Peat carbon stocks and potential microbial lability of boreal peatlands with varying permafrost histories

Abstract:

Large stores of C in the form of peat are stored in permafrost, particularly in the boreal discontinuous permafrost zone. Ongoing climate change is causing widespread permafrost thaw in boreal peatlands, a trend which is expected to continue this century and thus make large stores of soil C available for microbial processes and mineralization. Permafrost thaw in boreal peatlands is often associated with an ecosystem shift from dry peat plateau to wet bog surfaces, and the net C balance following thaw is determined by the balance between the mineralization of plateau peat and the new accumulation of bog peat on top. In this study we collected soil cores (~3 m deep) from one peat plateaus and four bogs that differed in time since thaw (approximately 10, 50 and 500 years since thaw). In order to assess the potential microbial lability, we incubated 25 soil samples from each core under aerobic conditions at 17.5 deg C. Mineralization rates were 1-2 order of magnitude higher near the surface than at depth, but near surface samples also had high variability among cores.  Variability in peat microbial lability near the surface was related to thaw history and to differences in characteristics between plateau and bog peat. Mineralization rates of peat samples from below 1 m depth and down to the interface with mineral soil at 3 m were consistently low and had no difference among cores. Mineralization rates during the first 3 months of incubation for deep plateau peat samples were equivalent to 1% soil C losses per year. Relatively low microbial lability of deep peat in combination with high rates of new peat accumulation during the initial stages of bog development suggests that there is net C accumulation immediately following thaw but that the sink strength weakens or reverses during later stages when new accumulation rates diminish.