INTRODUCTION
Grasslands are in significant decline globally (Bardgett et al. 2021). The productivity, diversity, and resilience of these ecosystems is heavily shaped by their soil microbiota (Koziol & Bever 2017; Wanget al. 2019; Liu et al. 2022). Despite strong plant-soil interactions in grasslands (i.e., plant-soil feedbacks), these interactions are under acknowledged and underutilised in conservation and restoration efforts (Robinson et al. 2023; Peddle et al. 2024). As climate change and land-use pressures intensify, understanding how soil microbiota support grassland productivity and stress tolerance is increasingly important to aid conservation and restoration efforts (Trivedi et al. 2022; Fadiji et al. 2023).
Carbon and nutrient cycling are among the many microbial-driven processes in soil that can shape plant communities (Bever et al. 2010; Wagg et al. 2014). Plants also form direct symbioses with soil microbiota in their rhizospheres (areas around plant roots) and endospheres (inside plant roots) (Bulgarelli et al. 2013). The colonisation of these plant compartments by soil microbiota is described by the two-step selection process (Bulgarelli et al. 2012; Lundberg et al. 2012; Bulgarelli et al. 2013). This process involves initial resource provision through plant roots which support microbial assemblages from the bulk soil to colonise host rhizospheres (step 1). Microbiota are then filtered into the endosphere with plant immune system regulation (step 2) (Bulgarelli et al. 2013). These rhizosphere and endosphere microbiota aid in plant nutrient acquisition and metabolic processes, but we currently lack a clear understanding of how recruitment is affected by plants growing under stressful conditions, such as drought. We also lack knowledge of how plant recruitment of these microbiota is affected by ecological contexts (e.g., high vs low aridity) (Ling et al. 2022; Santoyo 2022).
Harnessing soil biodiversity is increasingly recognised for its potential to enhance plant growth in applied ecology contexts (Mariotteet al. 2018; Porter & Sachs 2020; Peddle et al. 2024). One promising method to do this is through whole soil inoculations via the translocation of soil, including their microbiota, into new areas (Gebhardt et al. 2017; Wolfsdorf et al. 2021; Han et al. 2022). This approach leverages positive soil legacies where plant populations naturally cultivate soil microbiota that support the offspring of these plants (Kaisermann et al. 2017; Pinedaet al. 2017; Buchenau et al. 2022). Positive soil legacies can improve plant tolerance to water stress and herbivory (Kaisermannet al. 2017; Hannula et al. 2021), but we lack theoretical understanding of the colonisation mechanisms within soil and plant compartments. Experimental testing of how different soils and their microbiota influence plant growth along with comprehensive characterisation of bacterial colonisation patterns can address these knowledge gaps, especially when accounting for stress scenarios.
Themeda triandra (Forssk.) is a globally important keystone C4 grass species with a pan-palaeotropical distribution (Snyman et al. 2013; Dunning et al. 2017; Pascoe 2018). Currently, the processes by which microbiota colonise and influence the growth ofT. triandra remain poorly understood. To address this, we conducted a greenhouse experiment on how soil microbiota from high and low aridity regions affected the germination and growth of T. triandra under both water-available and drought-like (i.e., water stress) conditions. We used 16S rRNA amplicon sequencing to characterise the T. triandra -associated microbiota of high and low aridity soils under live versus sterilised, and water stress treatment conditions, plus the recruitment patterns of these microbiota from the bulk soils into T. triandra rhizospheres and endospheres. We hypothesised that: (1) soil microbiota sourced from arid sites would enhance T. triandra growth under stress conditions by providing mutualistic microbiota that support growth under drought-like conditions; (2) distinct microbial communities would be recruited into the rhizosphere and endosphere under each water treatment, reflecting shifts in host plant requirements; and (3) the presence of T. triandra plants would alter the bacterial community in soil due to a cumulative influence of microbe-root interactions. By assessing how microbiota impact the drought responses of this important grass, and monitoring their recruitment across root compartments, we can better understand the value of soil biodiversity as a tool for improving the resilience of grassland ecosystems.

MATERIALS AND METHODS