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.