Identifying Priority Nutrients for Achieving Water Quality Improvement and Climate Change Mitigation

Riverine and lake ecosystems are sources of global non-CO₂ greenhouse gases (GHGs) and serve as sinks for pollutants, facing the dual challenges of mitigating GHG emissions and controlling water pollution. However, interactions between pollutant inputs and GHG emissions remain unclear. Herein, relying on a compiled data set of global non-CO₂ GHGs and robust modeling, a watershed dissolved CH₄ and N₂O estimator is developed and validated on the global scale. Using the Dongting watershed (DTW) as a modeling example, various pollutant input scenarios were developed to explore the influence of changes in pollutant inputs on CO₂-equivalent (CO₂e) emissions from CH₄ and N₂O. Simulation results indicate that implementing pollutant inputs reduction measures in dissolved GHG hotspot areas will yield more efficient CO₂e emission reduction benefits. Moreover, a critical paradox was revealed: while decreasing pollutant inputs leads to a sustained decline in direct CO₂e (CO₂e_D) emissions, indirect CO₂e (CO₂e_I) emissions from aquatic systems may show a minimal reduction in some cases. This paradox is closely tied to carbon-nitrogen ratio variations in aquatic system and can be well explained by carbon and nitrogen limitation principle, as defined by the Redfield ratio. Thus, our study suggests that cocontrol of carbon and nitrogen inputs within dissolved GHG hotspot areas is vital for achieving both water quality improvement and climate change mitigation simultaneously.