Effect of mineral sediments on carbon mineralization, organic matter composition and microbial community dynamics in a mountain peatland

Abstract

Carbon (C) dynamics in northern peatlands are an important factor in the global C balance under climate change scenarios. They are microbially driven and influenced by the chemical composition of organic matter. Peatlands in the Rocky Mountains are usually formed on mineral sediments or developed with interbedded mineral lenses, which have been found to affect soil properties such as volumetric water content, pH, TOC and TN. Our objective was to investigate whether the presence and relative depth of mineral horizons (i.e., stratified mineral horizons) affect microbial community structure and C composition, and in turn influence C mineralization. Three organic soil profile types were selected in the Sibbald research wetland of southwestern Alberta: peat over silty mineral over calcareous sediment (PMC), peat over silty mineral over peat (PMP), and sedge peat over moss peat profiles (PP). Peat samples were subjected to C composition and microbial community abundance and structure measurement and then incubated to test potential C mineralization. The main differences were detected in subsurface peat. In subsurface peat above mineral sediments (PMC, PMP) versus at equivalent depth in PP, the presence of a mineral horizon caused different C mineralization (mg C-CO2 kg−1 soil) among soil types (PP > PMC and PMP). In addition, specific C mineralization (mg C-CO2 kg−1 SOC) decreased with depth in subsurface peat in PP, but not in PMP, as greater volumetric water content (θv) above the mineral horizon created anaerobic conditions in PMP. Microbial community structures also differed between PMP and PP due to different θv in peat below mineral sediments. Recalcitrant C: labile C, bacteria: fungi, and microbial physiological stress were greatest in the subsurface peat above mineral sediments. Depth had an even greater effect: both C mineralization and microbial abundance decreased significantly with depth. Moreover, microbial community structure mainly grouped according to relative depth. Overall, our findings indicated that stratified mineral horizons affected C mineralization, microbial community structure, and peat chemistry in subsurface peat.