Across the world, humans are causing rivers and streams to release the potent greenhouse gas nitrous oxide at levels three times higher than used in climate change predictions. Known as laughing gas when used as an anaesthetic, nitrous oxide is also a greenhouse gas over 300 times more powerful than CO2 on a per-molecule basis. How much of this gas comes from streams and rivers wasn’t previously well known, and scenarios predicting future climate change were based on estimates. Now, writing in the journal Proceedings of the National Academy of Sciences of the United States of America on Monday, a large team of US scientists has filled this knowledge gap.
Humans cause chemicals with high nitrogen contents from sources like fertilizers and sewage to enter water bodies as dissolved inorganic nitrogen (DIN). Microbes help break the DIN down, first by converting it into nitrates, and then converting these nitrate chemicals into nitrous oxide and nitrogen gas in a process called denitrification. Only a tiny amount of nitrous oxide is produced during denitrification in comparison to nitrogen gas. Previously, climate change models produced by the Intergovernmental Panel on Climate Change (IPCC) assumed that just 1 part of nitrous oxide is produced from 400 parts of DIN on average worldwide.
Now, lead author Jake Beaulieu of the University of Notre Dame and the US Environmental Protection Agency in Cincinnati, Ohio, and 26 coworkers have produced measurements to test that assumption. They studied the nitrous oxide emitted from 72 streams in different landscapes across the country, finding that 55 of them did contribute the gas to the atmosphere. When nitrate concentrations exceeded a certain limit, then nitrous oxide emissions increased with them. Below that limit there was no relationship between the two, showing that the emissions are not exclusively down to denitrification.
While fertilizer is often thought of as a key source of nitrates, Beaulieu and colleagues found that farms weren’t the type of land that caused the most nitrous oxide emissions from water. “The highest emission rates were observed in streams draining urban basins,” they wrote. Based on their measurements, they found that climate change models should be assuming that 3 parts of of nitrous oxide are produced from 400 parts of DIN in streams. This tripling raises the estimated total human-caused emissions of nitrous oxide from these sources to 600 tonnes per year, which is one tenth of the total amount of this gas produced by people. Industry and burning of wood and agricultural waste are among the other main sources of human-caused nitrous oxide emissions.
“This experiment clearly establishes streams and rivers as important sources of nitrous oxide,” said Henry Gholz, program director in the US National Science Foundation’s Division of Environmental Biology, which funded the research. “This is especially the case for those draining nitrogen-enriched urbanized and agricultural watersheds, highlighting the importance of managing nitrogen before it reaches open water,” Gholz says. “This new global emission estimate is startling.”