Nitrate threatening watersheds in the US and elsewhere

Science Now, by Philip Berardelli: There's mixed news about how the country's streams and rivers are handling increased loads of nitrate from human activities. The ecosystems are normally highly tolerant of the chemical, which is good. But new research shows that nitrate absorption can reach a limit, and that's what is happening in many areas. Worse, the budding biofuels industry figures to release even more nitrate into watersheds in the coming years.

Livestock waste and nitrogen-based fertilizers constitute the biggest source of nitrate in U.S. watersheds. The chemical, also known as NO₃, works its way into streams and rivers, where it feeds nitrogen-loving plants and bacteria. Eventually, some of it ends up in bays and estuaries, nourishing plants and algae there as well. But in large amounts, nitrate can unbalance ecosystems, fueling rampant growth by plants and algae. Their subsequent decomposition by bacteria can pull so much oxygen out of the water that it causes a condition called hypoxia, which can stress fish and other marine creatures. In the worst cases, the oxygen can disappear altogether, resulting in dead zones and spawning dangerous algal blooms called red tides (ScienceNOW, 28 June 2006).

Scientists have long known that plants and bacteria in watersheds serve as a nitrate-disposal system, but up to now no one has been able to gauge how efficiently the system works. So a team of researchers from 15 states set out to study the process in action. They dumped nitrate spiked with a harmless nitrogen isotope as a marker into 72 streams across the continental United States and Puerto Rico. Then they collected samples at various points downstream continuously over a 24-hour period to see how well nitrate was absorbed.

"Our results confirmed our initial suspicions in that we found relatively high removal rates of nitrate from stream water in most streams," says aquatic ecologist Patrick Mulholland of Oak Ridge National Laboratory in Tennessee. As the team reports tomorrow in Nature, stream ecosystems were able to remove between 10% and 20% of the added nitrate within an hour. But the big surprise, Mulholland says, was that increasing nitrate concentrations caused a precipitous drop in absorption efficiency--meaning much more nitrate could make its way downstream if nitrate loads continue to increase.

The most likely cause of heavier nitrate loads in the future, Mulholland says, is expanded biofuels production, which could require large increases in nitrogen-based fertilizers. "These problems are likely to be most severe for biofuel crops such as corn," he says, but may be less severe for switchgrass and other perennials. "It's a difficult problem to quantify."

Ecologist Emily Bernhardt of Duke University in Durham, North Carolina, says the paper is important because it shows that even the short-term ability of streams to remove nitrogen is substantial. "Their results are no doubt highly conservative" because their experiments did not measure nitrogen removal over longer periods. Still, she says, the findings provide "conclusive evidence" that this capacity "is not unlimited" and can be overtaxed.

Holstein dairy cows, Scott Bauer, USDA, Wikimedia Commons