Biogeochemical and molecular biological characterization of nitrogen cycle processes in the Columbia River and estuary

Human activities now contribute over 50% of global reactive nitrogen (Nr) fixation, significantly increasing Nr flux to rivers and driving eutrophication and greenhouse gas emissions. This study examines nitrification within the Columbia River System (CRS), a major freshwater source to the North Pacific, using molecular characterization, chemical sensors, and stable isotope techniques. Results indicate ammonia-oxidizing archaea (AOA) play a key role in ammonia oxidation—the rate-limiting step of nitrification—particularly in salinity-influenced estuarine zones during upwelling and low-flow conditions. Findings highlight the spatial and temporal sensitivity of nitrification processes and their implications for Nr cycling and regional biogeochemistry.