DISCUSSION
We experimentally assessed the effects of high and low aridity soil legacies on the growth of the keystone grass species, Themeda triandra , under drought conditions. We show that microbiota from high aridity soils supported increased growth of this grass species under both drought-like, water stress and control treatments, highlighting the powerful impact of soil legacies and supporting our first hypothesis. We also show that bacterial alpha diversity was positively correlated withT. triandra biomass, and that each of our treatments (i.e., soil aridity, sterilisation, and water stress) led to distinct bacterial assemblages in soils, rhizospheres and endospheres. This supports our second hypothesis by highlighting the conditional relationships thatT. triandra forms with its bacterial communities that is dependent on its environment. Finally, we did not see meaningful differences across the bacterial communities of our soil-only versus plant-present pots, which goes against the expectations of our third hypothesis. Our findings highlight the importance of soil microbiota for host plant growth and fitness under climate change. Our study underscores the importance of protecting diverse soil communities to support grassland health, and highlights the potential of harnessing these communities to increase grassland restoration that is more resilient to climate change.