Water quality in areas of intensive farming has been carefully studied in Iowa. In the past few years as corn acres increased, water quality models predicted that nitrate levels in field runoff would increase, too. However, that hasn’t been the case. Research results from the Iowa Institute of Hydraulic Research published in the Journal of Soil and Water Conservation show that nitrate discharges have decreased even as corn acres increased.
Data from more than 7,000 water samples collected over 15 years in the Raccoon River watershed of Central Iowa matched with fertilizer application data from 700 fields in the watershed led researchers to believe that nitrate levels are less dependent on corn production acres than previously thought.
According to the IIHR, as more acres were planted in corn (and fewer in soybeans), fertilizer application increased some 24 percent in the watershed. Interestingly, river nitrate did not increase and may have even decreased slightly at most watershed locations.
“One might conclude from these data that fertilizer use efficiency improved,” said IHRR research Chris Jones. “But we believe that was not the case. The amount of nitrogen leaving the watershed in the harvested grain actually declined a little bit during our study.”
Jones added that plant biology and chemical reactions in the soil are probably at play.
Nitrate-nitrogen can accumulate and be immobilized in the soil under corn. On the other hand, dead and decomposing soybean plants can increase the amount of nitrate in the soil vulnerable to loss (more so than cornstalks), especially if accompanied by fall tillage. Also, there is evidence that tile discharge may increase under soybean fields as a result of reduced plant evapotranspiration compared to corn. Therefore, because tile nitrate concentrations are similar under both corn and soybeans, more tile flow under soybeans can mean more nitrate delivered to streams. As a result, Jones says he and his colleagues believe that declining soybean acres may have reduced the cropped areas most vulnerable to nitrate loss, more than compensating for the increased fertilizer inputs on corn acreage.
Further research by IIHR scientists shows that Raccoon River nitrate is dependent upon the previous year’s soybean area.
“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 says. “We know we can’t just focus on fertilization of corn. We need a systems approach to improve water quality. It also demonstrates the power of monitoring water quality. Without this data, we could easily have missed this important and counterintuitive conclusion.”