If the majority of farmers in a particular watershed planted cover crops, would it impact water quality?

“It’s possible,” says Shalamar Armstrong, assistant professor of agronomy, soil conservation and management at Purdue University, as he interprets data from his collaborative cover crop watershed research project with Catherine O’Reilly, hydrogeologist, Illinois State University. Their recent research reveals cover crops have the potential to reduce nitrate levels on a watershed scale. A watershed scale study, Armstrong says, consists of multiple farms, management systems, landscape position, soil types, and several hundred acres.

BIOMASS LEADER: Armstrong says cereal rye, a “winter hardy” cover crop terminated in the spring, together with daikon radish, produces a much higher biomass. In the spring, this cover crop combination “resulted in a greater percentage of soil ammonium relative to nitrates compared to the daikon radish and oats mixture,” Armstrong notes.

Armstrong explains the study, jointly funded by the Illinois Nutrient Research & Education Council and Illinois Environmental Protection Agency (EPA), compared two Lake Bloomington watersheds. Farmers aerially applied cover crops, cereal rye and radishes in corn and radishes and oats in soybeans, to almost 50% of the treatment watershed acres while the control was left without cover crops.

The cover-crop-treated watershed showed a positive impact on water quality from January to May 2016.

Armstrong’s team tracked nitrate levels from tile lines in both the treated and untreated watersheds. Armstrong notes the cover crop watershed kept nitrate levels from spiking as high as the untreated watershed during the spring thaw and rain events.

“That’s showing potential in the first year to reduce nitrate concentrations in tiles below the EPA standard, more often compared to no cover crops in the control watershed,” Armstrong says.

Crunching the numbers

Armstrong says his research team gathered biomass samples to calculate cover crop nitrogen uptake and soil samples at different depths to determine spring ammonium and nitrate levels. They also pulled and tracked tile nitrate samples from both watersheds.

Armstrong notes cover crops, and the species of cover crops, impact nitrate and ammonium levels in the spring. “A cover crop that winter terminates results in a greater percentage of soil nitrates relative to ammonium due to greater time for residue to decompose,” says Armstrong. “A winter hardy cover crop that is terminated in the spring results in a greater percentage of soil ammonium relative to nitrates, due to less decomposition time.” The research group also found fall cover crop growth was greater in soybean residue relative to corn residue, due to less shading.

“It may not work this beautifully every year,” Armstrong notes. “This was a great year for cover crops with timely rainfall, high growing degree days in the fall and spring, and a warm and wet winter.”

Armstrong says overall, cover crops show “great potential” for protecting water quality on a watershed scale.

BELOW THE REDLINE: Armstrong says the treatment watershed (left), with cover crops in nearly 50% of the acres, kept nitrate levels from spiking above Environmental Protection Agency’s standard more often than the control watershed.