Thawing Siberian permafrost stabilizes organic carbon from recent plant litter inputs

Abstract. Greenhouse gas release due to microbial decomposition of thawing permafrost organic matter receives ample attention but the other side of the permafrost soil carbon budget, the stabilization of organic matter due to rising plant litter input in a greening Arctic has hardly been addressed. Here we explore whether thawing permafrost material may act as a long-term sink of fresh plant litter carbon. To identify the magnitude and drivers of litter carbon stabilization in thawing permafrost material, we incubated samples from the permafrost layer under oxic and anoxic conditions with 13C-labelled plant litter. Subsequently, we used the microbial CO2 and CH4 production from the added litter carbon (litter-C) and from the carbon in the thawed permafrost material (permafrost-C) to calibrate a carbon decomposition model with a fast and a slow carbon pool. Beside the size of the different pools, their mean residence times (MRT) were calculated as an indicator for carbon stabilization in these soils. Finally, we fractionated the remaining organic matter into a dissolved, a mineral-associated and a particulate fraction. At the end of the experiment, after nine years, on average 40 % to 60 % of the added litter-C persisted in the thawed permafrost material. The MRT of the slow litter-C pool of 18 years (oxic) and 52 years (anoxic) indicate a substantial stabilization of fresh litter-C over the course of the experiment. More than 80 % of the remaining litter-C was part of the mineral-associated fraction, but in contrast to current understanding, litter decomposability was positively correlated with the size of the mineral bound litter-C pool. Although the fraction of mineral-bound permafrost-C (64 % to 68 %) was significantly smaller than of litter-C, the MRT of the slow permafrost-C pools was more than 10-fold higher. Hence, the size of the mineral bound carbon pool alone may not be a suitable measure of carbon stabilization. We furthermore identified interactions between new litter carbon and pre-existing mineral-bound carbon from the thawed permafrost material as an important driver of litter-C stabilization. Such interactions could reduce net carbon emissions from thawing permafrost and add complexity to the permafrost carbon climate feedback.