“Human activities, including fossil fuel combustion and intensive agriculture, have increased the availability of nitrogen in the environment,” said Jake Beaulieu of the University of Notre Dame and the U.S. Environmental Protection Agency in Cincinnati, Ohio, and lead author of the paper published this week in the journal Proceedings of the National Academy of Sciences.

　　“Much of this nitrogen is transported into river and stream networks,” Beaulieu said. There, microbes (微生物) convert the nitrogen into nitrous oxide (also called laughing gas) and an inert gas called dinitrogen (二氮).

　　The finding is important, the researchers say, because nitrous oxide is a powerful greenhouse gas that contributes to climate change and destruction of the stratosphere’s ozone layer, which protects us from the sun’s harmful ultraviolet (紫外线) radiation. Compared with carbon dioxide, nitrous oxide is 300-fold more powerful in terms of its warming potential, though carbon dioxide is a far more common greenhouse gas. Scientists estimate nitrous oxide accounts for about 6 percent of human-induced climate change.

　　Beaulieu and colleagues measured nitrous oxide production rates in 72 streams. When summed across the globe, the results showed rivers and streams are the source of at least 10 percent of human-caused nitrous oxide emissions to the atmosphere.

　　“Changes in agricultural and land-use practices that result in less nitrogen being delivered to streams would reduce nitrous oxide emissions from river networks,” Beaulieu said.

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