The delivery of nitrates in rivers and streams in the Raccoon River watershed stayed steady — or actually decreased in some areas — despite an increase in corn acreage during the 15-year period from 1999 to 2014, according to a new study published in late May by the University of Iowa (UI).
The study’s results surprised researchers. They predicted that nitrate levels in surface water would rise because more farmers in the watershed moved to a corn-on-corn planting pattern, which required more fertilizer applications than soybeans.
The study shows that nitrate levels in surface water are less dependent on corn production than previously thought, the researchers said.
In the study, conducted by the UI’s IIHR-Hydroscience & Engineering group and the Iowa Soybean Association and published in the Journal of Soil Water Conservation, researchers evaluated 7,000 water samples in the Raccoon River Watershed. The researchers also had access to fertilization data for 700 fields in the watershed.
As more acres were planted to corn, fertilizer applications increased 24 percent in the watershed. But river nitrate did not increase and even decreased slightly at most watershed locations.
The UI’s Chris Jones, lead author of the study, said clues to the reduction in nitrate levels can be found in the differences between corn and soybean plant growth, soil chemistry and the decay of stalks and other crop residues.
Nitrate-nitrogen can accumulate and be immobilized in the soil under corn stalks, he said.
"Understanding this process could prove important as we try to reduce the loss of nutrients to Iowa streams as part of the Iowa Nutrient Reduction Strategy," Jones said. "We know we can’t just focus on fertilization of corn. We need a systems approach to improve water quality."
The research team encouraged farmers to take a holistic management approach to improve water quality, including crop rotation, cover crops, minimal or no tillage, nutrient management as well as other practices outlined in the state’s water quality initiative, known as the Iowa Nutrient Reduction Strategy.
According to the Iowa Nutrient Reduction Strategy, cover crops can reduce nitrate loss by 31 percent on average and phosphorus loss by 29 percent on average.