A 12% reduction in nitrogen fertilizer use in the 1980s and 1990s could have produced a 33% reduction in the nitrate flux in the Mississippi River to the Gulf of Mexico. That's according to a new correlation model developed at the University of Illinois (U of I) and reported in the November 8, 2001, issue of Nature.
"This is important because increased delivery of biologically available nitrogen has been linked to eutrophication and seasonal hypoxia in the northern Gulf of Mexico," says Gregory McIsaac, U of I assistant professor of environmental sciences and lead author of the study.
An hypoxic zone – popularly known as the "dead zone" – is an area low in oxygen where marine life may also be reduced.
"An earlier study estimated that a 24% reduction in fertilizer use would be needed to meet conservation goals, but our results indicate that increasing fertilizer use efficiency may have a greater impact than previously thought," says McIsaac.
Although the precise cause and effect relationship between fertilizer use and the hypoxic zone in the Gulf of Mexico is still uncertain, the fact remains nitrogen input to the Mississippi River Basin increased faster than the amount of nitrogen harvested in the crops in the 1960s and 1970s. And the nitrogen that is not taken up by plants becomes available to leach into groundwater and rivers.
As the difference between nitrogen inputs to and outputs from the land became larger, so have nitrate concentrations in the Lower Mississippi River. But the U of I model produced a surprising result.
"As net nitrogen inputs increased in the basin, a greater proportion appeared in the river as nitrate," explains Mark David, U of I professor of biogeochemistry and collaborator on the study. "It seems that when the capacity of the land to use or store nitrogen is exceeded, further nitrogen inputs can lead to large losses to streams."
"So our model suggests 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," McIsaac says.
Although there has been improvement in fertilizer use efficiency since 1988, he says, data collected as recently as last year shows that some Illinois corn growers can still reduce nitrogen use without reducing crop yields.
"In a survey conducted in 2000, about 30% 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 says.