New study helps figure out how agricultural runoff contributes to global warming

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. We've changed how nitrogen moves through the environment."

Matthew Winnick, Study Sole Author and Professor of Geosciences, University of Massachusetts Amherst

Winnick's study was published recently in AGU Advances,

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.