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Bedrock Weathering a Large Mobilizer of Subsurface Nitrogen 3 min read

by Priyanka Runwal on April 28, 2021

Climate and Ecosystem Sciences Division

 

A study led by EESA staff scientist Jiamin Wan found that bedrock weathering contributes an astounding 78 percent to the mobilized subsurface nitrogen in Colorado’s East River Watershed, 14 percent of which is emitted as nitrous oxide into the atmosphere. The paper, “Bedrock weathering contributes to subsurface reactive nitrogen and nitrous oxide emissions,” was the subject of the cover of the April issue of Nature Geoscience, with photography by Jeremy Snyder, who worked as an intern at the Watershed Function SFA site in Crested Butte, Colorado, under the supervision of the project’s deputy director Ken Williams.

Beneath the earth’s surface, just below the soil layer, there’s hard, solid bedrock. When this subsurface rock is of marine sedimentary origin, it harbors relatively large quantities of nitrogen from millennia of accumulated dead plant and animal matter. Cracks within the bedrock allow groundwater, oxygen, and roots to percolate, which physically and chemically breakdown and dissolve the rock, thus mobilizing the locked-up nitrogen. Microbes present in these fissures transform and release the dissolved rock nitrogen, with some of it becoming gas, including nitrous oxide—a potent greenhouse and ozone-depleting gas.

While there’s a growing recognition among scientists that such rock-derived nitrogen emissions are important, their quantifications are somewhat lacking. Now, a new Berkeley Lab study published in Nature Geosciences finds that in their field site located in Colorado’s East River Watershed, bedrock weathering contributes an astounding 78 percent to the subsurface dissolved nitrogen,14 percent of which is emitted as nitrous oxide into the atmosphere.

“We were in disbelief when we saw the huge numbers,” said Jiamin Wan, a research scientist at Berkeley Lab’s Energy Geosciences Division and lead author of the study. “Quantitatively you cannot ignore it, as it has implications not just for our site but for many other regions with marine sedimentary bedrock.”

The current global predictive models for nitrous oxide emissions rely on estimates from sources including agriculture, animal husbandry and fossil fuel combustion, ignoring the nitrous oxide from transformation of rock nitrogen. But in “pristine” environments that lack human activity the nitrogen input into the system is assumed to be mainly from atmospheric deposition and biological fixation of nitrogen by soil microbes.

“Conventional wisdom in this environment would be to attribute 100 percent of the subsurface reactive nitrogen to atmospheric sources,” Wan said. But by monitoring nitrogen levels in water and gas samples collected at different depths from three 10-meter-deep boreholes along a hillslope in the East River Watershed, her team found otherwise. Only 22 percent of the reactive nitrogen was atmospheric in origin as compared to the remaining 78 percent, which was as a product of weathering occurring primarily below the top one-meter soil layer until four-meters-depth that sports moisture-rich and oxygenated conditions. 44 percent of this nitrogen was transported downslope through shallow groundwater flowing into the river, while 56 percent in gaseous forms including nitrous oxide.

“Modelers could not account for nitrous oxide emissions from weathering because there was no such quantification, until now,” Wan said. “Soil scientists rarely go below one meter. Also, quantitatively understanding the subsurface hydrology and biogeochemistry is complicated.”

Still, this study highlights the need to measure and account for below-ground weathering processes within sedimentary bedrocks. “Our research is just the beginning,” Wan said.

News & Events

Daniel Stolper Selected by DOE’s Early Career Research Program2 min read

June 22, 2022

Daniel Stolper is among five Berkeley Lab researchers to receive funding through the Department of Energy’s Early Career Research Program (ECRP), and is one of just 83 nationwide to be selected this year by the DOE for this prestigious award. Stolper is an EESA faculty scientist with a joint appointment at UC Berkeley, where he…

Wageningen Students Visit Ecology Department Team2 min read

On May 31, a delegation of students from Wageningen University & Research Center (WUR) Microbiology and Systems Biology Groups in the Netherlands came to visit EESA’s Ecology department. WUR is a highly esteemed world-class Dutch university that trains specialists in a variety of life sciences disciplines. WUR’s research and teaching activities range from sustainable agriculture…

Strengthening Wildland Fire Science and Scientific Collaboration through New Data Management Platform3 min read

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  Wildfires are increasing in severity and frequency worldwide. A new report called Spreading like Wildfire: The Rising Threat of Extraordinary Landscape Fires indicates that wildfires are responsible for significant economic, environmental, and sociopolitical damage (UNEP, GRID-Arendal, 2021). They also contribute significantly to greenhouse gas emissions – thereby further fueling climate change.  Researchers need to…

Bhavna Arora Describes Agricultural Managed Aquifer Recharge5 min read

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Managed Aquifer Recharge is a water management strategy used to store excess surface water underground and thereby replenish groundwater basins when and where possible. This strategy enables communities to use depleted groundwater basins as natural water storage to augment water supplies and prevent land subsidence. In coastal regions, MAR can be implemented to act as…

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