Researchers from the University of Illinois revisited wetlands created 20 years ago and found an overall 62% nitrate removal rate and little emission of nitrous oxide, a greenhouse gas.

Researchers revisited wetlands created 20 years ago and found an overall 62% nitrate removal rate and little emission of nitrous oxide, a potent greenhouse gas. Source: Science daily, Image credit: University of Illinois"Slowing down the rate of flow of the water by intercepting it in the wetland is what helps to remove the nitrate," says Mark David, a University of Illinois biogeochemist in the College of Agricultural, Consumer and Environmental Sciences. He says the vegetation that grows in the wetland does not make much of a difference because the grasses do not take up much nitrogen. Instead, the important function is to slow the water and allow microbes in the sediment to eliminate the nitrate. “It goes back into the air as harmless nitrogen gas," he says.

David has spent much of his career studying runoff from tile-drained fields and methods of reducing nitrate and phosphate loss. He says the runoff, especially nitrate from fields in the upper Mississippi River basin, is a major cause of hypoxia in the Gulf of Mexico.

The U.S. Department of Agriculture requested proposals on the effectiveness of wetlands and woodchip bioreactors to reduce nitrate losses from fields, but was also concerned about greenhouse gas emissions, says David. His team found the greenhouse gas emissions were low and that nitrous oxide was not a problem. “The other good news is that this research confirms that wetlands really do work to reduce nitrate runoff, and they work long term," he says.