Indirect regulation of heterotrophic peat soil respiration by water level via microbial community structure and temperature sensitivity

Northern peatlands contain a considerable share of the terrestrial carbon (C) pool, which climate change will likely affect in the future. The magnitude of this effect, however, remains uncertain, due mainly to difficulties in predicting decomposition rates in the old peat layers. We studied the effects of water level depth (WL) and soil temperature on heterotrophic soil respiration originating from peat decomposition (RPD) in six drained peatlands using a chamber technique. The microbial community structure was determined through PLFA. Within the studied sites, temperature appeared to be the main driver of RPD. However, our results indicate that there exist mechanisms related to lower WL conditions that can tone down the effect of temperature on RPD. These mechanisms were described with a mathematical model that included the optimum WL response of RPD and the effect of average WL conditions on the temperature sensitivity of RPD. The following implications were apparent from the model parameterisation: (1) The instantaneous effect of WL on RPD followed a Gaussian form; the optimum WL for RPD was 61 cm. The tolerance of RPD to the WL, however, was rather broad, indicating that the overall effect of WL was relatively weak. (2) The temperature sensitivity of RPD depended on the average WL of the plot: plots with a high average WL showed higher temperature sensitivity than did those under drier conditions. This variation in temperature sensitivity of RPD correlated with microbial community structure. Thus, moisture stress in the surface peat layer or, alternatively, the lowered temperature sensitivity of RPD in low water level conditions via microbial community structure and biomass may restrict RPD. We conclude that a warmer future climate may raise RPD in drained peatlands only if the subsequent decrease in the moisture of the surface peat layers is minor and, thus, conditions remain favourable for decomposition.