Fertilization, soil and plant community characteristics determine soil microbial activity in managed temperate grasslands

Recent studies in experimental grasslands indicated that declining plant species diversity negatively affects soil microbial communities. Here, we assessed if plant diversity effects also occur in "real-world" grasslands. We studied the influence of fertilization, soil, and plant community characteristics on soil microbial activity (microbial biomass carbon, basal respiration) in 12 managed temperate grasslands of varying plant species richness in two subsequent years. The most important variable explaining variation in microbial activity was soil water content, while positive effects of other soil characteristics (organic carbon and nitrogen concentrations) and fertilization became more important in one study year with generally moister soil conditions. Under moister conditions, fertilization also indirectly influenced soil microbial biomass C via negative effects on plant species richness, which itself increased soil microbial biomass C. Our results show that variation in soil microbial activity in managed grasslands involves direct effects of fertilization as well as indirect effects through changes in plant diversity and the amount of carbon and nitrogen stored in plants and soil. These results emphasize that increased nutrient inputs in grasslands entail complex changes in ecosystem processes and indicate that mechanisms driving soil microbial activity in experimental grasslands also apply to "real-world" grasslands.