Predicting microbially mediated plant coexistence is sensitive to vital rate identity and soil conditioning history

Understanding the mechanisms that maintain the coexistence of plant
species is critical to addressing the global biodiversity crisis.
Increasing attention has been paid to interactions between plants and soil
microbes (plant-soil feedback, PSF), which can promote plant coexistence
by increasing stabilizing effects, but also hinder it by generating
competitive fitness differences. However, the predictive power of the PSF
has been questioned in recent studies because estimates of microbially
mediated coexistence have correlated poorly with the outcomes of plant
interactions observed in the field. This discrepancy may be due to the
approaches typically used in PSF research, such as measuring PSF effects
on a single vital rate or using soil conditioned for a short time period
and without considering abiotic contexts. Here, I examined the effects of
soil inoculum with different training histories and training environments
(with and without added nutrients) on germination, seedling survival, and
biomass of four grassland species. I then examined whether predictions of
microbially mediated coexistence of four species pairs were sensitive to
the vital rate identity, conditioning history, and soil training
environment. I found that conspecific inoculum trained for longer had
increasingly positive and negative effects on germination and biomass,
respectively, although the effects of inoculum history varied across
species and training environments. Estimates of microbially mediated
outcomes were directly related to the vital rate used: when based on
biomass and seedling survival, all four pairs were predicted to coexist,
but only two pairs could do so when based on germination due to much
reduced or even negative stabilization. Although coexistence predictions
were not significantly related to conditioning history (including the
effects of both variable conditioning durations and combinations of
conditioning species) or nutrient treatments, both factors had a
significant effect on stabilization. These results suggest that
predictions of microbially mediated coexistence may be biased when based
on a single vital rate, such as plant growth. To obtain more realistic and
accurate outcome estimates, PSF effects should be integrated across
different life stages, considering the temporal and abiotic contexts of
these effects specific to a focal study system.