and urban areas. Their model revealed that although land use explained variations across the basin, rainfall was responsible for example, more fertilizer use as a Heavier rainfall due to climate variations from year to year. result of increased corn planting change will exacerbate the effects The authors also found that a is expected to raise the levels of of increased fertilizer use for 7-fold increase in nitrogen input nitrogen in the Gulf of Mexico, corn-based ethanol production, from humans caused river nitrowhich in turn fuels algal growth causing a significant increase in gen levels to increase 8-fold, but and creates oxygen-free dead nitrogen levels in rivers, according when the rainfall increases by zones, according to a study (Proc. to a new study in ES&T (DOI 7-fold, nitrogen levels double in Natl. Acad. Sci. U.S.A. 2008, DOI 10.1021/es801985x). The good the rivers. This suggests news is that if farmers that human impact is choose organic practices greater than that of cliand reduce fertilizer use, mate on the nitrogen levthe impact of heavy rains els, especially in will lessen and nitrogen agricultural areas. pollution levels might The researchers prodrop to below present-day jected future nitrogen levels, the study fluxes under different concludes. land-use and climate The study by research scenariossbusiness as fellow Haejin Han of the usual, increased depenUniversity of Michigan dence on organic farming, and colleagues is the first increased fertilizer use to simultaneously model from corn-based ethanol the changes in nitrogen production, a 5% increase runoff across spaces18 Excess nitrogen from agricultural fields fuels coastal dead in rainfall, and a 10% inwatersheds in the Lake Michigan basinsand over zones; here, red and orange indicate algal blooms that lead to crease in rainfall. The low-oxygen water along the Gulf of Mexico. analysis revealed that the time, the past 20 years. combined effect of 10% Using a model they devel10.1073/pnas.0708300105) by remore rainfall and more ethanol oped, the researchers then project searcher Simon Donner of the production would increase nitrofuture variations in nitrogen loadUniversity of British Columbia gen levels in rivers by 24%. But ing under different land-use and (Canada). more use of organic farming pracclimate scenarios. But rainfall and water discharge tices could slash nitrogen levels in The study reinforces an old also influence the amount of nirivers by 7%, even if rainfall inmessage for lawmakers: current trogen entering rivers, and many creased by 10%. decisions on land use and agriculplaces are expected to experience The study also suggests a nonture will have a strong impact on heavier rainfall due to climate linear relationship between nitrothe future availability of freshwachange. “If you want to control gen application by humans and ter and the health of our aquatic the amount of nitrogen leaving nitrogen fluxes in rivers, says and coastal ecosystems, says the watershed, you need to unecologist Robert Howarth of CorDavid Allan, one of the coauthors. derstand...the dominant control nell University. Although that idea It also reinforces the notion that factors,” says Donald Scavia, a is not newsHowarth himself protwo factors drive the future of nistudy coauthor. “Because you posed it in 2006sHan and coltrogen pollution, he notes: clihave a lot more control over, say, leagues have used a larger data mate change and how much land-use practices, than you do set, “which gives them more stanitrogen humans use on land. over the hydrology.” tistical power,” Howarth says. “If Scientists have known that Allan, Scavia, and Han used exthey are correct and the relationroughly 20-25% of the nitrogen isting data on nitrogen levels in ship is nonlinear, then the benapplied to land by humans evenrivers across 18 watersheds in the efits of reducing nitrogen inputs tually makes its way into rivers. Lake Michigan basin. They used [say, by decreasing fertilizer use] But that percentage changes denumbers from five time intervals are even greater than we and othpending on the type of land use, between 1974 and 1992. The land ers had earlier concluded,” because the more fertilizer-dearound some watersheds was Howarth adds. pendent the agriculture is, the mostly farmland, but other waterhigher the concentrations of ni—RHITU CHATTERJEE sheds included forests, wetlands, trogen entering water bodies. For NASA
Projecting the future of nitrogen pollution
10.1021/es9002179
2009 American Chemical Society
Published on Web 02/04/2009
March 15, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 1659
