Seasonal Responses of Extracellular Enzyme Activity and Microbial Biomass to Warming and Nitrogen Addition

Soil microbial responses to climate warming in temperate regions may interact with the effects of increased atmospheric N deposition. In addition, the combined effects of these factors on microbial activity during the plant growing season may differ from the effects over winter, when reduced plant soil C inputs and soil freezing can alter microbial nutrient availability and demand. We examined seasonal changes in soil extracellular enzyme activity (EEA), microbial biomass C and N, and soil fungal and bacterial content in a warming and N addition experiment in a temperate old field. For PEA, we examined both hydrolases (organic C degrading enzymes, a chitinase and phosphatase) and ligninases (phenol oxidase and peroxidase). While both hydrolase and ligninase activities exhibited significant seasonal variation, LEA was unresponsive to the experimental treatments. Microbial biomass C increased with warming year round, however, and microbial biomass N increased with N addition but only over summer. Despite increased microbial biomass in response to warming, phosphatase was the only enzyme that exhibited a significant change in specific activity (enzyme activity per unit of microbial biomass) in response to warming. Likewise, soil fungal and bacterial biomass varied seasonally, but treatment effects on these variables were minimal. Overall, while the effects of N addition on microbial N varied seasonally, microbial responses were relatively insensitive to the warming and N addition treatments in our experiment. This insensitivity was unexpected given the large treatment effects on plant productivity and soil N dynamics documented during the same time frame in the field experiment.