Marie Mollenkopf, Monique Patzner, Eva Voggenreiter, Thomas Scholten, Carsten Müller, Thomas Borch, Casey Bryce, Prachi Joshi, Sigrid van Grinsven, E. Marie Muehe, Andreas Kappler
As permafrost thaws, vast stocks of organic carbon accumulated over long periods within these soils are vulnerable to microbial decomposition, and carbon may be released as greenhouse gases such as CO2 and CH4. The timescale and magnitude of the permafrost-climate feedback are highly uncertain as knowledge gaps remain regarding the rate of decomposition of permafrost organic carbon. These knowledge gaps stem, in part, from lacking understanding of the association between organic carbon (in the form of natural organic matter) and minerals, especially high surface area iron (oxyhydr)oxide minerals, but also how a change in plant species composition due to thawing affect the carbon cycling in such a dynamic ecosystem. Iron-carbon associations have the potential to stabilize organic matter, lower its bioavailability, and therefore protect it from biodegradation – representing a “rusty carbon sink”. Our initial work showed that up to 20% of the organic carbon in intact permafrost sites may be associated with iron(III) (oxyhydr)oxides and thereby protected from microbial decomposition. We then found that this association is broken down at the onset of thaw, likely due to the microbial reduction and dissolution of iron(III) minerals under decreasing redox conditions. Consequently, the previously protected organic carbon is thus released and becomes bioavailable. Highly productive graminoid plants dominating the thawed soils affect soil redox conditions mainly by introducing readily available organic molecules via root exudation. This further decreases soil redox conditions when graminoid plants are present, increasing CH4 production and release.
We conclude that redox shifts upon thawing result in organic matter mobilization by reductive dissolution of (iron) minerals and increased input of plant-derived organic carbon, which is likely susceptible to microbially mediated release as CO2 and CH4.