Nitrous oxide (N2O) is a potent greenhouse gas, with 300 times the warming ability of carbon dioxide. Due to fertilizer runoff from farm fields, an increasing load of nitrogen is washing into rivers and streams, where nitrogen-breathing microbes break some of the fertilizer down into N2O, which the river releases into the atmosphere as it tumbles toward the ocean. But, until now, scientists haven't had a clear picture of how the process works, what fraction of the runoff winds up as N2O or what steps might be taken to mitigate N2O emissions.
"Humans are fundamentally altering the nitrogen cycle," says Matthew Winnick, sole author of the new paper, published recently in AGU Advances, and professor of geosciences at the University of Massachusetts Amherst. "We've changed how nitrogen moves through the environment." Much of this change can be attributed to enormous amounts of nitrogen-rich chemical fertilizers, spread upon agricultural fields, which run off into streams and rivers when it rains, and get converted to nitrate.
Scientists have long known that microbes in the soil and streambed contribute to the "denitrification process," whereby nitrate is converted to either harmless dinitrogen gas or N2O. But the exact mechanics of the conversion processes have remained a mystery, as evidenced by the wide range of N2O emissions estimates-;somewhere between .5% and 10% of global emissions-;annually attributable to streams.
Winnick's innovation was to revisit a large experimental dataset that quantified N2O in 72 streams across the US using a combination of chemical reaction models, which can trace how nitrogen is transformed through a stream system, and stream turbulence models, which capture how the mechanical forces of the river itself deliver nitrate to the stream's bed, which is where denitrification occurs.
This novel combination, pairing the high resolution of the chemical reaction model with the turbulence model, allowed Winnick to see how nitrate moved from the stream to the streambed and was key to his discovery.
It turns out that what effectively determines the production of N2O is "denitrification efficiency," or the fraction of nitrate, delivered to the streambed, that is subjected to the various reactions in the denitrification process. The greater the streambed's efficiency in converting nitrate, the less N2O is released. But where denitrification efficiency is low, Winnick found comparatively higher levels of N2O emissions.
Furthermore, the bed of the stream to which the nitrate is delivered also plays an important role. Streambeds studded with small anoxic zones, or patches starved of oxygen, also help prevent the release of N2O.
Winnick suggests that this new understanding of nitrogen cycling could help inform efforts at climate-change mitigation. "Increasing the ability of streams to process anthropogenic nitrogen may also reduce proportional N2O emissions," he writes.