Plants reverse the positive effect of nutrient addition on the drought resistance of soil multifunctionality

Global change affects soil microbial communities and the multiple functions they control in soil. However, our understanding of the combined effects of multiple global change factors on soil multifunctionality (SMF), and how plant-soil interactions shape these effects remain limited. In this study, we used a mountain grassland soil to test the interactive effect of mineral nutrient (Nitrogen and Phosphorous) addition and drought on SMF with and without plant in a mesocosm experiment. We calculated SMF based on 8 microbial properties associated with the capacity of soil microbes to store carbon (C), nitrogen (N) and phosphorous (P) in their biomass, and to process these elements through organic matter depolymerization, mineralization, nitrification and denitrification processes. To investigate mechanisms underlying the SMF response we characterized the associated changes in soil nutrients stoichiometry and microbial community composition using 16S and 18S rRNA amplicon sequencing. Our results showed that nutrient addition decreased the SMF drought resistance when plants were present, but the opposite was observed in unplanted soil. We suggest this was due to the interaction of plant, fertilization and drought in influencing four coupled properties related to high SMF: high soil moisture, low microbial C limitation, high bacterial diversity and low bacteria gram positive:gram negative ratio. Our study revealed that plant presence can reverse the response of SMF to interacting global change factors, and further showed that combining stoichiometric and biodiversity assessment represents a powerful approach to disentangle the underlying mechanisms.