More crop diversity, less nitrate pollution

The changing face of America’s farmland—from smaller farms growing a wide diversity of crops to very large farms growing one crop—is significantly contributing to increased nitrate levels in rivers and lakes, according to new research published online in Frontiers in Ecology and the Environment (2009, DOI 10.1890/080085).

Too much nitrate in the water can lead to prolific growth of algae, which use up most of the water’s dissolved oxygen when they die and decompose; this process creates “dead zones” that cannot support life, including most famously the enormous hypoxic zone in the Gulf of Mexico.

Whitney Broussard of the University of Louisiana Lafayette and R. Eugene Turner of Louisiana State University compared water-quality data from the past 100 years with information from the same time period about farms, including size, crops, and agricultural methods. Next, the researchers studied 56 watersheds that varied in size from the Illinois Cache River basin at 400 square miles to the Mississippi River basin at more than 1 million square miles.

The past century has witnessed major changes in the average U.S. farm, which doubled in size while the number of farms shrank by almost two-thirds. Average nitrate concentrations in the 63 rivers the researchers studied were 3–4 times higher at the end of the 20th century than at the beginning.

The watersheds where farming is most intense seem to have driven the rise in nitrate in the latter half of the century, and there is evidence that commercial fertilizer, mechanical tillage, and intensive drainage dramatically increase nitrate loads, the authors note.

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