Do plant-soil feedbacks promote coexistence in a sagebrush steppe?

Recent studies have shown the potential for negative plant-soil feedbacks
(PSFs) to promote stable coexistence but have not quantified the
stabilizing effect relative to other coexistence mechanisms. We conducted
a field experiment to test the role of PSFs in stabilizing coexistence
among four dominant sagebrush steppe species that appear to coexist
stably, based on previous work with observational data and models. We then
integrated the effects of PSF treatments on focal species across
germination, survival, and first-year growth. To contribute to stable
coexistence, soil microbes should have host-specific effects that result
in negative feedbacks. Over two replicated growing seasons, our
experiments consistently showed that soil microbes have negative effects
on plant growth, but these effects were rarely host-specific. The uncommon
host-specific effects were mostly positive at the germination stage and
negative for growth. Integrated effects of PSF across early life-stage
vital rates showed that PSF-mediated self-limitation occasionally had
large effects on projected plant biomass but occurred inconsistently
between years. Our results suggest that while microbially-mediated PSF may
not be a common mechanism of coexistence in this community, it may still
affect the relative abundance of dominant plant species via changes in
host fitness. Our work also serves as a blueprint for future
investigations that aim to identify underlying processes and test
alternative mechanisms to explain important patterns in community ecology.