Overlooked nitrous oxide emissions driven by bedrock weathering

Abstract:

Nitrous oxide (N₂O) in the atmosphere contributes directly to global warming, yet current models overlook bedrock-contained nitrogen (rock-N), the largest terrestrial N pool, as a source of N₂O. Although estimated rock-N release rates are large, lack of mechanistic understanding on bedrock weathering and fate of the released rock-N has obscured connections between rock-N and atmospheric N₂O. This connection emerged through our field studies of a pristine mountainous watershed hillslope underlain by marine shale. We found that rock-N releases occur primarily in the subsurface, not at the land surface, and weathering depth is controlled by the seasonally fluctuating water table depth. Bedrock weathering was found to contribute most of the subsurface reactive-N, with less than 25% derived from atmospheric deposition and biological nitrogen fixation, commonly regarded as the sole sources of reactive-N in pristine environments. About 56% of the reactive-N denitrifies, with about 25% of that lost to the atmosphere as N₂O, while reactive-N remaining in porewaters discharging to an adjoining floodplain where additional denitrification likely occurs. We estimate a global N₂O contribution of about 1.3–2.1 Tg N y^-1 sourced from denitrification of reactive-N released from diverse bedrock types, a significant fraction of the global terrestrial emission rate of ~13 Tg N y^-1.