A national-scale redox clustering for quantifying CO₂ emissions from groundwater denitrification

Nitrate pollution from agriculture poses a significant global threat to the environment and to public health. In groundwater, nitrate can be reduced through denitrification, a process that produces dissolved inorganic carbon (DIC) via organic carbon (OC) mineralization and/or carbonate dissolution. This DIC acts as a net anthropogenic source of atmospheric CO2; however, its overall climatic impact remains poorly constrained. In this study, we quantified CO2 production from groundwater denitrification across Denmark using extensive observational datasets and national-scale modeling tools. A set of machine learning techniques was applied to cluster groundwater redox conditions and map the dominant electron donors for denitrification at the national scale. At the redox interface, denitrification was predicted to be mediated by pyrite oxidation in approximately 76 % of Denmark, with the remainder dominated by OC oxidation. Our results underscore the central role of hydrogeology in controlling the distribution of dominant electron donors. Spatial variability in CO2 production from groundwater denitrification was governed by nitrogen leaching and prevailing denitrification pathways. Assuming complete denitrification, we estimated that groundwater denitrification produces approximately 204 ktCO2eq.(carbondioxideequivalent)yr-1 as DIC, of which ∼50 % is likely released into the atmosphere. The Intergovernmental Panel on Climate Change (IPCC) guidelines account for agricultural CO2 emissions from liming, urea, and other carbon-containing fertilizers, estimated at 246, 16 and 6 ktCO2eq.yr-1, respectively, for Denmark in 2022. Although CO2 comprises a minor share (∼2 %) of the total agricultural GHG emissions, our findings suggest that denitrification-derived CO2 should be included in agricultural GHG inventories.