Patterns and drivers of organic matter decomposition in peatland open-water pools

Peatland pools are unvegetated, inundated depressions that cover up to 30 % of the surface of many temperate and boreal peatlands and that are net carbon (C) sources within C-accumulating ecosystems. The emission of carbon dioxide (CO2) and methane (CH4) from peatland pools comes from the degradation of organic matter (OM) that comprises the surrounding matrix. It is, however, not clear how decomposition rates in pools, which define their function and distinguish them from other aquatic ecosystems, vary spatially and what mechanisms drive these variations. We first quantified rates of OM decomposition from fresh litter at different depths in six pools of distinct morphological characteristics in a temperate ombrotrophic peatland using litterbags of Sphagnum capillifolium and Typha latifolia over a 27-month period. Rates of decomposition were faster for T. latifolia than S. capillifolium and, overall, faster at the pool surface and decreased with increasing depth. We then measured potential CO2 and CH4 production from the sediments of the same six pools by performing 35 d laboratory incubations. Pool sediment chemistry was variable among pools and influenced the production of CH4 and CO2 from sediments, with decreasing CO2 production with increasing OM humification and decreasing CH4 production with increasing nitrogen-to-phosphorus ratio. Both CH4 production and CO2 production from pool sediments were higher in the 1 m deep pools but similar in the shallow < 1 and the > 1.5 m deep pools. When combining both experiments, our results indicate that OM decomposition in peatland pools is highly variable and mostly related to the environmental conditions in which it occurs as a function of general pool depth rather than to OM chemistry. Overall, we show that OM degradation and C emissions in peatland open-water pools may increase over time in warmer and drier climate conditions.