While research on soil microbial health has mainly been done in a lab, greenhouse or in short-term field experiments, University of Illinois crop scientists have captured valuable crop rotation and tillage data from a 20-year field experiment.

This long-term field data demonstrates significant risks associated with continuous corn rotations, both from the standpoint of the soil microbial community and for environmental health on a larger scale.

Nitrogen Loss Concerns

“In order to maintain yield levels under continuous corn, more inorganic nitrogen is required, thereby intensifying the nitrogen cycle and causing a dangerous loop,” adds Gevan Behnke, a crop science researcher. “The result is acidification and potential increases in nitrogen loss and harmful nitrous oxide emissions.”

On the other hand, the researchers found rotating corn with soybeans for a year or more tempers the effect.

Some fields were in continuous corn, others in continuous soybeans and still others rotating annually between the crops. Within each system, fields were either no-tilled or chisel-plowed .

The process of microbial nitrification turns ammonia fertilizer into nitrate, a much more mobile form associated with runoff and leaching. On the other hand, denitrification turns nitrate into gaseous forms, including nitric and nitrous oxide.

The researchers found continuous corn increased soil organic matter and acidity. It also led to an increase in soil microbes involved with both nitrification and denitrification and demonstrated the role of soil microbes in emitting more greenhouse gases.

However, the researchers found the opposite was true with continuous soybeans. There were intermediate results with the corn and soybean rotation.

No-Till Benefits

No-till increased soil organic matter, which led to more acidic soils and an increase in the abundance of bacteria that can turn ammonia into nitrate.

“Nitrogen is left on the soil surface in no-till fields, whereas tillage can really spread out that nitrogen throughout the top 10 centimeters,” says Behnke. “However, tillage by itself wasn’t as strong a driver of microbial abundance as crop rotation. Ultimately, reducing the number of corn years will help decrease the negative effects of continuous corn on soil health.”