NITROGEN GAS EMISSIONS FROM SHALE CRITICAL ZONE ENVIRONMENTS

Global atmospheric concentrations of N₂O are rising largely due to human activities, but uncertainty remains in natural sources and their response to climate change. In particular, geologic nitrogen accounts for approximately 25% of global unmanaged inputs to terrestrial systems; however, the reactions and physical transport that govern the fate of this geologic nitrogen remain largely uncharacterized. Shale bedrock environments, which are a significant source of geologic nitrogen, may act as potential hotspots for denitrification and N₂O production due to an abundance of electron donors such as organic carbon and ferrous iron. In this study we assess the conditions that control and promote N₂O and N₂ gas production within the Mancos shale in the East River watershed, Colorado in subsurface environments. To characterize Mancos shale denitrification dynamics and N₂O emissions, we measured N₂O concentrations from gas sampling wells at three monitoring sites within the East River watershed in June 2021.The measured N₂O concentrations and isotopic composition will be used to drive a diffusion-reaction model to assess depth-resolved N₂O production and consumption dynamics. Field observations are supported with laboratory incubation experiments to characterize the magnitude of Mancos shale denitrification, in which N₂O production and denitrification rates were measured over the course of 30 days using the acetylene inhibition method. Additional chemical amendments (ferrous iron, organic carbon, and nitrate) are included to assess potential substrate limitations on denitrification rates and impacts of N₂O emissions. Our results will provide an important first step towards characterizing shale denitrification processes and the factors that promote N₂O emissions.