Despite many thermokarst (thaw) lakes having exceptionally high methane (CH4) emission rates, few studies have examined biodegradability of thawed permafrost organic carbon (Corg) in deep anaerobic environments, such as thermokarst lake sediments and taliks (thaw bulbs). We examined bulk sediment organic matter (SOM) and water-extractable organic matter (WEOM) composition in a full-talik lake core (590 cm) collected from the center of a thermokarst lake in interior Alaska. These properties were then correlated with CH4 production measured on sediments from the same core.
The surface organic-rich sediments horizon of our lake core (0-150 cm depth) had the highest CH4 production potentials and rates. This section of the core also had: higher levels initial substrate, indicated by ligher levels of total soil carbon (Ctot), Corg, and total soil nitrogen (Ntot); less decomposed OM, indicated by higher Corg:Ntot ratios and lighter δ13Corg; and greater proportions of alkanes, lignin, and phenols and phenolic precursors in its bulk SOM compared to the remainder of the core. We observed statistically significant (p < 0.05) correlations between each of these bulk SOM parameters and anaerobic CH4 production measured at an incubation temperature of 3°C. Proportions of aromatic proteins and fulvic-acid like WEOM correlated with initial CH4 production rates (t = 0-34 d) but not longer-term CH4 production potentials (incubation length = 175 d), suggesting that initial WEOM composition influences short-term C mineralization rates but not longer term anaerobic C mineralization potentials. Some of the relationships we observed in our anaerobic incubations, such as a strong positive correlation (r = 0.72) between CH4 production potentials and phenols and phenolic precursors, differed from previously observed relationships in aerobic incubations. This suggests OM is utilized differently in aerobic and anaerobic environments, emphasizing the importance of examining each of these environments separately to improve our estimates of potential C emissions from thawing permafrost soils.