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 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.
“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 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.
Other studies have shown benefits of cover cropping on the soil
microbial community, but most of them have been one-offs
influenced by specific site conditions, unique seasonal effects,
idiosyncratic management regimes, and the researchers’ chosen
analysis methods. Kim’s work is different in that he looked for
universal patterns among dozens of these one-off studies.
“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 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
reanalyzed data from 60 studies reporting on 13 soil microbial
parameters.
“That’s why the criteria of selection had to be so strict. We
wanted to compare studies that were solid, and with enough
replications that we could make valid claims about global
patterns,” Villamil says.
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 percent, and diversity
increased 2.5 percent.
“All the categories are important, but especially
diversity, because a diverse microbiome is more resilient.
Considering the close linkage between microbial diversity and
the provision of ecosystem services, small impacts could go a
long way to increase sustainability. In that sense, I think the
cover crops are really helping,” Kim says.
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The researchers were also able to tease out several
factors that layered on top of the main effect of cover crops. For
example, how did climate, cover crop termination method, or tillage
regime affect the ability of the cover crops to benefit the soil
microbial community?
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.”
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.”
Because their effects were indirect, these secondary factors need
more research before real claims can be made. Villamil’s research
team already has studies in the works to get more definitive
answers. But in the meantime, she’s heartened by the results of the
analysis as a whole.
“For me, it was surprising to see the consistent, positive effect of
cover crops – surprising but good. Finally! I’ve been researching
cover crops in our typical corn-soybean rotations in Illinois since
2001, yet in these high-fertility environments, it has proven
difficult to show any effects beyond cereal and annual rye capturing
nitrogen (weather permitting). Changes in chemical and physical
properties related to cover crop use are difficult to see,” Villamil
says. “But the microbiome, that’s where it’s at. That’s how
everything is related. Thanks to this work, I have something to look
forward to when I put in cover crops, and have generated many more
questions in need of research.”
The article, “Do cover crops benefit soil microbiome? A
meta-analysis of current research,” is published in Soil Biology and
Biochemistry [DOI: 10.1016/j.soilbio.2019.107701]. Authors include
Nakian Kim, Maria Zabaloy, Kaiyu Guan, and Maria Villamil. The
research was supported by USDA-NIFA. The Department of Crop Sciences
is in the College of Agricultural, Consumer and Environmental
Sciences at the University of Illinois.
[Sources: Nakian Kim/Maria Villamil
News writer: Lauren Quinn] |
