More Efficient Fertilizer Use Reduces Nitrate Loads
Published: Nov. 7, 2001
November 7, 2001
Urbana - A 12 percent reduction in nitrogen fertilizer use in the 1980s and 1990s could have produced a 33 percent 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 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," said 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," said 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," explained 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 said.
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 said.
The U of I 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 utilizes state 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, who led one of six scientific studies of hypoxia that were published in 1999.
"We conducted a very comprehensive uncertainty assessment of the model and its inputs," said George Gertner, U of I professor of biometrics and another collaborator on the study. "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. In other words, we assumed that there has not been a large change in soil organic nitrogen over the years," said David.
This assumption is supported by data from the U of I Morrow Plots and the Sanborn Plots in Missouri, and it produced a better correlation between net nitrogen inputs to the watershed and river nitrogen. However, correlation studies do not prove cause and effect, and more studies are needed.
"Even with the uncertainty of correlations, it still makes sense to continue to improve efficiency of fertilizer use by following university recommended practices, such as fertilizing for five year average corn yields, not maximum yield and counting all forms of applied nitrogen such as manure and credit from soybeans and other legume crops," David said.
The full effect of reducing nitrogen fertilizer application to U of I recommended levels will depend on weather conditions and could take up to nine years to be observed.
"So, our model provides some insights, but we need to continue to refine our understanding of nitrogen processes in soils and rivers by monitoring to see what happens to in response to changes in fertilizer use and weather," McIsaac said.
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