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.