Plant biomass and stress responses
Water stress and soil sterilisation treatments significantly reduced the
total T. triandra biomass recorded compared to control
(no-stress) and live soil conditions (both p<0.001; Figure 1b;
Table S2). The water stress-sterilisation interaction was significant,
with the most notable difference being increased biomass in the live
control (no-stress) soil treatment (p<0.001; Table S2; Figure
S6). Plants in the live high aridity soils had greater biomass than
those in live low aridity soils (p<0.001; Figure 1b; Table S2),
and the soil aridity-sterilisation interaction was also significant, and
showed plant biomass was higher under high aridity soil conditions
(p=0.022; Table S2; Figure S6).
The aboveground biomass of T. triandra plants was lower in the
water stress (p<0.001; Figure 2c; Table S2) and sterilisation
treatments (p<0.001; Figure 2c; Table S2). A water
stress-sterilisation interaction was also present, where aboveground
plant biomass was greater under live soil, with control water
conditions, while unaffected by sterile soil conditions under both water
availabilities (p<0.001; Table S2; Figure S7). Like total
biomass, we found higher T. triandra aboveground biomass for
plants grown under high compared to low aridity soil conditions
(p<0.001; Figure 2c; Table S2). We also found significant
interactions between soil aridity-sterilisation (p=0.032; Figure S7;
Table S2), soil aridity-water stress (p<0.001; Table S2; Figure
S7), and soil aridity-sterilisation-water stress (p=0.046; Table S2;
Figure S7). Here, there was a stronger increase in aboveground biomass
in the live high aridity soils compared to the sterile high aridity
soils. We also found that the aboveground biomass increase was greater
between the water stress and the control treatments in high aridity
soils than in the low aridity soils (Figure S7).
We found that belowground biomass decreased when under water stress
(p<0.001; Figure 2d; Table S2) and sterilisation treatments
(p<0.001; Figure 2c; Figure 2d; Table S2). High aridity soils
also led to increased belowground biomass than low aridity soils
(p<0.001; Figure 2d; Table S2). Belowground biomass was also
affected by a water stress-sterilisation interaction (p= 0.014; Figure
2d; Table S2; Figure S8). Here, sterilisation reduced belowground
biomass more under low aridity soil conditions than under high aridity
soil conditions.
There was no effect of water stress on root-mass fraction. However,
sterilisation of low aridity soils increased the root-mass fractions
(p=0.003; Figure 2e; Table S2) and sterilisation of high aridity soils
reduced the root-mass fraction (p=0.002; Figure 2e; Table S2).
All plant soil feedback ratios were positive in each treatment, though
we found significantly higher plant soil feedback ratios in the low
aridity soils compared to high aridity soils for total, aboveground, and
belowground biomass and root mass fractions (see Table S3; Figure S10).
The elevated plant soil feedback ratios in low aridity soils appear to
be driven by the very low biomass outcomes when these soils were
sterilised (Figure 1b-d). In the low aridity soils, the plant soil
feedback ratios were higher in the control treatments compared to water
stress treatments for total, aboveground, and belowground biomass (Table
S3; Figure S10a-c).