Agricultural runoff contributes to global warming, but a new study offers insight on climate-change mitigation

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The greenhouse gas Nitrous oxide (N2 O) has a 300-fold warming capacity. Increasing amounts of fertilizer runoff from farms is causing a rise in nitrogen levels. Streams and rivers are flooded with nitrogen, which is broken down by nitrogen-breathing microbes into N 2 O. The river then releases the nitrogen into the atmosphere as the river tumbles towards the ocean. Scientists have not been able to determine how this process works, which fraction ends up as N2 O, and what steps can be taken to reduce N2 O emissions.

Matthew Winnick, the sole author of the new paper and professor of geosciences at University of Massachusetts Amherst, says that "Humans fundamentally alter the nitrogen cycle." "We have changed the way nitrogen moves through the environmental environment." This is largely due to the enormous quantities of nitrogen-rich chemical fertilisers that are spread on agricultural fields. These fertilizers run off into rivers and streams when it rains and become nitrate.

Scientists know for a long time that soil and streambed microbes play a role in "denitrification," when nitrate is converted into harmless dinitrogen or N2 O. The wide range of N2 O emissions estimated to be attributableto streams has made it difficult to determine the precise mechanism of these conversions.

Winnick's innovative approach was to revise a large experimental dataset, which quantified N 2O in 72 streams across the US. He used a combination of stream turbulence models and chemical reaction models to track how nitrogen is transported through stream systems. This captures how stream forces deliver nitrate to stream's beds, where it is responsible for denitrification.

Winnick was able to observe how nitrate moves from stream to streambed by combining the high resolution of both the chemical reaction model and the turbulence modeling. This combination was crucial to his discovery.

The key to N 2 O production is the "denitrification efficacy", or the amount of nitrate delivered to the streambed and subject to the reactions during the denitrification process. The streambed's ability to convert nitrate is more efficient, so less N2 O is released. Winnick discovered that N 2 O emissions are higher in areas with low denitrification efficiency.

The bed to which the nitrate is delivered plays an important part. Streambeds that are contaminated with small anoxic areas, or areas without oxygen, can also prevent the release N 2 O.

Winnick believes that this new understanding about nitrogen cycling may help to inform climate-change mitigation efforts. He writes that increasing the capacity of streams to process anthropogenic Nitrogen may reduce proportional N2 O emissions.

Continue reading Microbial nitrogen consumption promotes HONO emission in agricultural soils

More information: Stream Transport and Substrate Controls on Nitrous Oxide Yields From Hyporheic Zone Denitrification, AGU Advances, 10.1029/2021AV000517 Journal information: AGU Advances Stream Transport and Substrate Controls on Nitrous Oxide Yields From Hyporheic Zone Denitrification,