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

Belowground microbial communities are vital to ecosystem nutrient cycling, plant health, and resource acquisition, yet belowground 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 fully 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: Belowground microbial symbionts improved savanna tree growth in limited resource environments and could be critical for plant growth in the field. However, drought substantially affected tree growth and the benefit of native soil microbes to tree growth, indicating that extreme droughts could create lasting consequences for both aboveground tree growth and belowground beneficial microbial communities.