Microbial necromass formation, enzyme activities and community structure in two alpine elevation gradients with different bedrock types

Abstract

Microbial necromass is an important part of soil organic carbon (SOC), but the changes in its contribution along elevational gradients and soil types are poorly understood. At two sites, the alkaline site Hochschwab and the acidic site Rauris, soil samples from three elevation levels and depths were analysed for amino sugars as microbial necromass indices, extracellular enzymes, and phospholipid fatty acids (PLFA) as microbial biomass indices. The three elevation levels at each site represented similar temperature and precipitation regimes. Microbial necromass contribution to SOC was lower (26%) at the alkaline site than at the acidic site (37%), indicating lower transformation of microbial biomass into stable SOC and support the increased accumulation of fungal necromass under N limitation at Hochschwab. Fungi generally dominate microbial necromass, with 88% at Hochschwab and 77% at Rauris. At the alkaline site Hochschwab, phenoloxidase, peroxidase and phosphatase enzymes activities were lower by 96%, 98% and 75%, respectively than at the acidic Rauris site, but not those of the hydrolytic enzymes β-glucosidase, exo-glucanase, exo-chitinase, and protease, indicating retarded decomposition of plant residues. Gram positive bacteria dominate bacterial biomass at both sites, with 70% at Hochschwab and 72% at Rauris. The higher fungal to bacterial PLFA ratio suggest increased contribution of fungi to total microbial PLFA at Hochschwab due to the better adaptability of fungi to the site-specific environmental conditions. Effects along alpine elevation gradients depend on vegetation induced changes in soil properties, which interact with bedrock properties.