Nitrogen Gas Emissions from Shale Critical Zone Environments

Nitrous oxide (N2O) is a greenhouse gas with approximately 300 times the warming potential of CO2. 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 unmanaged global 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 subsurface environments of the Mancos shale in the East River watershed, Colorado. To characterize Mancos shale denitrification dynamics and N2O emissions, we measured N2O concentrations from gas sampling wells at three monitoring sites during June 2021.The measured N2O concentrations and isotopic composition constrain a diffusion-reaction model to assess depth-resolved N2O production and consumption dynamics. To supplement the field observations, we used laboratory incubation experiments to characterize the magnitude of Mancos shale denitrification, measuring N2O production and denitrification rates over the course of 30 days with the acetylene inhibition method. Additional chemical amendments (ferrous iron, organic carbon, and nitrate) were included to assess potential substrate limitations on denitrification rates and impacts of N2O emissions. Our results provide an important first step towards characterizing shale denitrification processes and the factors that promote N2O emissions.