The influence of water table position on soil microbial processes and carbon mineralization in a mid-latitude spruce peatland

High latitude forest and peatland soils represent a major global carbon store sensitive to the impacts of global climate change. While increased temperatures may impact rates of microbial enzyme activity and greenhouse gas release from peat soils, the interaction between increased temperatures and changing precipitation patterns is projected to simultaneously reduce soil moisture and water table (WT) height in high latitude peatlands. WT reduction increases oxygen diffusion within the peat profile and potentially impacts (1) microbial activity and enzyme production, and (2) the rate of carbon mineralization and greenhouse gas emission. We performed an experiment investigating the influence temperature and available oxygen on rates of microbial enzyme activity and carbon mineralization across a 0-40 cm depth-to-water-table gradient in Caribou Bog, Orono, ME. We incubated peat samples acrotelm and catotelm peat samples at four temperatures and three oxygen concentrations for 28 days in order to investigate the temperature and oxygen sensitivity of extracellular enzyme activity and carbon gas emission. We assayed rates of four hydrolytic and two oxidative exoenzymes that depolymerize carbon (C), nitrogen (N), or phosphorus (P) and compared enzymatic activity to rates of carbon mineralization and CO2 production in incubated samples. Microbial biomass increased significantly with water table depth and incubation temperature, but did not vary significantly with sampling depth or [O2]. In contrast, hydrolytic and oxidative enzyme activity consistently decreased with sampling depth, but did not typically vary significantly with site water table position. The chitinase, N-acetyl-glucosaminidase, however, demonstrated significantly higher activity at low water table sites than high water table sites, potentially due to high fungal abundances at low water table height. Enzyme activity increased with temperature, although increases were not significant above 21°C and weakly increased with [O2]. CO2 mineralization increased with depth to water table, sampling depth, oxygen concentration, and incubation temperature, with microbial CO2 production most sensitive to increased temperature at the highest water table sites. Although total microbial biomass did not differ significantly between samples taken above and below the water table, CO2 production was significantly higher in the acrotolm and associated with higher rates of oxidative and hydrolytic enzyme activity. These distinct functional responses despite identical total biomass suggest microbial community-level differences driving distinct patterns in enzyme expression and C release. As peatland water tables fall, shifts in the proportion of catotelm-associated microbial communities to acrotelm-associated microbial communities could drive C losses and contribute to a reduction in the long-term peatland carbon store.