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.