Agriculture

Despite more fertilizer, nitrate levels not up in Raccoon River

Experts find concentrations have actually declined

Corn and soybeans, growing side by side in this Iowa field, tend to lose nitrates to surface water at different rates, according to a new study by seven Iowa scientists. The study reached the surprising conclusion that soybean fields in the Raccoon River watershed are more susceptible than cornfields to nitrate loss. (IIHR — Hydroscience & Engineering photo)
Corn and soybeans, growing side by side in this Iowa field, tend to lose nitrates to surface water at different rates, according to a new study by seven Iowa scientists. The study reached the surprising conclusion that soybean fields in the Raccoon River watershed are more susceptible than cornfields to nitrate loss. (IIHR — Hydroscience & Engineering photo)

A substantial expansion of corn acreage in the Raccoon River watershed during the recent ethanol boom has not — despite experts’ predictions to the contrary — increased nitrate levels in the nutrient-plagued river, according to recently published research by seven Iowa scientists.

Although corn acreage increased 19 percent and nitrogen fertilizer inputs increased 24 percent in the watershed since 1999, the researchers found that nitrate concentrations in the Raccoon River actually had declined slightly in the years since.

“It’s a counterintuitive conclusion. You would think with more nitrogen fertilizer being applied, more nitrates would flow into the river,” said Christopher Jones, a research engineer at the University of Iowa, IIHR-Hydroscience and Engineering, and lead author of the report published in the Journal of Soil and Water Conservation.

The study results “contradict what many people consider to be the problem and the solution,” said Roger Wolf, director of environmental programs and services for the Iowa Soybean Association, a key participant in the research.

Those results indicate that soybean fields in the Raccoon River watershed are more susceptible than cornfields to nitrate loss.

Both Jones and Wolf said the advantages of rotating crops, which include less reliance on chemicals to control weeds and insects, far outweigh the reduced nutrient losses associated with continuous corn.

“What we are really saying is that we need to look at ways we can initiate soybean cultivation practices that reduce nitrate losses,” Jones said.

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Both Jones and Wolf said nutrient losses could be reduced substantially by eschewing fall tillage of soybean fields and planting cover crops on them immediately after harvest.

Jones said he and his colleagues believe that declining soybean acres resulting from increased corn plantings may have reduced the cropped areas most vulnerable to nitrate loss, more than compensating for the increased fertilizer inputs on corn acreage.

Farmers typically apply from 150 to 200 pounds of nitrogen fertilizer per acre of corn ground, while they apply little if any to fields intended for soybeans. Since 1980, about 80 percent of the watershed’s 3,625 square miles above Des Moines has been planted to corn and soybeans.

Jones said differences between corn and soybean plant growth, soil chemistry and the decay of stalks and other crop residues likely explain why soybean fields tend to lose more nitrates than cornfields.

Dead and decomposing soybean plants, when compared with cornstalks, can increase the amount of nitrate in the soil vulnerable to loss, especially if accompanied by fall tillage, he said.

Jones said the soil in the Des Moines lobe region of Northern Iowa, much of which lies within the Raccoon River watershed, contains exceptionally high concentrations of intrinsic organic nitrogen that is susceptible to leeching when in contact with water.

Because corn has considerable more biomass than soybeans and a longer growing season, it uses more soil moisture than soybeans, which translates to more water flowing through underground tile drainage systems beneath soybeans than beneath corn, he said.

“Because tile nitrate concentrations are similar under both corn and soybeans, more tile flow under soybeans can mean more nitrate delivered to streams,” he said.

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The scientists examined nitrate concentration and loading data from more than 60 monitoring sites in the Raccoon watershed from 1999 to 2014. Their data were derived from nearly 7,000 samples collected by the Des Moines Water Works, the Iowa Soybean Association and the Agriculture’s Clean Water Alliance.

Wolf said the research is based on “an unprecedented data set” that includes not only extensive water sampling but also climate and river flow data, as well as crop rotations and fertilization rates.

“The big take-home for me,” he said, is that without integrated data on a large scale, “you could chase the wrong solution for a long time and wonder where the results are.”

The study results, he said, also underscore that in a naturally nutrient rich system with variable climate — a system like the Raccoon River watershed — rainfall influences nutrient losses much more than fertilizer applications.

The Raccoon, a principal source of drinking water consumed by 500,000 central Iowans, is considered impaired for use as drinking water. The Des Moines Water Works since 1992 has operated a nitrate removal plant to ensure compliance with the Environmental Protection Agency’s minimum nitrate standard of 10 parts per million.

As of last week, all 10 of the Iowa Water Quality Information System’s 10 real-time gauges on the Raccoon and Des Moines rivers above Des Moines were recording nitrate concentrations above the EPA’s minimum standard, and nitrate concentrations were above 15 parts per million at four of them.

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