Soil microbial influences over coexistence in multispecies plant communities in a subtropical forest

Soil microbes have long been recognized to substantially affect the
coexistence of pairwise plant species across terrestrial ecosystems.
However, projecting their impacts on the coexistence of multi-species
plant systems remains a pressing challenge. To address this challenge, we
conducted a greenhouse experiment with 540 seedlings of five tree species
in a subtropical forest in China and evaluated microbial effects on
multispecies coexistence using the structural method, which quantifies how
the structure of species interactions influences the likelihood of
multiple species to persist. Specifically, we grew seedlings alone or with
competitors in different microbial contexts and fitted individual biomass
to a population dynamic model to calculate intra- and inter-specific
interaction strength with and without soil microbes. We then used these
interaction structures to calculate two metrics of multispecies
coexistence, structural niche differences (which promote coexistence) and
structural fitness differences (which drive exclusion), for all possible
communities comprising two to five plant species. We found that the soil
microbes generally increased both the structural niche and fitness
differences across all communities, with a much stronger effect on
structural fitness differences. A further examination of functional traits
between plant species pairs found that trait differences are stronger
predictors of structural niche differences than structural fitness
differences and that soil microbes have the potential to change
trait-mediated plant interactions. Our findings underscore that soil
microbes strongly influence the coexistence of multispecies plant systems,
and also add to the experimental evidence that the influence is more on
fitness differences rather than niche differences.