Germination and growth trial
The greenhouse was set at 16 h – 8 h day-night cycle, with temperatures set to 30ºC and 18ºC, respectively. In each pot (14 cm diameter, 2 L pots), we sowed eight T. triandra seeds and recorded their weights before placing them in identifiable wells in each pot on 13 December 2023 (Figure S3; 80 seeds per treatment, 640 seeds total). We monitored seedling emergence rates across each treatment. After 8 weeks, seedlings were randomly thinned to one plant per pot (6 February 2024), and water stress conditions were imposed at 10 weeks (21 February 2024). At the conclusion of the experiment (23 weeks; 21 May 2024), we recorded aboveground and belowground biomass, root-mass fraction (Pérez-Harguindeguy et al. 2013), and plant-soil feedback ratios (described below). Soils were collected immediately post-harvest for both physicochemical and bacterial community profiling in 40 pots (5 pots per treatment). We also collected rhizosphere and endosphere samples from 40 pots for microbial profiling (described below).