Unraveling the Overlooked Dissolved Methane and Nitrous Oxide Hotspots and Their Causes in High C/N Environments

Inland waters are sources of methane (CH4) and nitrous oxide (N2O) emissions and act as nutrient sinks. However, the interaction between nutrients inputs and these greenhouse gases (GHGs) dynamics remains unclear. Here, based on field investigations and simulations, we sampled two regions of Dongting Lake (DTL) with distinct carbon and nitrogen loads. Unlike previous studies, we found that long-term nutrient loading in carbon-dominated areas increased CH4 production and inhibited its oxidation, and enriching incomplete denitrifiers. As a result, dissolved CH4 and N2O concentrations were 30.76% and 33.33% higher than in nitrogen-dominated regions, respectively. A global dataset of 34,519 inland dissolved GHG samples is compiled and generated response curves between dissolved GHG concentrations and environmental variables, confirming that the response curves within DTL closely align with global trends. Using SHAP and microbial analyses results, we identified a four-stage impact of dissolved organic carbon (DOC) on dissolved N2O, reconciling discrepancies in GHG hotspot indicators between our study and previous research. Threshold analysis revealed that DOC and the carbon-to-nitrogen ratio are critical in determining greenhouse gas emission hotspots, highlighting the limitations of relying solely on substrate concentrations. This study enhances our understanding of the relationship between nutrient inputs and dissolved greenhouse gas dynamics.