Thawing arctic permafrost contains 30-50% of global soil carbon and is expected to drive substantial alterations to carbon (C) cycling that will accelerate climate change. As permafrost thaws, old C may decompose and be released as carbon dioxide (CO2) and methane (CH4, a more potent greenhouse gas). However, thaw can also increase new C inputs to the soil from plant litter when perennial shrub communities transition to more productive annual wetland plants. The impacts of these combined changes on the C balance depend not only on total C inputs, but also on their influence on microbial degradation pathways such as those that drive priming effects. Priming effects at the aerobic-anaerobic interface are not well studied but may be especially important since this is where litter deposition takes place. Here we test the hypothesis that litter deposition may alter microbial activity and production of CO2 and CH4 at an aerobic-anaerobic interface by 1) fueling rapid oxygen drawdown from aerobic decomposition, thus expanding the anaerobic zone and 2) increasing substrate availability for methanogenesis. These processes can be expected to increase CH4 production both from decomposition of the litter itself and from increased anaerobic decomposition of SOM (priming).
We tested these ideas by allowing decomposition of 13C-labeled plant material in arctic peats from pre- and post-permafrost thaw areas in incubations under near-in situ conditions (which included oxic headspace and potentially anoxic regions within soil). We traced the labeled C into CO2 and CH4 emissions, fermentation products (using NMR) and the microbial community (using SIP-metagenomics).
Results/Conclusions
We observed rapid release of labeled C from litter decomposition and strong stimulation of CH4 production in post-thaw habitats. Microbial and NMR results supported all the hypothesized mechanisms of litter influence on CH4 production in the fully thawed sedge-dominated system, but only the first three in the sphagnum-dominated partial thaw system, presumably due to chemical inhibition of decomposition by sphagnum litter. If these processes operate at similar rates in the field, litter decomposition at the peat surface could account for more than 10% of the total CH4 fluxes measured in post-thaw fen areas with an additional 2% due to priming, versus only 1% and 0.2% in bog areas. These results indicate that litter deposition impacts on microbial activity may be a key driver of CH4 production, thus playing an important role in determining net climate forcing of thawed permafrost peatlands.