"So our model," McIsaac said, "also suggests the reverse that a relatively modest reduction in nitrogen fertilizer use, while maintaining crop yields, could substantially reduce the amount of nitrate found in the Mississippi River."
Although there has been improvement in efficiency since 1988, he said, data collected as recently as last year show that some Illinois corn growers could still reduce nitrogen use without reducing their crop yields.
"In a survey conducted in 2000, about 30 percent of Illinois farmers indicated that they apply more nitrogen than is recommended for economically optimum crop production. Eliminating that over-application will maintain yields, reduce costs and, according to our analysis, reduce the nitrate in the Mississippi River," McIsaac said.
The UI model accounts for 95 percent of the annual variation in nitrate-nitrogen delivered to the Gulf of Mexico as measured by the U.S. Geological Survey. It also incorporates the states level crop production statistics and estimates of nitrogen input from the atmosphere in rainfall. Most of the data were compiled under the leadership of Donald Goolsby, a scientist with the U.S. Geological Survey, another collaborator.
"We conducted a very comprehensive uncertainty assessment of the model and its inputs," said George Gertner, UI professor of biometrics and co-author. "The empirical model proved to be very robust."
"A main difference between our model and some others is that we assumed a steady state of soil organic nitrogen," David said. "In other words, we assumed that there has not been a large change in soil organic nitrogen over the years.
The assumption is supported by long-term soil monitoring data from the UI Morrow Plots and the Sanborn Field in Missouri. That data produced a more accurate correlation between net nitrogen inputs to the watershed and river nitrogen
Contact: Jim Barlow
University of Illinois at Urbana-Champaign