Stabilization of fresh organic matter from recent plant litter in thawing permafrost

The response of greenhouse gas fluxes from the circum-Arctic tundra to rapidly rising temperatures is one of the deep uncertainties in climate change research. Numerous studies have substantially increased our understanding on the production of greenhouse gases from thawing permafrost. However, we do not know how fast and to which extent fresh organic matter (OM) from decaying plant litter, the amounts of which are increasing in warming permafrost landscapes, may be stabilized in thawing permafrost.

To investigate the stabilization of fresh plant litter in thawing permafrost, we incubated permafrost samples from two Siberian islands for nine years with ¹³C-labelled plant litter under oxic and anoxic conditions. Within this experiment, we quantified CO₂ and CH₄ formation from two carbon sources: soil organic carbon (SOC) and litter carbon (litter-C). These data were used to calibrate a two-pool carbon decomposition model for determining the mean residence times (MRT) of the labile and stable carbon pools of SOC and litter-C. At the end of the incubation, we fractionated the remaining OM into the dissolved, the particulate and the mineral associated SOC and litter-C.

Roughly halve of added litter-C were mineralised to CO₂ and CH₄ after the nine years. Most of the SOC and of the litter-C were bound to the mineral fraction. However, the final litter-C mineralisation rates were 10-fold higher than those of SOC, indicating that the mineral associated carbon fraction contains OM of different decomposability. The median MRT of the stable litter-C pool was 18yr (oxic) and 52yr (anoxic), indicating a substantial stabilization of litter-C in thawing permafrost. However, the MRT of the stable permafrost SOC pool was one order of magnitude higher, demonstrating that permafrost OM is dominated by relatively stable OM. Our data shows that carbon from fresh plant litter is preferentially bound to pre-existing SOC in the mineral fraction and that this carbon still has a relatively high decomposability. Furthermore, we did not find evidence that the particulate OM is more labile than the mineral associated OM, or that the particulate OM fraction may be used as a proxy for a more decomposable carbon pool in thawing permafrost.