Predicting soil microbial responses to drought: a laboratory analysis

Soils are fundamental components of terrestrial ecosystems, sustaining plant productivity, nutrient cycling, and carbon storage. Microbial communities play a central role in these processes through decomposition and nutrient mineralization, but their activity is highly sensitive to drought, an increasingly frequent climatic stressor. This study aims to assess how soil microbial activity responds to varying degrees of water stress across sites differing in soil properties and climatic regimes. Soil samples were collected from five Italian grassland sites spanning a gradient of mean annual precipitation (MAP), organic matter content, and pH. Controlled laboratory incubations were conducted at five moisture levels (10 - 80% water holding capacity) to measure soil respiration, eight extracellular enzymatic activities, microbial biomass carbon (MBC) and nitrogen (MBN), and available nutrients. We hypothesized that microbial communities from drier sites would be more resistant to water stress than those from wetter regions. In agreement to this expectation, sites with higher MAP showed stronger reductions in respiration under low moisture, but all sites converged to similar relative respiration rates at the lowest moisture levels, even after normalization by soil organic matter. Microbial communities exhibited two contrasting nutrient-acquisition strategies under drought - either increasing or decreasing enzyme production - yet these shifts did not translate into differences in CO2 efflux, indicating that drought imposes a dominant physiological constraint regardless of community strategy or climatic origin. Respiration rates were strongly and positively associated with dissolved nitrogen, revealing a tight coupling of microbial C and N cycling and a potential role for available N in mediating drought effects on soil metabolic activity. Overall, our results demonstrate that microbial functional responses to drought are governed more by intrinsic moisture limitation than by long-term climatic adaptation, providing mechanistic insights into how soil carbon losses may accelerate in a drying world.