New estimates place 1330-1580 billion tons of soil carbon (C) in the northern circumpolar permafrost zone, more than twice as much C than in the atmosphere. Permafrost thaw and the microbial decomposition of previously frozen organic C is considered one of the most likely positive feedbacks from terrestrial ecosystems to the atmosphere in a warmer world. Alaskan tundra soils were exposed to experimental field warming for two consecutive winters, increasing soil temperature by >2°C down to 40 cm depth. Soil layers from three depths within the active layer, and separate soil samples from deep in the permafrost, were then incubated under aerobic conditions at 15ºC and 25ºC for over 1100 days in the laboratory. Carbon fluxes were measured periodically to estimate relative sizes and decay rates of organic matter fractions, separated into fast and slow decomposing C based on flux kinetics. Temperature sensitivity was estimated using both a short-term temperature manipulation performed at 14, 100 and 280 days of incubation, and via the equal-C method calculated continuously. Functional diversities of the soil microbial communities were measured using a microbial functional gene array, with over 20,000 probes target genes involved in the degradation of varying C substrates.
Results/Conclusions
The slow decomposing C pool, representing close to 95% of total C in the top 25 cm soils, had a higher temperature sensitivity than the fast decomposing C pool and also dominated the total amount of C released by the end of the incubation. Overall, the slow pool had similar temperature sensitivities, represented by Q10, of 2.55 ± 0.03 and 2.19 ± 0.13, estimated by the short-term and equal-C method respectively. In contrast, the fast C pool had a temperature sensitivity of 1.16 ± 0.30 for the equal-C method. Kinetics modeled after 3 years of incubation estimated that a larger fraction of C was in the fast pool but that the turnover time of that pool decreased, in comparison to kinetics modeled after 1 year. In contrast to 15ºC, the 25ºC microbial communities showed reduced diversities of C-degradation functional genes in the early stage of the incubations. However, as the incubations continued the 25ºC communities converged with the 15ºC communities with respect to the microbial genes utilized in the degradation of labile to recalcitrant C substrates. Two winter seasons of experimental warming did not affect the dynamics and temperature sensitivity of SOM decomposition or the microbial C-degradation genes during incubation.