Experimental design
We prepared a germination and five-month growth trial to test the
influence of microbiota in soils collected from either high aridity or
low aridity locations by growing T. triandra under sterilised and
live microbiota conditions. We also assessed the germination and growth
of T. triandra plants in these soils under water-available versus
water-stress conditions (mimicking a drought). Each of the eight
treatments (i.e., 2 x soil aridity levels, 2 x sterilisation treatments,
2 x water availability) had 10 replicate pots, making 80 pots in total
(see Figure S1a-b). Each pot received an equal 1,190 g dry weight of its
assigned soil. We calculated the relative soil water content for each
soil treatment to give a standardised measure of moisture, with 0%
corresponding to oven-dry soil and 100% to maximum water-holding
capacity. For our control treatment, we watered each pot to 100%
relative soil water content, while we kept the water stress treatment to
40% relative soil water content by regularly weighing and watering
according to methods described by Earl (2003). Relative soil water
content generally sat between 75-88% for the control (no-stress)
treatment, and 35-38% for the water stress treatment (Figure S2). We
included an additional 24 soil-only pots to account for changes in
microbiota across each treatment in the absence of T. triandra (Figure S1c).
To capture naturally occurring soil microbiota associated with T.
triandra , bulk soil was collected from around the roots of T.
triandra plants in two undisturbed remnant sites (Figure 1a) with
different levels of aridity: Kuitpo Forest Reserve at 35.2279ºS,
138.7199ºE (the mesic, low aridity site; aridity index = 0.658 –
henceforth low aridity soil) and Quorn Floral Reserve at 32.3434ºS,
138.0182ºE (the semi-arid, high aridity site; aridity index = 0.227 –
hereafter high aridity soil) on 14 and 16 November 2023, respectively
(Table S1). Seeds were collected from the remnant T. triandra in
Kuitpo Forest Reserve in December 2020. After collection, soils were
sieved at 5 mm to remove large stones and litter and then stored at 4ºC
for one month prior to setting up the growth trial. Half of the soil
volumes were sterilised by autoclaving them twice at 121ºC, two days
apart.