Farm-derived nutrients in the Mississippi River that create a huge dead zone in the Gulf of Mexico could be substantially reduced if farmers simply used a little less fertilizer, a new analysis suggests.
When fertilizer-fed algae blooms die and decompose, they sop up much of the oxygen available in the water and create a zone that often has too little dissolved oxygen to support most marine animals. The Gulf of Mexico’s so-called hypoxic zone is an 18,000-square-kilometer region of ocean that stretches westward from the mouth of the Mississippi toward the Texas coast.
The concentration of nitrates appearing in river water at St. Francisville, La., more than doubled between 1960 and 1998, says Gregory F. McIsaac, an environmental scientist at the University of Illinois at Urbana-Champaign. He and his colleagues have analyzed year-to-year trends in river nutrients sampled at St. Francisville, about 50 km upstream of Baton Rouge. The researchers also examined contributing factors such as fertilizer use, automobile and industrial emissions of nitrogen oxides, and lightning and other natural sources of nitrates. The team presents its findings in the Nov. 8 Nature.
In the 1990s, a 1-hectare plot of cropland in the Mississippi River watershed received, on average, about 3 kilograms of nitrate each year from industrial emissions and natural sources. That same plot–about 1.5 times the area of a soccer field–gained about 20 kg from fertilizer, says McIsaac. Much of this nitrate ends up in crops, but about one-fourth of the excess leaches into rivers and is transported to coastal waters. The rest is stored in soil or groundwater or is converted to nitrogen and returned to the atmosphere.
Nitrogen-bearing chemicals are the number-one pollution problem in U.S. coastal waters, says Robert W. Howarth, a biogeochemist at the Marine Biological Laboratory at Woods Hole, Mass. About a third of these coastal areas have been severely degraded by these chemicals, and another third, moderately affected. The Gulf of Mexico hypoxic zone is a “poster child” for this problem, he notes.
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McIsaac’s model suggests that if farmers now begin to trim their total use of nitrogen-bearing fertilizers by 12 percent–a change that wouldn’t necessarily reduce crop yields, the researchers contend–the amount of nitrates transported to the Gulf would drop by 33 percent by 2010.
To reach this goal, farmers could apply fertilizer more precisely in the spring and not use the chemicals after harvest in the autumn, says Otto C. Doering III, an agricultural economist at Purdue University in West Lafayette, Ind.
Farmers typically administer about 30 percent more fertilizer than necessary, says Howarth, adding that they view this overapplication as cheap insurance against poor crop yields. Howarth says that the new analysis by McIsaac and his colleagues shows that farmers could rapidly decrease nitrate concentrations in the river without sacrificing crop yields.