Identity of Iron-Carbon Associations across a Permafrost Thaw Gradient and their Contribution to Greenhouse Gas Release

Abstract

Permafrost soils store a vast organic carbon (OC) stock (ca. 1300 Pg). Increasing global temperatures are causing permafrost to thaw, which releases large quantities of C in the form of greenhouse gases (GHG), such as carbon dioxide (CO2) and methane (CH4), amplifying global climate change. However, accurate predictions of the total extent of GHG release from permafrost soils are not yet possible, since the degradation of SOC depends largely on poorly constrained microbial degradation rates under these changing conditions. One factor influencing microbial degradation is the association of OC with soil minerals. On the one hand, minerals with high surface area, such as poorly crystalline ferric iron (Fe(III)) (oxyhydr)oxides, could lower OC bioavailability under oxic conditions. On the other hand, increasingly waterlogged, anoxic conditions due to permafrost thaw could lead to microbial Fe(III) reduction which could increase CO2 emissions and affect CH4 emissions. In this PhD work, we simulated the formation of Fe(III)-OC associations in permafrost soils over a thaw gradient to draw conclusions on the bioavailability of Fe-bound OC and assessed the impact of formed Fe(III)-OC associations on GHG release. To accomplish this, soil samples were collected across a representative permafrost thaw gradient at Stordalen Mire (Abisko, Sweden). Adsorption and coprecipitation experiments of soil-derived dissolved OC with poorly crystalline Fe(III) (oxyhydr)oxides were conducted in order to determine the maximum binding capacities of OC and assess which kind of OC compounds are preferentially bound. Afterwards, synthesized Fe(III)-OC associations were exposed to anoxic conditions in soils across the thaw gradient to investigate their potential reductive dissolution and quantify following changes to CO2 and CH4 emissions. The results of this work therefore advance our understanding of the role of Fe(III)-OC associations for OC bioavailability and GHG emissions during future permafrost thaw.