Unveiling the Disproportionate Impact of Nitrogen Deposition on Aquatic Nutrient Loading and Greenhouse Gas Emissions

Existing research has confirmed that nitrogen deposition is a key source of nutrient input in watersheds, but its contribution to riverine carbon, nitrogen loads, and aquatic greenhouse gas (GHG) emissions has not been thoroughly quantified. By developing a multimodel composite framework, this study investigates these contributions in a test watershed and reveals that nitrogen deposition is a disproportionately efficient contributor to aquatic pollutants. While its load (43.11 kg N ha^-1 yr^-1) is significantly lower than that from agricultural nonpoint sources (248.18 kg N ha^-1 yr^-1), nitrogen deposition delivers substantially more total nitrogen (TN) and dissolved organic carbon (DOC) per unit of nitrogen input. Consequently, it accounts for considerable portions of riverine loads of nitrate (NO₃^-, 11.6-17.2%), TN (16.0-19.8%), and DOC (39.2-39.8%), thereby driving 16.34% of aquatic methane (CH₄) and 18.56% of nitrous oxide (N₂O) emissions. Analysis revealed that reductions in nitrogen deposition have lagged far behind decreases in atmospheric pollutants within the test watershed, highlighting the urgent need to accelerate a cocontrol strategy for atmospheric ammonia, acidic gases, and nitrogen oxide emissions. Moreover, integrating such a strategy with climate change mitigation is crucial for reducing nitrogen deposition. This study provides insights into the relationships among nitrogen deposition, water quality, aquatic GHG emissions, and climate change in agricultural watersheds.