Native soil microbes buffer savanna trees against nutrient limitation but are drought sensitive

Below-ground microbial communities are vital to ecosystem nutrient cycling, plant health, and resource acquisition, yet below-ground plant–soil interactions in savannas remain understudied, especially in their responses to environmental stressors like drought and nutrient limitation. Here, we evaluate if native soil microbiomes have positive or negative effects on tree growth and if these effects are dependent on the level of resource availability. We grew 6 tree species from Kruger National Park, South Africa, for 8 weeks under factorial soil inoculant, water stress, and nitrogen-limitation treatments (i.e. sterile/inoculated soils, droughted/non-droughted water supply, and low/high rate of nitrogen supply). In all resource treatments, inoculated plants grew significantly more than sterile plants. Under low nitrogen, trees increased investment in nitrogen-fixing nodules and mycorrhizal associations, leading to increased mass gain. Soil inoculant was most beneficial in non-droughted water conditions, indicating that microbial symbiont effects decreased under drought. Synthesis. Below-ground microbial symbionts increased savanna tree growth in limited resource environments and could be critical for plant growth in the field. However, drought substantially affected both tree growth and the effects of native soil microbes on tree growth, indicating that extreme droughts could create lasting consequences for both above-ground tree growth and below-ground beneficial microbial communities.