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

<p>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 (Q<sub>10</sub>) 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 Q<sub>10</sub> of CO<sub>2</sub>, CH<sub>4</sub> and N<sub>2</sub>O production increases with lake size as OM becomes increasingly refractory. Rates of CO<sub>2</sub> 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<sup>-1</sup> day<sup>-1</sup>), as expected for the more labile OM. However, the Q<sub>10</sub> of CO<sub>2</sub> production (1.8-2.2) was unexpectedly similar between two lakes. The small lake sediment emitted 2-3 orders of magnitude larger amount of CH<sub>4</sub> (20-583 ng C g<sup>-1</sup> day<sup>-1</sup>) as compared with large lake. The Q<sub>10</sub> values and activation energy (E<sub>a</sub>) of CH<sub>4</sub> production in small lake sediment significantly decreased from 4-10°C (Q<sub>10</sub> = 6.7; E<sub>a</sub> = 124 kJ mol<sup>-1</sup>) to 10-16°C (Q<sub>10</sub> = 3.1; E<sub>a</sub> = 76 kJ mol<sup>-1</sup>). This suggests that methanogenesis is a strongly temperature-dependent process that is more sensitive in the low-temperature range. However, Q<sub>10</sub> of CH<sub>4</sub> production in the large lake did not reveal a sensitivity to temperature probably due to too low CH<sub>4</sub> concentrations. In contrast to low CH<sub>4</sub> production, the N<sub>2</sub>O emission rates were dramatically high (0.1-1.3 μg N g<sup>-1</sup> day<sup>-1</sup>) in the sediment of the large lake. Interestingly, there was no N<sub>2</sub>O detected in the small lake sediment. Presumably, intensive denitrification in the large lake sediment outcompeted methanogenesis for substrate and energy, or enhanced CH<sub>4</sub> oxidation occurred with NO<sub>3</sub><sup>-</sup> 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 CH<sub>4</sub> and the large lakes are hotspots of N<sub>2</sub>O production.</p>