Microbial activity and litter decomposition under snow cover in a cool-temperate broad-leaved deciduous forest

Winter CO₂ emissions from soil and snow-covered surfaces can represent a significant fraction of the annual carbon cycle in cool-temperate broad-leaved deciduous forests. The sources of this CO₂ production are, however, poorly understood. To clarify the role of saprophytic microorganisms in the carbon cycle during the winter (December-April), mass loss rates of organic substrates (leaf litter and cellulose sheets), and microbial activity and fungal biomass in the litter layer of a cool-temperate broad-leaved deciduous forest in Japan were investigated. The mass loss rate of leaves reached 13%, which accounted for 26% of the annual mass loss. Microbial respiration in the leaf litter, measured in the laboratory, was detected even at -2°C. The ergosterol content (an indicator of fungal biomass) of the cellulose sheets increased significantly in the snow-covered season. Seven genera of fungi including Varicosporium elodeae, Epicoccum purpurascens and Cylindrocarpon destructans were isolated from leaf litter collected from under snow cover. Microbial respiration during the winter was estimated based on the relationships between the amount of leaf litter available, the temperature dependency of microbial respiration and the temperature in the litter layer. The microorganisms in the litter layer emitted 31 gCO₂-C m^-2, which accounts for 38% of the total CO₂ efflux during the winter. These results suggest that through soil respiration, microorganisms in the litter layer play an important role in the carbon cycle during the winter.