Mobilization of stable organic carbon in thawing permafrost by fresh organic matter from recent vegetation

Abstract:

Permafrost affected soils contain 1,300 Pg organic carbon which is about twice the amount of the global vegetation. Most of this carbon (C) is locked in the perennially frozen ground (permafrost) and only a minor part is stored in the seasonal surface thaw layer (active layer). Rising arctic temperatures will cause deeper active layer thaw and permafrost degradation, which liberates additional soil organic matter (SOM) for microbial mineralization. After thaw, old permafrost C will be mixed with fresh organic matter from plant residues, e.g. by cryoturbation or leaching. Recent incubation studies have increased our understanding on how fast permafrost SOM may be mineralized to the greenhouse gases (GHG) carbon dioxide (CO₂) and methane (CH₄). After initial maximum GHG production from labile SOM components (labile C pool) mineralization rates slow down since the remaining SOM is more recalcitrant (stable C pool). The current study investigates if this stabile C pool may be mobilized by fresh organic matter from recent vegetation (“priming effect”). Therefore, permafrost samples (¹⁴C ages 0.1 - 17 ka BP) from the Siberian tundra were spiked with a ¹³C-labeled sedge (Carex aquatilis) after the samples were pre-incubated for 4 years. The amount of C released from permafrost SOM was calculated from the δ¹³C-values of produced GHG using a mixing model. Under aerobic conditions, all samples showed an accelerated mineralization of SOM after the addition of C. aquatilis (positive priming). After 4 months, which is about one vegetation period, the measured CO₂ production exceeded the estimated CO₂ release without labile plant material by 60 ± 28%. Under anaerobic conditions, priming was more pronounced increasing CO₂ production by 100 ± 67% and CH₄ production by 33 ± 32%. The CO₂/CH₄ ratio increased from 0.9 before priming to 1.3 after priming. The total mineralization of SOM over 4 months was significantly higher under aerobic (14.2 ± 6.1 µmol CO₂-C gdw^-1) than under anaerobic conditions (5.8 ± 2.1 µmol CO₂-C+CH₄-C gdw^-1). However, if considering the higher global warming potential of CH₄, the anaerobic release of C expressed as CO₂ equivalents (28.2 ± 11.5 µmol CO₂-C equiv. gdw^-1) was twice the aerobic release.