Source-specific carbon dioxide production and acetoclastic methanogenesis upon anaerobic degradation of dissolved organic matter

Lakes receive and process diverse sources of dissolved organic matter (DOM), which contribute differently to the production of carbon dioxide (CO2) and methane (CH4) within the system. Here, we isolated DOM from soil, algae, submerged macrophytes, and sewage, then tracked their anaerobic fate for 56 days with high-frequency isotope and ultrahigh-resolution mass spectrometry analyses. Biolabile macrophyte-, sewage-, and algal-DOM lost approximately 80% of DOC within 56 days, leading to CO2 concentrations of up to 1422 µmol L–1 and CH4 concentrations of up to 1.59 µmol L–1 for the macrophyte-DOM group. These concentrations were 6 to 9-fold higher than those found for soil-DOM and 30 to 90-fold higher than their initial levels. The calculated carbon isotope fractionation factor (αC) decreased from initial values of 1.05–1.10 to 1.03–1.04 by day 56, and δ13C-CH4 rose to -40‰, indicating the immediate dominance of the acetoclastic methanogenic pathway. Extreme gradient boosting models identified δ13C-DOC as a key predictor of both CO2 and CH4 concentrations. Degradation of 1 mg algal DOC yielded 47 µmol CO2 and 0.04 µmol CH4, surpassing soil-derived yields by 7- and 4-fold, respectively. By quantifying yield coefficients and identifying δ13C-DOC as a key predictor, we move beyond qualitative patterns; the immediate onset of acetoclastic methanogenesis across all sources provides a mechanistic framework for modeling emissions from mixed DOM pools in natural systems.湖泊接收并处理多种来源的溶解性有机质（dissolved organic matter, DOM），不同来源的DOM对系统内二氧化碳（carbon dioxide, CO₂）与甲烷（methane, CH₄）的生成贡献存在差异。本研究从土壤、藻类、沉水植物及污水中提取DOM，随后采用高频同位素与超高分辨质谱分析技术，追踪其56天的厌氧降解过程。易生物降解的沉水植物源、污水源及藻类源DOM在56天内流失了约80%的溶解性有机碳（dissolved organic carbon, DOC）；其中沉水植物源DOM组的二氧化碳浓度最高可达1422 μmol·L⁻¹，甲烷浓度最高可达1.59 μmol·L⁻¹。上述浓度较土壤源DOM组高出6~9倍，较各组初始水平高出30~90倍。计算得到的碳同位素分馏因子（αC）从初始的1.05~1.10降至第56天时的1.03~1.04；同时δ¹³C-CH₄升高至-40‰，表明此时乙酸型产甲烷途径已占据主导地位。极端梯度提升模型识别出δ¹³C-DOC是二氧化碳与甲烷浓度的关键预测因子。每降解1 mg藻类源DOC可生成47 μmol二氧化碳与0.04 μmol甲烷，分别较土壤源DOM的生成量高出7倍与4倍。本研究通过量化产率系数并明确δ¹³C-DOC作为关键预测因子，突破了仅定性描述的局限；所有来源DOM均快速启动乙酸型产甲烷过程，这一发现为自然系统中混合DOM库的碳排放建模提供了机制性框架。