Baseflow and Coupled Nitrification-Denitrification Processes Jointly Dominate Nitrate Dynamics in a Watershed Impacted by Rare Earth Mining

Abstract Mining activities cause severe nitrogen pollution in watersheds, yet our understanding of the transport pathways, transformation processes, and control mechanisms of nitrate (NO3–) in these areas is limited. Based on nearly 4-year observations of groundwater and river in China’s largest ion-adsorption rare earth mining watershed, we revealed the dynamics of NO3– and its drivers using stoichiometry-based load model, molecular biological, and multi-isotope approaches. Results indicated that the NO3– dynamics were jointly controlled by sources (precipitation, terrestrial inputs, and sediment supply) and processes (hydrological and biological). The monthly NO3– export load from the 444.4 km2 watershed was 3.72 × 105 kg. Groundwater (36 ± 26%) and soil nitrogen (25 ± 17%) were the primary exogenous sources of NO3–. Baseflow was the main hydrological pathway for legacy nitrogen into the river, contributing 66.8% of the NO3– load. Coupled nitrification-denitrification were key biological processes affecting the NO3– transformation, with denitrification contributing 58%. Burkholderia were most associated with NO3– transformation. Dissolved organic carbon and oxygen were major drivers affecting the NO3– production and consumption. This study highlights effective control and management strategies for nitrogen pollution in mining-affected watersheds, considering not only reducing nitrogen inputs but also integrating hydrological pathways and nitrogen transformation mechanisms.