The study, appearing in the Nov. 8 issue of the journal Nature, concluded that had there been a 12 percent reduction in nitrogen fertilizer use in the last two decades, there could have been a 33 percent reduction in the nitrate flux in the Mississippi River and Gulf of Mexico. In the gulf, excess nitrogen has been suspected as a major contributor of seasonal dead zones where oxygen levels are depleted and marine life is reduced.
"An earlier study estimated that a 24 percent reduction in fertilizer use would be needed to achieve the same level of reduction of nitrate movement to the gulf, but our results indicate that increasing the efficiency of fertilizer use may have a greater impact than previously thought," said Gregory McIsaac, a professor of environmental sciences at the University of Illinois and lead author of the Nature article. Although the precise cause-and-effect relationship between fertilizer use and the hypoxic zones is still uncertain, McIsaac said, the fact remains that nitrogen going into to the Mississippi River Basin increased faster than the amount of nitrogen harvested in crops in the 1960s and 1970s. Nitrogen that is not taken up by plants becomes available to leach into groundwater and rivers.
As the difference between nitrogen inputs and outputs onto the land became larger, so have nitrate concentrations in the lower Mississippi River. But the UI model produced a surprising result.
"As net nitrogen inputs increased in the basin, the percentage that appeared in the river as nitrate also increased," said study collaborator Mark David, a UI professor of biogeochemistry. "It seems that when the capacity of the land to use or store
Contact: Jim Barlow
University of Illinois at Urbana-Champaign