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　　湿地可长期吸收硝酸盐

　　Wetlands created 20 years ago between tile-drained agricultural fields and the Embarras River were recently revisited for a new two-year University of Illinois research project. Results show an overall 62 percent nitrate removal rate and little emission1 of nitrous oxide2, a potent3greenhouse gas. "Slowing down the rate of flow of the water by intercepting4 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. "The vegetation that grows in the wetland doesn't make much of a difference because the grasses don't take up much nitrogen. It's just about slowing the water down and allowing the microbes in the sediment5 to eliminate the nitrate. It goes back into the air as harmless nitrogen gas."

　　David was involved with research on these same wetlands from 1994-98 but didn't take any measurements after that. He has spent much of his career studying the runoff from tile-drained fields and methods to reduce losses of nitrate and phosphorus. The runoff, particularly nitrate, from fields in the upper Mississippi River basin, is believed to be the major cause of the hypoxia in the Gulf6 of Mexico.

　　"The USDA requested proposals on the effectiveness of wetlands and woodchip bioreactors to reduce nitrate losses from fields, but was also concerned about greenhouse gas emissions7," says David. Working with graduate student and lead author Tyler Groh, "We found the greenhouse gas emissions were really quite low. 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."

　　David says that, along with fertilizer management, cover crops, and bioreactors, wetlands are an integral part of the Illinois Nutrient8 Loss Reduction Strategy. Building a wetland costs about the same as installing a bioreactor.

　　One of the reasons David prefers wetlands to help solve the nitrogen pollution problem is that they work reasonably well in the winter when the water temperatures are low.