Long Term Thawing Experiments on Intact Cores of Arctic Mineral Cryosol: Implications for Greenhouse Gas Feedbacks from Global Warming

Abstract:

Mineral cryosols comprise >87% of Arctic tundra. Much attention has focused on high-organic carbon cryosols and how they will respond to global warming. The biogeochemical processes related to the greenhouse gas release from mineral cryosols, however, have not been fully explored. To this end, seventeen intact cores of active layer and underlying permafrost of mineral cryosol from Axel Heiberg Island, Nunavut, Canada, were subjected to 85 weeks of thawing at 4.5°C under various treatment regimes. The fluxes of CO₂ and CH₄ across the atmosphere-soil boundary and vertical profiles of the gas and water chemistry and the metagenomes were determined. The flux measurements were compared to those of microcosms and field measurements. The main conclusions were as follows: 1) CO₂ emission rates from the intact cores do not behave in the typical fast to slow carbon pool fashion that typify microcosm experiments. The CO₂ emission rates from the intact cores were much slower than those from the microcosm initially, but steadily increased with time, overtaking and then exceeding microcosm release rates after one year. 2) The increased CO₂ flux from thawing permafrost could not be distinguished from that of control cores until after a full year of thawing. 3) Atmospheric CH₄ oxidation was present in all intact cores regardless of whether they are water saturated or not, but after one year it had diminished to the point of being negligible. Over that same time the period the metagenomic data recorded a significant decline in the proportion of high-affinity methanotrophs. 4) Thaw slumps in the cores temporarily increased the CH₄ oxidation and the CO₂ emission rates. 5) The microbial community structures varied significantly by depth with methanotrophs being more abundant in above 35 cm depth than below 35 cm depth. 6) Other than the diminishment of Type II methanotrophs, the microbial community structure varied little after one week of thawing, nor even after 18 months of thaw.