Overlooked nitrous oxide emissions driven by bedrock weathering

Abstract Atmospheric nitrous oxide (N2O) contributes directly to global warming, yet current models1-5 overlook bedrock-contained nitrogen (rock-N), the largest terrestrial N pool6, as a N2O source. Although rock-N release rates are large6-9, incomplete understanding on the fate of released rock-N has obscured connections between rock-N and atmospheric N2O. This connection emerged through our field studies of a hillslope underlain by marine shale. Bedrock weathering within the zone of the seasonally fluctuating water table controls the weathering depth, hence the release of rock-N. At this site, rock-N weathering contributes 78% of the subsurface reactive-N, with ~22% derived from atmospheric deposition and biological nitrogen fixation, commonly regarded as the sole sources of reactive-N in pristine environments10,11. About 56% of reactive-N denitrifies, including 14% emitted as N2O into the atmosphere. The remaining reactive-N discharges in porewaters to a floodplain where additional denitrification likely occurs. Using global rock-N releases of 11–18 Tg y-1 8, our measurements extrapolate to a weathering driven efflux of 1.3–2.1 Tg N-N2O y-1, consistent with a flux of 1.0–1.7 Tg N- N2O y-1 solely derived from the literature. Thus, bedrock weathering contributes approximately 10-17 % of nitrous oxide to the current global estimate of ~10 Tg y-1.