Highly trapped terrestrial dissolved organic matter fuels dissolved greenhouse gases of ice-covered boreal steppe lakes

Although lakes are widely recognized as a significant source of greenhouse gas (GHG) emissions, it is worth nothing that boreal ice-covered period lakes are frequently overlooked in annual GHG budgets, leading to considerable uncertainly in estimating their fluxes. This uncertainty is closely linked to the dominant pool of dissolved organic matter (DOM), as its biodegradability and stability can significantly influence GHG budgets. For better understanding the potential impact of DOM sources and structure on GHG emissions, this study systematically investigated the primary dissolved GHGs, as well as the sources and distribution of DOM in a boreal steppe lake basin during the ice-covered period. The concentrations of dissolved CO₂, CH₄, and N₂O ranged from 27.9 to 33.1, 0.070–0.139, and 0.054–0.056 μmol L^-1, respectively, and exhibited spatial similarities with higher levels observed in the inflowing rivers compared to those in lakes. Three aromatic humus components were identified via the spectral characteristics of DOM, and subsequent molecular composition analysis further revealed that tannin and lignin were the primary components of DOM. The aromatic humus DOM showed a significant positive correlation with GHGs, as did the microbial indicators, suggesting that the trapped terrestrial DOM in the boreal steppe lake basin during the ice-covered period contributed to the storage of dissolved GHGs. Microorganisms isolated under the ice utilized DOM for respiration, degradation, nitrification and denitrification, resulting in the production of a considerable amount of GHGs. Furthermore, the presence of ice cover accelerates the accumulation of dissolved GHGs. Therefore, it is crucial to consider the massive GHG release during the thawing period to accurately evaluate GHG emissions in freshwater bodies.