Treatment effects on the two-step selection process
We show a decrease in bacterial diversity in the rhizosphere and endosphere versus bulk soils, which is consistent with previous findings expected under the two-step selection process (Bulgarelli et al. 2012; Lundberg et al. 2012; Urbina et al. 2018). T. triandra plants recruited different communities of bacteria from the soil into their rhizospheres and endospheres depending on whether they underwent soil sterilisation or water-stress treatments. These findings show that the plant’s growth environment alters the recruitment dynamics of soil bacteria. It also shows that T. triandra plants under stress appear to alter their entry screening strategies of soil bacteria when growing under drought-like conditions.
Endosphere recruitment dynamics were most sensitive to the long-term effects of soil sterilisation, compared to soil aridity or water-stress treatments. In all sterilised treatments, endosphere diversity was lower and bacterial communities were differently structured to the unsterilised soils. However, it remains unclear how bacteria from sterilised soils were selectively recruited into the endospheres – whether they originated from the seed microbiome or were microbiota that were not entirely removed from the soils during sterilisation (Kimet al. 2022; Ling et al. 2022; He et al. 2024). Given the reduced T. triandra growth rates (biomass) in the low aridity sterilised soils (which was much lower than in high aridity sterilised soil treatments), we suspect that this grass may also be more susceptible to colonisation by microbial pathogens that possible thrive under the low competition environment created by soil sterilisation (Mallon et al. 2015; Mawarda et al. 2022). Shotgun metagenomic analysis could help identify properties of microbial endosphere colonisation, such as the acquisition of growth-promoting functions. Alternatively, it could reveal whether colonisation dynamics are being hijacked by pathogenic or opportunistic microbes (i.e., ‘cheater’ organisms) that do not provide the same host plant services, despite other shared traits (Kiers et al. 2002; Kiers et al. 2011). The consequences of these interactions could help inform the vulnerability of T. triandra to soil degradation, making this an important avenue for future research.