - People have assumed climate change solutions that sequester carbon from the air into soils will also benefit crop yields.

But a new study from Cornell University finds that most regenerative farming practices to build soil organic carbon – such as planting cover crops, leaving stems and leaves on the ground and not tilling – actually reduce yields in many situations.

The computer model analysis showed that global adoption of such practices to improve soil health can benefit either greenhouse gas mitigation or crop yields but rarely both.

The predictions will help farmers, policymakers and sustainability professionals mix and match optimal management plans based on location, as different practices will work better or worse depending on local conditions. For example, the model predicted that climate mitigation and improved yields had the best chance of occurring together when grains are planted, especially in soils with high clay content or that have limited nutrients.

“For the first time, we can have contextualized information about how farmers can choose the optimal mix of practices that meet their needs to maintain crop yields while also providing climate change mitigation,” said Dominic Woolf, senior research associate in the School of Integrative Plant Science, Soil and Crop Sciences Section, in the College of Agriculture and Life Sciences at Cornell University.

Woolf is principal investigator of the project and senior author of the study published May 19 in Nature Climate Change. Shelby McClelland, a postdoctoral researcher at New York University’s Department of Environmental Studies, formerly in Woolf’s lab at Cornell, is the paper’s first author.

For farmers, climate mitigation strategies include cover crops that are planted and left in place. Cover crops benefit farms by adding soil organic carbon (carbon from organic matter in soils), improving soil health, reducing soil erosion, cycling nutrients and converting nitrogen to forms usable by plants (when legumes are planted). They also offer off-farm benefits of protecting surface water quality and mitigating climate change, by pulling carbon from the air for growing steams, leaves and roots, and sequestering it from being released back into the atmosphere. Other practices, such as eliminating tillage, reduce erosion, limit soil carbon losses and disruption of soil structure.

The global computer model compared soil organic carbon changes, greenhouse gas release and yield outcomes of cropland climate mitigation practices with conventional cropland management. The researchers simulated a set of scenarios through the end of the century, including various combinations of four common management practices: planting grass cover crops, planting legume cover crops, zero-tillage, and leaving crop residues in fields.

The analysis showed that grass cover crops combined with no tilling led to the highest potential for limiting greenhouse gasses, but were the worst for crop yields. Legume cover crops with no tilling provided higher crop yields but close to 70% lower climate benefits. Reduced yields were found to be most likely in drier climates where cover crops compete for available water. Also, in some regions, these climate mitigation practices led to higher greenhouse gas emissions than conventional farming due to increased soil nitrous oxide, which is 273 times more potent as a greenhouse gas than CO2.

“We found a strong synergy in many locations between cover cropping and conducting no till,” McClelland said. “If you do both those practices together, in many cases, that allows you to increase soil organic carbon much faster than individual practices alone, which offsets negative effects from things like nitrous oxide emissions,” she said. Lowering nitrogen inputs into soil may also help address nitrous oxide emissions.

The authors found that in order to maintain crop yields to feed a growing global population, the maximum greenhouse gas mitigation through 2100 would be about 85% lower than if yields were not considered and farming practices centered around optimal climate mitigation strategies. “So tradeoffs have a massive impact in terms of what’s achievable at the global scale,” Woolf said.

Co-authors include  researchers from The Nature Conservancy, the Environmental Defense Fund,  Colorado State University, and the Woodwell Climate Research Center.

The research was funded by the National Institute of Food and Agriculture, a U.S. Forest Service contract through an interagency agreement with the U.S. Environmental Protection Agency, the Nature Conservancy, the Environmental Defense Fund, the Bezos Earth Fund, King Philanthropies, and Arcadia, a charitable fund.

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