Nutrient availability and pH jointly constrain microbial extracellular enzyme activities in nutrient-poor tundra soils

Tundra soils, which usually contain low concentrations of soil nutrients and have a low pH, store a large proportion of the global soil carbon (C) pool. The importance of soil nitrogen (N) availability for microbial activity in the tundra has received a great deal of attention; however, although soil pH is known to exert a considerable impact on microbial activities across ecosystems, the importance of soil pH in the tundra has not been experimentally investigated. We tested a hypothesis that low nutrient availability and pH may limit microbial biomass and microbial capacity for organic matter degradation in acidic tundra heaths by analyzing potential extracellular enzyme activities and microbial biomass after 6 years of factorial treatments of fertilization and liming. Increasing nutrients enhanced the potential activity of beta-glucosidase (synthesized for cellulose degradation). Increasing soil pH, in contrast, reduced the potential activity of beta-glucosidase. The soil phospholipid fatty acid concentrations (PLFAs; indicative of the amount of microbial biomass) increased in response to fertilization but were not influenced by liming. Our results show that soil nutrient availability and pH together control extracellular enzyme activities but with largely differing or even opposing effects. When nutrient limitation was alleviated by fertilization, microbial biomass and enzymatic capacity for cellulose decomposition increased, which likely facilitates greater decomposition of soil organic matter. Increased soil pH, in contrast, reduced enzymatic capacity for cellulose decomposition, which could be related with the bioavailability of organic substrates.