Drought responses of root biomass provide an indicator of soil microbial drought resistance in grass monocultures

Plant species may exert a strong influence on soil biological properties, but the linkages between plant and soil responses to severe drought remain unclear. We conducted an outdoor mesocosm experiment with five upland grass species and one Mediterranean drought-resistant grass cultivar to investigate the effects of root biomass and rhizosphere conditions on the drought responses of soil microbial biomass in the topsoil. In particular, we assessed whether variation in the drought resistance of microbial biomass could be linked to root biomass, soil inorganic nitrogen (N) or dissolved organic carbon (DOC). Experimental drought decreased microbial biomass but increased soil inorganic N and DOC across plant species. Root biomass responses to drought were less predictable, and varied depending on species. Microbial biomass resistance to drought showed a negative relationship with the drought resistance of root biomass across species, possibly via changes in rhizodeposition. Moreover, the drought resistance of microbial biomass showed a negative relationship with soil nutrient availability under droughted conditions. Our findings highlight the importance of root biomass as a predictor of soil microbial resistance to drought in grass-dominated systems, and suggest that trade-offs between plant and microbial processes could have significant implications for ecosystem function in a changing environment.