Thermokarst lake size controls greenhouse gases production but not its temperature sensitivity

Thermokarst lakes formed form permafrost thawing under the global warming are an important source of greenhouse gases (GHG). However, the driving mechanisms and temperature sensitivity (Q₁₀) of GHG emissions from the sediments of thermokarst lakes require deeper understanding. From existing studies of organic matter (OM) turnover and thermodynamic theory, it is known that more refractory OM has a higher temperature sensitivity of decomposition. To test the relevance of such effects in thermokarst lakes, sediments of two differently sized lakes (small = young, DOC rich; large = mature, DOC poor) from Western Siberia were anoxically incubated under three temperatures (4, 10, 16°C) for 49 days. We hypothesized that the Q₁₀ of CO₂, CH₄ and N₂O production increases with lake size as OM becomes increasingly refractory. Rates of CO₂ production increased exponentially with temperature in sediments from lakes of both sizes, whereas the highest rates were observed for sediments of the small lake (4.2-9.7 μg C g^-1 day^-1), as expected for the more labile OM. However, the Q₁₀ of CO₂ production (1.8-2.2) was unexpectedly similar between two lakes. The small lake sediment emitted 2-3 orders of magnitude larger amount of CH₄ (20-583 ng C g^-1 day^-1) as compared with large lake. The Q₁₀ values and activation energy (E_a) of CH₄ production in small lake sediment significantly decreased from 4-10°C (Q₁₀ = 6.7; E_a = 124 kJ mol^-1) to 10-16°C (Q₁₀ = 3.1; E_a = 76 kJ mol^-1). This suggests that methanogenesis is a strongly temperature-dependent process that is more sensitive in the low-temperature range. However, Q₁₀ of CH₄ production in the large lake did not reveal a sensitivity to temperature probably due to too low CH₄ concentrations. In contrast to low CH₄ production, the N₂O emission rates were dramatically high (0.1-1.3 μg N g^-1 day^-1) in the sediment of the large lake. Interestingly, there was no N₂O detected in the small lake sediment. Presumably, intensive denitrification in the large lake sediment outcompeted methanogenesis for substrate and energy, or enhanced CH₄ oxidation occurred with NO₃^- as the electron acceptor. In summary, the temperature sensitivity of GHG production in thermokarst lake sediments depended more on gas species than on lake size. Nevertheless, the size of thermokarst lakes can serve as an indicator of biogeochemical processes in the sediments, as the small lakes are hotspots of CH₄ and the large lakes are hotspots of N₂O production.