Microbial community response to permafrost thaw in Alaskan soils: Implications for C cycling

Background/Question/Methods

The fate of carbon contained within permafrost in boreal forest environments is an important consideration for the current and future carbon cycle. Boreal forest soils contain large amounts of organic carbon that may be metabolized as soils warm in northern latitudes. Yet little is known about the factors that may limit or stimulate decomposition once soils thaw. We tested the hypothesis that microbial populations and /or chemical quality of organic matter are lower in permafrost soils compared to active layer soils, thereby limiting decomposition rates in permafrost soils. We examined permafrost and active layer soils from three regions of interior Alaska: the Hess Creek watershed near the Yukon River (high carbon storage in permafrost); Smith Lake, in Fairbanks, AK (moderate carbon storage in permafrost), and Coldfoot, just north of the Arctic Circle (low carbon storage in permafrost). Gas fluxes from permafrost and active layer soils from these regions were measured in the lab under aerobic and anaerobic conditions during a slow warm up: two days at -5 °C, two days at 0 °C, followed by 3 months at 5° C to calculate Q10 values. Results/Conclusions Temperature sensitivity of decomposition was lower in permafrost soils compared to active layer soils. Respiration per unit soil C over 100 days was higher in permafrost soils compared to active layer soils. Soil nutrients, leachable dissolved organic carbon quality and quantity, and NMR spectroscopy of the soils revealed that the organic matter within permafrost soils is as labile, or even more so, than surface soils. Microbial communities were examined using a combination of qPCR and microarray techniques. Microarray data from permafrost, thawed soils, and anaerobic thawed soils is currently being processes to examine microbial response to thaw at the population level. Microbial abundances (qPCR for fungi, bacteria, and subgroups: methanogens and Basidiomycetes), and exoenzyme activities involved in decomposition were lower in permafrost soils compared to active layer soils, which could have cause the reduced Q10 values in permafrost soils. In the anaerobic treatments, surprisingly low methane fluxes were measured, and were associated with low methanogen abundances. Taken together, these results suggest that permafrost soils have high inherent decomposability, but low microbial abundances and activities reduce the temperature sensitivity of carbon fluxes.