The impact of cover crops on soil organic matter, nutrient cycling

Steven Hall is dedicated to exploring how farmers can get the most out of their soil.

Hall, an assistant professor in the Department of Ecology, Evolution, and Organismal Biology at Iowa State University, runs a biogeochemistry lab where students examine the various factors that can affect soil health.

“Our group is focused on both basic research related to soil organic matter and nutrient cycling in a wide range of ecosystem types,” Hall said, adding that the group is also involved in “applications specific of these ideas to agricultural systems”.

He said the lab is made up of graduate students, postdoctoral researchers, professional scientists and undergraduate students.

One of Hall’s studies in November 2019 explored the impact of cover crops and grassland plants on microbial activity in soil.

He and visiting doctoral student Chenglong Ye from Nanjing Agricultural University in China published results that show that cover crops may not provide the high level of carbon sequestration benefits expected.

“Systems with cover crops or perennials generally increase the total amount of plant residue inputs compared to typical cereal cropping systems, but in some cases their presence can also increase organic matter decomposition rates. “Hall said.

“That is, residue management and nutrient availability can play a key role in controlling carbon gains and losses in these systems. Where nitrogen becomes limiting, microbes can actually “pull” it out of soil organic matter, helping to increase decomposition rates.

Hall stressed that this shouldn’t make cover crops any less attractive to farmers interested in the practice. He noted that microbial activity in these cover crops can provide benefits to soil fertility and reduce the loss of other nutrients.

The lab has three themes posted on its website, and Hall said the items they address are largely influenced by student and researcher interest. One of the themes — “what factors control the persistence and microbial transformations of soil organic matter?” – has been one of Hall’s interests since his thesis.

“We have a collaborative project with the Iowa Nutrient Research and Education Council that focuses on evaluating the environmental sustainability and agronomic impacts of a new microbial nitrogen fertilization technology produced by California-based startup Pivot Bio,” said Hall said.

Other themes focus on plant microbial interactions in the rhizosphere and the impact of nutrient cycling on greenhouse gas emissions.

He said these are complex topics, but lab researchers have found that there is a strong correlation between the rate of nitrogen fertilizer applied and the physical and biological conditions of the soil in controlling these emissions.

“All other things being equal, we would expect higher nitrous oxide emissions where nitrogen fertilizer rates are high and soils are moist and warm,” he said. “More importantly, as nitrogen fertilizer rates increase, nitrous oxide emissions increase exponentially.”

He suggested that to reduce nitrous oxide emissions, farmers could keep nitrogen levels as low as possible “but within the profitable range”.

Through research from the lab, Hall said there are still uncertainties about how tillage practices affect these issues, as well as how the form of nitrogen and cover crops can affect the nitrous oxide.

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