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 (α_C) 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.