Is soil organic carbon always responsible for the anomalous greenhouse gases emissions recorded from the Antarctica permafrost?

Recent studies carried out on permafrost in both Arctic and Antarctic regions have highlighted how these areas represent among the largest natural carbon (C) reservoirs at a global scale. The gradual but steady thawing of permafrost, as projected by various climate change scenarios, can lead to the release of a significant amount of greenhouse gases (i.e., CO2 and CH4) into the atmosphere. Currently, this aspect is not often considered in climate studies. Unlike Arctic permafrost, Antarctic permafrost is characterized as a cold desert, being characterized by a surface not covered by ice lower than 1%, the absence of vascular plants, and an extremely low concentrations of soil organic carbon (SOC). At the same time, recent studies have highlighted significant and anomalous greenhouse gas emissions in such a context (i.e., up to 3.4 %v and 18.4 ppmv for CO2 and CH4, respectively). The main objective of this work, which is part of the SENECA project (SourcE and impact of greeNhousE gases in AntarctiCA), is to contribute understanding the origin of these emissions, distinguishing potential biological contributions (e.g., SOM mineralization) from physico-chemical (e.g., carbonate dissolution) and geogenic (i.e., deep brines) factors. Sixty-six soil samples, collected from two valleys (Taylor Valley and Wright Valley) at different depths (0-10, 10-20, 20-30, 30+ cm), were investigated from the physical (i.e., temperature, texture, density), chemical (i.e., pH, EC, organic C, total N and S, major and trace elements, mineralogy) and biological (i.e., total DNA, bacterial community composition via 16S-metabarcoding) point of view. Obtained data underline a very low SOC (0.09±0.05 %) and total N (0.012±0.007 %) content, which occurs almost exclusively in form of particulate organic matter. The almost undetectable mineral-associated organic matter is consistent with the coarse soil texture (sand: 88±14 %) and mineralogy (33±9 % quartz, 31±3 % NaCa-Feldspar). No correlation among SOC, total N and DNA was found. The structure of the bacterial communities observed was consistently pointing out at unrelated and stochastic assemblage processes leading to a passive, non-deterministic accumulation of taxa which are not expectably involved in an active physiology of the site. In conclusion, results seem to support the geogenic origin of CO2 and CH4 (i.e., deep brines) without a consistent involvement from the occurring biota.