Soybeans may play bigger role in nitrate levels than previously thought


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IIHR’s water-quality monitoring network has generated interesting data that contradicts some widely-held beliefs regarding corn and soybeans and their impact on nitrate in Iowa’s streams. (IIHR)
Jake Slobe | September 28, 2016

New research shows that soybeans may play a key role in the transport of nitrate from farmed fields to the stream network.

As Iowa farmers have planted more acres of corn to meet the increasing demand, many models predicted that nitrate concentrations in Iowa streams would increase as a consequence. However, a new study conducted by the UI’s IIHR-Hydroscience & Engineering group and the Iowa Soybean Association, published in the Journal of Soil and Water Conservation, challenges many of these predictions.

As the amount of corn planted increased and the amount of soybeans decreased, fertilizer application increased by 24 percent in the watershed. Surprisingly, the nitrate levels in the river saw no increase and in some cases saw slight decreases.

The study evaluated 7,000 water samples in the Raccoon River Watershed from 1999- 2014 and had access to fertilization data for 700 fields in the watershed. The result from the study has led researchers to believe that nitrate levels are less dependent on corn production than previously thought.

IIHR —Hydroscience & Engineering researcher Chris Jones says that clues to the reduction in nitrate levels can be found in the differences between corn and soybean  growth, soil chemistry, and the decay of other crop residues. Conversely, the dead and decomposing soybean plants can increase the amount of nitrate in the soil vulnerable to loss.

“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 counter-intuitive conclusion.” 

As a result, Jones says he believes that declining amount of soybeans planted may have reduced the cropped areas most susceptible to nitrate loss, more than compensating for the increased fertilizer inputs on corn production.

 

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