Projecting the future of nitrogen pollution

Heavier rainfall due to climate change will exacerbate the effects of increased fertilizer use for corn-based ethanol production, causing a significant increase in nitrogen levels in rivers, according to a new study in ES&T (DOI 10.1021/es801985x). The good news is that if farmers choose organic practices and reduce fertilizer use, the impact of heavy rains will lessen and nitrogen pollution levels might drop to below present-day levels, the study concludes.

The study by research fellow Haejin Han of the University of Michigan and colleagues is the first to simultaneously model the changes in nitrogen runoff across space18 watersheds in the Lake Michigan basinand over time, the past 20 years. Using a model they developed, the researchers then project future variations in nitrogen loading under different land-use and climate scenarios.

The study reinforces an old message for lawmakers: current decisions on land use and agriculture will have a strong impact on the future availability of freshwater and the health of our aquatic and coastal ecosystems, says David Allan, one of the authors on the study. It also reinforces the notion that two factors drive the future of nitrogen pollution, he notes: climate change and how much nitrogen humans use on land.

Scientists have known that roughly 20−25% of the nitrogen applied to land by humans eventually makes its way into rivers. But that percentage changes depending on the type of land use, because the more fertilizer-dependent the agriculture is, the higher the concentrations of nitrogen entering water bodies. For example, more fertilizer use as a result of increased corn planting is expected to raise the levels of nitrogen in the Gulf of Mexico, which in turn fuels algal growth and creates oxygen-free dead zones, according to a study (Proc. Natl. Acad. Sci. U.S.A. 2008, DOI 10.1073/pnas.0708300105) by researcher Simon Donner of the University of British Columbia (Canada).

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