Cover Crops Provide an Often-Overlooked Reduction in Soil Carbon Erosion, Study Finds

Planting ground cover in fields between cash crop growing seasons is an effective way to prevent farmland from losing soil carbon from erosion, a factor that’s underestimated in considering the carbon sequestration potential of cover crops, according to a new study by an Iowa State University ecologist.

Soil scientists have repeatedly shown that cover crops reduce erosion, but an analysis by a team led by Wenjuan Huang, an assistant professor of ecology, evolution and organismal biology, is the first study to synthesize data on carbon loss from erosion in individual cover crop field trials. The research, recently published in Global Change Biology, showed that across more than 150 experiments they studied, cover crops reduced the average annual loss of soil carbon through erosion by 68%. 

Understanding the impact that planting cover crops has on soil carbon erosion is important because one of cover crops’ expected benefits is a boost in soil carbon levels, which could help mitigate climate change by acting as a carbon sink for atmospheric carbon dioxide. But most research has studied the effects of the carbon added to soil via the roots and biomass of the overwintering plants, Huang said.

“We think a lot about how cover crops affect soil carbon levels. But we’ve often focused only on carbon inputs and have ignored outputs,” she said. 

Adding more organic matter to soil isn’t a straightforward solution for increasing its carbon stock, in part because increased carbon inputs can stimulate microbial activity that generates more carbon dioxide emissions. Another recent study co-authored by Huang found longer, more diverse rotations of crops fertilized with livestock manure didn’t affect soil carbon levels.

But cover crops tend to produce a modest though highly variable increase in soil carbon, prior research has shown. In the studies analyzed in the new paper, carbon was up 14% on average in the top six inches of soil. The analysis showed no relationship, however, between how much soil carbon erosion was reduced and the changes in soil carbon. That’s why carbon market incentives that pay farmers for planting cover crops should consider carbon erosion reduction as a benefit separate from changes in soil carbon levels driven by plant carbon input, Huang said. 

“It’s two independent processes, so we really should account for them both,” she said. 

Researchers also analyzed the studies to determine which factors were the biggest influence on soil carbon erosion and soil carbon changes. Topography – steeper slopes, especially – correlated strongest with cover crops’ ability to reduce soil carbon erosion.

Applying that data to a machine-learning generated map of global farmland, Huang’s team built a model that estimated that planting cover crops would reduce soil carbon erosion by an average of 25% worldwide. The estimate for the central U.S., including Iowa, was a 20% reduction. 

That’s a significant effect that would be even more pronounced on hilly farms and on the edges of fields, Huang said. Cover crops also have numerous other benefits, including improving water quality and the overall health and resiliency of soil, Huang said. 

“It’s a valuable conservation practice here in Iowa,” she said.