Microbially mediated effects on biomass
We show that live soil communities enhanced plant growth in high and low aridity soils, and under control and water-stress conditions. Additionally, bacterial alpha diversity across the soils, rhizospheres and endospheres were positively correlated with T. triandra biomass, suggesting that a greater variety of unique bacteria, either naturally present in the soil or recruited into the rhizospheres and endospheres, leads to greater plant growth. Alpha diversity is a well-known driver of plant productivity and is associated with greater ecosystem functionality (Schnitzer et al. 2011; Byrnes et al. 2014; Wang et al. 2019). Our findings support previous research which shows that host-benefiting microbial functions are present within T. triandra soils, rhizospheres and endospheres (Hodgson et al. 2024a), and the importance of the habitat source of microbes (e.g., arbuscular mycorrhizal fungi) for T. triandra drought response (Petipas et al. 2017). As such, there is now a strong body of evidence to suggest that soil microbiota support T. triandra growth across diverse ecosystems, under both stress and non-stress conditions.
Our T. triandra plants developed larger root-mass fractions in the sterilised high aridity soils, compared to the live high aridity soils. This shows that a higher proportion of plant resources were allocated to the development of roots under sterilised soil conditions, perhaps in response to an absence of microbiota which typically aid the acquisition or unblocking of nutrient resources in the soil (Pérez-Harguindeguy et al. 2013; Bai et al. 2022; Wanget al. 2024). Interestingly, we observed the opposite trend in the low aridity soils, where greater root investment occurred in the live low aridity soils compared to those grown in sterilised low aridity soils. As the bacterial communities were distinct, the low aridity soil microbiota may not provide the same functional benefits as those found in the high aridity soils – where different soil conditions, like available moisture or organic matter, could create different host needs (Hodgson et al. 2024a). Plants growing in the low aridity soils may not typically produce such strong microbial-root interactions, given the potential absence of these stress-tolerance benefits by the microbiota (Comas et al. 2013; Hodgson et al. 2024a). The importance of microbiota for plant growth, and the strength of the interaction they form, may therefore depend on the aridity of the soil and plant growth environment (De Long et al. 2019).