Iron mineral dissolution releases iron and associated organic carbon during permafrost thaw

Abstract

It has been shown that reactive soil minerals, specifically iron(III) (oxyhydr)oxides, can trap organic carbon in soils overlying intact permafrost, and may limit carbon mobilization and degradation as it is observed in other environments. However, the use of iron(III)-bearing minerals as terminal electron acceptors in permafrost environments and thus their stability and capacity to prevent carbon mobilization during permafrost thaw is poorly understood. We have followed the dynamic interactions between iron and carbon, using a “space for time” approach, across a thaw gradient in Abisko (Sweden), where wetlands are expanding rapidly due to permafrost thaw. We show through bulk (selective extractions, EXAFS) and nanoscale analysis (correlative SEM and nanoSIMS) that organic carbon is bound to reactive Fe primarily in the transition between organic and mineral horizons in palsa underlain by intact permafrost. During thaw, water-logging and O2 limitation lead to reducing conditions and an increase in abundance of Fe(III)-reducing bacteria which favor mineral dissolution and drive mobilization of both iron and carbon along the thaw gradient. By providing a terminal electron acceptor, the “rusty carbon sink” is effectively destroyed along the thaw gradient and cannot prevent carbon release with thaw.

Cryoturbation leads to iron-organic carbon associations along a permafrost soil chronosequence in northern Alaska

“…In parallel, a dithionite-citrate bicarbonate extraction was carried out to selectively extract reactive Fe minerals (defined here as reductively dissolvable by dithionite-citrate extraction) such as ferrihydrite, goethite, lepidocrocite, akaganeite and hematite nanoparticles (Cornell & Schwertmann, 2003;Coward et al, 2017;Mehra & Jackson, 1960;Raiswell et al, 1994). This extraction also allows for the quantification of OC that is mobilized during the reductive dissolution of reactive Fe minerals (in the following referred to as Fe-associated OC) (Lalonde et al, 2012;Mu et al, 2016;Mu et al, 2020;Patzner et al, 2020). We followed the dithionite-citrate extraction which is performed for 16 h on a rolling shaker under room temperature and anoxically in the dark (Coward et al, 2017;Wagai et al, 2013; and combined different approaches to account for potential difficulties when using this extraction method as previously discussed in detail by Patzner et al (2020) and shortly discussed in the following.…”

“…This extraction also allows for the quantification of OC that is mobilized during the reductive dissolution of reactive Fe minerals (in the following referred to as Fe-associated OC) (Lalonde et al, 2012;Mu et al, 2016;Mu et al, 2020;Patzner et al, 2020). We followed the dithionite-citrate extraction which is performed for 16 h on a rolling shaker under room temperature and anoxically in the dark (Coward et al, 2017;Wagai et al, 2013; and combined different approaches to account for potential difficulties when using this extraction method as previously discussed in detail by Patzner et al (2020) and shortly discussed in the following. Due to the instability of dithionite in solution (Varadachari et al, 2006), powdered dithionite was added to the sample to reach a final concentration of 0.1 M by adding 3.125 mL of a 0.27 M trisodium citrate, 0.11 M sodium bicarbonate solution (pH 7, N2:CO2 (90:10, v:v) headspace).…”

“…A pre-test carried out under the same conditions (pH, ionic strength) showed that without citrate, 43.5±17.6 % less reactive Fe and 37.2±4.4 % less reactive Fe-associated OC was obtained in an organic horizon of a medium aged soil core (Table SI 1). To account for effects of soil drying on reactive Fe minerals and Fe-associated OC, comparable permafrost soil material from a desiccating palsa in northern Sweden (Patzner et al, 2020) was dried under anoxic and oxic conditions at room temperature and 60°C. We obtained similar reactive Fe and Fe-associated OC contents by the dithionite-citrate extraction method for all drying techniques (Table SI 2).…”

“…Previous studies on permafrost soils have intensively focused on the decomposition of the stored OC (Estop-Aragones et al, 2020;Hopple et al, 2020;Schädel et al, 2016), whereas only a few have focused on the stabilization mechanisms of OC which can mitigate the permafrost carbon feedback (Mu et al, 2016;Mu et al, 2020;Mueller et al, 2017;Osterkamp & Romanovsky, 1999;Patzner et al, 2020;Prater et al, 2020;Schuur et al, 2015;Wang et al, 2020). In soils, the fate of OC is determined by an interplay of various physical, chemical and biological components (Lehmann et al, 2020).…”

“…Recent studies point towards a significant proportion of reactive Fe associated OC in permafrost soils (Herndon et al, 2017;Mu et al, 2016;Mu et al, 2020;Patzner et al, 2020;Sowers et al, 2020). In the permafrost soil chronosequence of drained thaw lake basins in northern Alaska we have studied, found the oldest OC fraction directly associated with minerals, especially with Fe (oxyhydr)oxides (Mueller et al, 2017;Mueller, Steffens and Buddenbaum, 2020).…”