These days, you don't have to look hard to read or hear about the benefits of cover crops. Many of these benefits have been measured anecdotally. However, more recently, research by growers and universities has aimed to put some numbers behind these touted benefits.
Over the past two years, Katja Koehler-Cole, research assistant professor in the Department of Agronomy and Horticulture at the University of Nebraska-Lincoln, has researched the effect of cereal rye cover crops on soil microbial communities and soil nutrients after corn and before soybeans.
"The period where we actually lose the most soil nitrate in a corn-soybean rotation is the period before planting soybeans," Koehler-Cole says. "Because the soil warms up, microbes decompose organic matter, and there are no plants there to take up the nitrogen that's released. So, there is potentially quite a bit of nitrogen lost."
Of course, there's also the risk of loss of soil structure and organic matter because of erosion when there's nothing growing in the field, and cover crops are one of the most obvious ways to mitigate this.
However, cover crops also are a food source and a habitat for soil microbes.
"Their root exudates are simple sugars, amino acids, and they're preferred food for microbes," Koehler-Cole says. "The decomposing cover crop residue also provides carbon and nitrogen to microbes.
"Roots are the favored habitat for microbes. Most microbes live in the root zone — either near or on roots, so having living roots in the ground increases habitat for microbes. Cover crops can help influence soil surface microclimates, for example, by lowering the temperature, preventing temperature extremes, preventing wind erosion or slowing evaporation."
Search for microbial life
Koehler-Cole explains that soil microbes are responsible for nutrient retention, exchange and cycling. The biggest group of decomposers in ag fields is bacteria, which preferentially decompose simple organic compounds that are easy to break down, such as freshly terminated cover crops. Fungi decompose organic compounds that are tougher, like cornstalks or roots.
Koehler-Cole notes that while fungi are less numerous than bacteria, they bring big benefits to the soil.
"[Fungi] have threadlike growth, called hyphae, and they also release a gluelike substance called glomalin," she says. "Those two together help combine little soil particles and make those nice aggregates. Fungi improve aggregate size, they improve aggregate stability, and that makes the soil much more resilient to erosion."
The two fungi groups of interest are saprophytic fungi, which decompose cellulose and lignin, and arbuscular mycorrhizal fungi, which colonize roots and increase crop nutrient uptake.
"They can increase nutrient uptake because they can reach nutrient sources that are otherwise not accessible to plant roots," Koehler-Cole says. "When you have a cover crop growing over the winter, it can be a host to AMF. It can live on the cover crop roots, and when the cover crop is terminated, and your soybean or corn is planted, AMF can actually spread from the cover crop roots to the crop roots, colonize the crop roots, and have benefits for the crops there as well."
Putting rye to the test
Koehler-Cole compared cereal rye and a control treatment at two sites in Arlington and Shelby, Neb. Rye was planted in mid-November 2019, and terminated at soybean planting in early- to mid-May. Soil was tested for nitrogen, phosphorus, potassium and organic carbon. Koehler-Cole also conducted a phospholipid fatty acid analysis to determine which microbial groups are in the soil.
This year, Koehler-Cole explains, the total biomass produced was relatively low — with 1,082 pounds per acre at the Arlington site, and 1,310 pounds per acre at the Shelby site.
"These cover crops took up between 25 and 40 pounds of nitrogen per acre, and had a carbon-to-nitrogen ratio between 19 and 14," she says. "Anytime the ratio is below 25-to-1, we expect it to decompose pretty quickly. With the ratios we have here, we think that cover crop will decompose relatively fast, which means it also releases the nitrogen it took up back to the crop."
Overall, the cereal rye treatment saw a significant reduction in soil nitrate at both sites. However, there was no influence on phosphorus, potassium or organic carbon.
"With organic carbon, we expected that a cereal rye cover crop would not influence it within the short amount of time that we've grown it," Koehler-Cole says. "You really need four, five years or more to really see differences in organic carbon."
They also tested for microbial biomass and diversity with the phospholipid fatty acid analysis. While there were no differences between treatments this year, Koehler-Cole says in 2019, the cereal rye treatment did result in an increase in microbial population.
"I looked at my test results again, and actually realized that the microbial biomass at our sites, even in the controlled treatment, was already pretty high, or at least average," she says. "Organic carbon in these sites was already relatively high already. These sites had relatively good soil health, and that's probably one reason why we didn't see more treatment differences."
In 2019, Koehler-Cole tested rye’s effect on soil microbial communities at three sites in May, just before cover crops were terminated, and again in July. In May, soil microbial abundance was greater under cover crops, likely because of the food source provided by cover crop root exudates.
Soil bacterial biomass was significantly increased; however, fungi biomass did not increase. Bacteria reproduce quickly, so when living conditions improve — like with a cover crop — their numbers go up, while fungi are slower to respond.
Room for growth
How can growers increase soil microbial populations? Koehler-Cole notes that her research has, so far, focused on cereal rye, and using diverse cover crop mixes may be one way to increase microbial diversity. In addition, increasing cover crop biomass by planting earlier or terminating later provides microbial populations a food source and a habitat for a longer period of time.
"We think we could use the right cover crop to really reduce soil nitrate levels, which could help reduce soil nitrate contamination, and may reduce leaching," she says. "At least this year, we did not see any improvements in soil microbial abundance. We have seen it in the past, but this year, we did not have very high cover crop biomass. Increasing cover crop biomass, and increasing the amount of plant species we're growing, may lead to greater benefits for soil microbes."