Just a tad less fertilizer on Midwestern farms could lead to big drops in nitrogen pollution in the Mississippi River without sacrificing crop yields, a new study shows. Experts say such cuts could shrink the so-called dead zone in the Gulf of Mexico, which becomes inhospitable to wildlife almost every summer.
Crops such as corn and soybeans flourish when nitrogen-containing fertilizer is applied to Midwestern fields, but many farmers routinely apply more fertilizer than their crops can take up. That excess nitrogen, in the form of nitrate, leaches from the soil into streams, wending its way to the Mississippi River and the Gulf of Mexico. Most years, it fuels algal blooms that suck up oxygen from a zone of coastal water the size of New Jersey, killing the creatures that can't swim away. But it's been hard to measure exactly how much of the nitrate comes from fertilizer, rather than other sources such as dead plants and soil.
To account for all the nitrogen entering and leaving the Mississippi River, environmental scientist Gregory McIsaac of the University of Illinois, Urbana-Champaign, and his colleagues combined and adapted two earlier mathematical models. They fed the new model almost 4 decades of data from 20 state agriculture departments and other sources on fertilizer use, animal waste, natural nitrogen fixation by crops, crops harvested, and atmospheric fallout from fossil-fuel combustion. The model indicated that a 12% cut in fertilizer, which wouldn't diminish yields, could slash nitrate in the lower Mississippi River by 33%, McIsaac's team reports in the 8 November issue of Nature. An earlier prediction had argued for a 24% cut in fertilizer.
Although the model makes assumptions about the environmental fate of nitrogen that risk oversimplifying matters, says oceanographer Don Boesch of the University of Maryland Center for Environmental Science in Cambridge, Maryland, he calls the conclusions "robust." That's because experimental studies of field plots and smaller watersheds show that these assumptions are valid, he says. Boesch adds that this model could reassure farmers that fertilizer cuts that reduce water pollution don't have to harm their crops.