Microbial community and C cycling across permafrost peatlands in the Hudson Bay Lowlands: feedbacks to in-situ degradation and simulated warming

Abstract

Peatlands in the Hudson Bay Lowlands (HBL) constitute a sensitive and globally significant store of carbon, estimated at approximately 30 Pg, where in the northern portion of the HBL a significant amount of this carbon is stored in permafrost. Particularly, near-surface permafrost in the HBL occurs as palsas which are omnipresent in the HBL. However, permafrost degradation in the HBL is proceeding at an accelerated pace as changing sea-ice dynamics in Hudson Bay amplify the regional effects of climate change. The degradation of permafrost leads to decomposition of organic carbon, which produces two important greenhouse gases; carbon dioxide (CO2) and methane (CH4). Under the context of climate change and degrading permafrost conditions in the HBL, this research investigated the microbially mediated production of CO2 and CH4 from palsas and adjacent thermokarst features from the Hudson Bay Lowlands, and the effects of palsa evolution on these processes. In August of 2017, cores of active layer, permafrost, and thermokarst were collected from five replicate palsas in the continuous zone of permafrost within a 150 km2 watershed selected for monitoring by the Ontario Ministry of Natural Resources and Forestry. Short-term aerobic incubations (7 days) at field moisture conditions were used to assess methane production and consumption, and longterm (225 days) anaerobic incubations were used to assess methane production. Peat chemistry was characterized through elemental analysis, including peat substrate chemistry through the use of Fourier-Transform Infrared (FTIR) Spectroscopy. Finally, microbial communities from samples pre- and post-incubation were characterized via Illumina highthroughput sequencing for 16s rRNA. This study finds that active layer samples are capable of oxidizing CH4 under field conditions, whereas permafrost samples generally produced CH4. Under anaerobic conditions, thermokarst samples are much more prolific producers of CH4 in comparison to permafrost and active layer samples. This study suggests that it is peat chemistry and changes in vegetation associated with the evolution of palsas responsible for increased production of CH4 from thermokarst. This is the result of combined physical and chemical conditions both conducive to methanogenesis, where differing peat chemistry increases the availability of substrates utilized by methanogens, as well as increased zones of anaerobiosis. This research highlights the need for permafrost carbon models to include degradation of palsas via thermokarst encroachment, as this study demonstrates that it has important implications for the production and release of CH4.