Cover crops struggle to overcome conventional soil management
Cover crops can help farmers build healthier soil, but they may not work well on fields where farmers have continuously grown corn for decades and applied large amounts of nitrogen fertilizers, according to two new studies.
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“In the Midwest, our soils are healthy and resilient, but we shouldn’t overestimate them. A soil under unsustainable practices for too long might reach an irreversible threshold,” said Nakian Kim, a doctoral graduate student in the University of Illinois’s Department of Crop Sciences who led the studies.
For the experiment, Kim used a university field that has had 40 years of continuous corn planted under different nitrogen fertilizer rates. In 2018, cover crops were added to the field, and, in 2020, Kim took samples to analyze for potential changes to the soil microbial community, comparing those with earlier samples when no cover crops were on the field.
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He found in one study that after two years of cover crops, the soil had more microbial diversity but still favored microbes that could increase the risk of nitrous oxide emissions — a powerful greenhouse gas. And, although research has shown that cover crops can reduce soil nitrate leaching and nitrous oxide emissions, Kim did not find that effect in his second study. Instead, his results showed that decades of applying nitrogen fertilizer caused soil acidification and an overabundance of soil nutrients that, in turn, disrupted the soil microbes’ ability to fix nitrogen.
“If a system is exposed to disruption of nitrogen-cycling microbial communities long enough,” he said, “it may develop resistance to conservation practices.”
His findings on the limits of cover crops echo the opinions of soil health experts in a recent Environmental Working Group webinar. During the event, scientists said research shows cover crops do not sequester carbon well because their roots stimulate microbial activity and led to carbon cycling: the microbes eat the carbon provided by the plants and then breathe out carbon dioxide, emitting the greenhouse gas back into the atmosphere.