Agricultural runoff contributes to global warming - New study helps us
figure out how and what we can do about it
Mechanics of how rivers, bearing increased loads of nitrogen, emit
greenhouse gasses revealed

Date:
October 19, 2021
Source:
University of Massachusetts Amherst
Summary:
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.



FULL STORY ========================================================================== 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.

========================================================================== Story Source: Materials provided by
University_of_Massachusetts_Amherst. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. M. J. Winnick. Stream Transport and Substrate Controls on Nitrous
Oxide
Yields From Hyporheic Zone Denitrification. AGU Advances, 2021 DOI:
10.1029/2021AV000517 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211019110540.htm

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