Do PSF-causing microorganisms respond more to vegetation than to
soil?
Indicator OTUs causing PSF were not more strongly associated with plant
community structure than with soil properties, as compared to the rest
of the soil microbiome (Fig. 3). In fact, procrustean analyses revealed
a very strong correlation between the predictors of microbial community
structure when comparing indicator vs non-indicator prokaryotes
(r = 0.86, P = 0.002). A similar result was found for
fungi (r = 0.88, P < 0.001). For both
prokaryotic and fungal communities, the strongest predictors were soil
pH, moisture and N levels, as well as two plant community structure
ordination axes associated mainly with the abundances of Bromusinermis , Nassella viridula and Carex stenophylla(plant ordination axes 2 and 3, jointly explaining ~
34% of plant community structure, Fig. 3, Table S1). Although we had
found that high-pH, high-moisture and low-fertility soils were conducive
to negative PSF in these soils (Chagnon et al. , 2018), we did not
find strong associations between these soil properties and pathogenic
indicator OTUs in our canonical ordinations (Fig. S6). Likewise, we did
not find that the proportion of fungal sequence reads belonging to
pathogenic OTUs was significantly influenced by soil properties
(assessed through generalized additive mixed models), except for a very
weak unimodal trend with soil moisture (Fig. S7). However, this trend
explained less than 5% of the variance. Collectively, these results
further confirm that pathogens were not strongly associated with the
decrease in plant growth observed with inocula collected from
low-nutrients, high-pH and high-moisture soils (Chagnon et al. ,
2018).