A new global analysis from the University of Illinois shows cover crops can boost soil microbial abundance by 27%, reports Science Daily.
Soil microbial abundance important metric
The result adds to cover crops’ reputation for nitrogen loss reduction, weed suppression, erosion control, and more. Although soil microbial abundance is less easily observed, it is a hugely important metric in estimating soil health, according to Nakian Kim, doctoral student in the Department of Crop Sciences at the University of Illinois and lead author on a new paper in Soil Biology and Biochemistry.
“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,” says Kim.
985 scientific articles
The research team performed a search of the existing studies on cover crops, and wound up with some 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 reanalysed data from 60 studies reporting on 13 soil microbial parameters.
Consistent 27% increase in microbial abundance
“Our analysis shows that across 60 field studies, there was a consistent 27% increase in microbial abundance in fields with cover crops versus no cover crops. It’s across all these studies from around the world,” says Maria Villamil, associate professor in crop sciences and co-author on the paper.
The research team divided the 13 microbial parameters into 3 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%.
Burndown herbicides
Kim says the use of burndown herbicides as a cover crop termination method had a strong moderating effect on the microbial community. “The results were very interesting. With chemical termination, the effect sizes were consistently smaller compared to mechanical termination. In other words, the benefits from the cover crops are diminished somehow from the herbicides. I think that’s one big takeaway.”
Conservation tillage reduced effect of cover crops
Tillage also made a difference, according to Kim. He 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. So the difference between the control treatment and the cover crop may decrease because of the weeds.”
Cover crops increase soil health in semi-arid region of New Mexico
In another new study, researchers from New Mexico State University and the United States Department of Agriculture show that cover crops can increase soil health in a semi-arid region of New Mexico.
To determine soil health, the researchers measured soil carbon dioxide emissions. These emissions were higher in test plots with cover crops compared to fallow plots. Carbon dioxide is released from soils during plant root and soil microbial respiration. “The higher the biological activity is in soils, the greater the carbon dioxide emissions,” says Rajan Ghimire, researcher at New Mexico State University.
The plots were located in Clovis, New Mexico – about 200 miles east of Albuquerque. Ghimire and colleagues tested a variety of cover crops over 2 growing seasons. They also tested combinations of cover crops, such as growing peas and oats together.
Plots with peas alone, and a combination of peas and canola, showed the highest soil carbon dioxide emissions during one of the study years. However, the emissions trend was not consistent in the second year, making results difficult to interpret.
Unchecked soil carbon dioxide emissions can be a problem, because carbon dioxide is a greenhouse gas. “Soil carbon dioxide release needs to be balanced with soil carbon storage,” says Ghimire. Cover crops increase soil carbon storage in two ways. First, their root and aboveground biomass are largely made of carbon, which will eventually decompose into soil organic matter. They also provide housing and food for the soil microbes. These microbes, especially fungi, are associated with even more carbon storage.
Microbes living in the roots of legumes can fix atmospheric nitrogen to make it available to crops. However, this activity can also increase soil carbon dioxide emissions when legumes are grown as cover crops. Grassy cover crops, such as oat and barley, contribute well to soil carbon accumulation without the extra emissions from fixing nitrogen. But that means plants will need to get nitrogen elsewhere, and these grasses also tend to need more water than legumes.
“Finding a balance is key,” says Ghimire. “Mixing grasses with legumes may help increase soil carbon and nitrogen while minimising carbon dioxide release.” The researchers plan to continue this experiment as a long-term study. “Cover crops are a great way to sequester carbon, reduce global warming and increase agricultural resilience,” says Ghimire. “But there is still a lot to learn about cover cropping, especially in semi-arid environments.”
