Plant drought resistance is mediated by soil microbial community structure and soil-plant feedbacks in a savanna tree species

Soil microbial communities have the potential to modify plant performance and condition plant species responses to environmental change, but the role of soil microorganisms for plant drought responses remains unclear. We used a novel experimental approach to examine the interactive effects of drought and presence of soil microbes on biomass production and plant traits in a savanna tree species. Seedlings of Bauhinia brachycarpa were grown in sterilized or 'live' soil, with or without drought, during a 24-week greenhouse experiment. Soil microbial community structure was assessed with phospholipid fatty acid analysis and soil-plant feedback effects were measured. Both drought and the presence of soil microbes decreased plant growth and biomass produced per gram nitrogen (a proxy for N use efficiency) but increased biomass allocation to roots. However, the presence of soil microbes increased plant drought resistance, driven by weakened soil-plant feedbacks under dry conditions. Experimental drought was associated with an increase in the Gram positive: Gram negative bacteria ratio, but did not affect the fungi: bacteria ratio or total microbial biomass. Our results suggest that soil microbes mediate plant responses to drought via soil-plant feedbacks and drought-induced changes in microbial community structure. These findings highlight the importance of plant-soil interactions for improved mechanistic understanding of savanna function, and confirm that characteristics of the soil microbial community could have significant implications for ecosystem stability in a changing environment.