Dramatic greenhouse gas emissions from microbially mediated carbon-nitrogen co-mineralization in Middle-Lower Yangtze shallow lakes.

Lake eutrophication significantly alters the biogeochemical cycles of inland waters, however, current assessments often underestimate its climate impacts by treating carbon and nitrogen processes separately. In a study of 12 shallow lakes across a trophic gradient in the Middle-Lower Yangtze River basin, we found that carbon-nitrogen (C-N) coupling heavily drives greenhouse gas (GHG) emissions. As lakes became hyper-eutrophic, diverse microbial communities were replaced by a specialized group dominated by Dechloromonas (>80%). This shift caused a"C-N coupling priming effect,"where excess nitrogen stimulated microbes to break down recalcitrant organic carbon. Consequently, the mineralization rates of sedimentary organic carbon (40.14-598.51 μg g-1 d-1) and nitrogen (0.1-11.64 μg g-1 d-1) in hyper-eutrophic lakes increased drastically. This intensified microbial metabolism sharply accelerated methane (CH4) production. Driven by abundant substrates and oxygen depletion, the mean dissolved CH4 in hyper-eutrophic lakes (1.19 μmol/L) nearly tripled compared to moderately eutrophic lakes, with peaks reaching 2.94 μmol/L. Meanwhile, N2O levels remained stable, and CO2 concentrations were high at both oligotrophic and hyper-eutrophic extremes. Ultimately, neglecting this microbially driven C-N synergy leads to a significant underestimation of lake GHG emissions. Future carbon budget assessments must evaluate these coupled C-N mechanisms rather than focusing solely on individual elements.