Drought impacts on plants and microbes are moderated by leaf litter via species specific shifts in plant and soil biotic interactions

Prolonged drought affects plant–soil interactions, with cascading impacts on ecosystems. We conducted a pot experiment with soils from a seven-year Australian grassland rainfall manipulation to test how litter mediates drought effects on plant growth, nutrient cycling, and microbial communities. Two common pasture species ( Plantago lanceolata , forb; Microlaena stipoides , grass) were grown for three months in soil maintained at 70% or 40% water holding capacity, reflecting field manipulations, with or without leaf litter of the same species. Litter increased shoot, but not total, biomass of both species. Further, litter increased M. stipoides root biomass under drought but decreased root biomass under ambient watering. Litter increased microbial biomass carbon, and drought reduced microbial biomass carbon and nitrogen, consistently across both species. Further, in M. stipoides , litter increased most microbial biomarkers (e.g. PLFA, NLFA), while drought increased Gram-positive and Actinobacteria. By contrast, in P. lanceolata , litter reduced biomarker contents under ambient conditions for Gram-positive and Actinobacteria and under drought conditions for Gram-negative, Protozoa and Arbuscular Mycorrhizae. Litter and drought further moderated plant-soil biotic relationships, particularly in M. stipoides , where biomass was negatively related to several microbial biomarkers under ambient conditions without litter, indicative of resource competition. Litter mass loss was positively correlated with M. stipoides root biomass and an arbuscular mycorrhizal biomarker under drought conditions, indicating an important plant–soil biotic feedback. Our findings suggest that drought moderate plant community dynamics via species-specific changes in plant-soil biotic interactions, including feedbacks mediated by litter decomposition.