A new global analysis by the University of Illinois shows cover crops planted after harvest can boost soil microbial abundance by 27%.

The result adds to cover crops’ reputation for nitrogen loss reduction, weed suppression, erosion control, and more.

“A lot of ecological services are done by the soil microbiome, including nutrient cycling. It’s really important to understand how it functions and how agriculture can form a healthier soil microbiome,” said Nakian Kim, doctoral student in the U of I Department of Crop Sciences and lead author on a new paper in Soil Biology and Biochemistry.

The research team performed a search of existing studies on cover crops, and wound up with about 985 scientific articles. Of these, they only kept studies that directly compared cover crops and bare fallow soils, and omitted studies conducted in greenhouses or that treated crop residues as cover crops. They also ensured that the studies were statistically sound, with reasonably large sample sizes. In the end, they mined and reanalyzed data from 60 studies reporting on 13 soil microbial parameters.

The research team divided the 13 microbial parameters into three categories: microbial abundance, activity and diversity. Microbial abundance wasn’t the only category to show a significant increase with cover cropping compared to bare fallow soils. Microbial activity was also up 22%, and diversity increased 2.5%.

“All the categories are important, but especially diversity, because a diverse microbiome is more resilient,” Kim said.

Kim said the use of burndown herbicides as a cover crop termination method had a strong moderating effect on the microbial community. “The benefits from the cover crops are diminished somehow from the herbicides. I think that’s one big takeaway,” he noted.

Tillage also made a difference. Kim expected conventional tillage to reduce the effect of cover crops on the soil microbes, but instead, conservation tillage did that.

“My guess is that because conservation tillage included not tilling at all, that allowed weeds to grow on the land. The weeds could have mimicked what the cover crops do,” said Kim. “So, the difference between the control treatment and the cover crop may decrease because of the weeds.”

Because their effects were indirect, these secondary factors need more research before real claims can be made. The research team already has studies in the works to get more definitive answers.