Causes and consequences of differences in soil and seed microbiomes for two alpine plants.

Seed and soil microbiomes strongly affect plant performance, and these effects can scale-up to influence plant community structure. However, seed and soil microbial community composition are variable across landscapes, and different microbial communities can differentially influence multiple plant metrics (biomass, germination rate), and community stabilizing mechanisms. We determined how microbiomes inside seeds and in soils varied among alpine plant species and communities that differed in plant species richness and density. Across 10 common alpine plant species, we found a total of 318 bacterial and 128 fungal operational taxonomic units (OTUs) associated with seeds, with fungal richness affected by plant species identity more than sampling location. However, seed microbes had only marginally significant effects on plant germination success and timing. In contrast, soil microbes associated with two different plant species had significant effects on plant biomass, and their effect depended both on the plant species and the location the soils were sampled from. This led to significant changes in plant-soil feedback at different locations that varied in plant density and richness, such that plant-soil feedback favored plant species coexistence in some locations and opposed coexistence at other locations. Importantly, we found that coexistence-facilitating feedback was associated with low plant species richness, suggesting that soil microbes may promote the diversity of colonizing plants during the course of climate change and glacial recession.

Changes in precipitation patterns can destabilize plant species coexistence via changes in plant-soil feedback.

Climate change can alter species coexistence through changes in biotic interactions. By describing reciprocal interactions between plants and soil microbes, plant-soil feedback (PSF) has emerged as a powerful framework for predicting plant species coexistence and community dynamics, but little is known about how PSF will respond to changing climate conditions. Hence, the context dependency of PSF has recently gained attention. Water availability is a major driver of all biotic interactions, and it is expected that precipitation patterns will change with ongoing climate change. We tested how soil water content affects PSF by conducting a full factorial pairwise PSF experiment using eight plant species common to southeastern United States coastal prairies under three watering treatments. We found coexistence-stabilizing negative PSF at drier-than-average conditions shifted to coexistence-destabilizing positive PSF under wetter-than-average conditions. A simulation model parameterized with the experimental results supports the prediction that more positive PSF accelerates the erosion of diversity within communities while decreasing the predictability in plant community composition. Our results underline the importance of considering environmental context dependency of PSF in light of a rapidly changing climate.

Native soil amendments combined with commercial arbuscular mycorrhizal fungi increase biomass of Panicum amarum.

Coastal dune restorations often fail because of poorly performing plants. The addition of beneficial microbes can improve plant performance, though it is unclear if the source of microbes matters. Here, we tested how native soil amendments and commercially available arbuscular mycorrhizal (AM) fungi influenced performance of Panicum amarum, a dominant grass on Texas coastal dunes. In a greenhouse experiment, we manipulated the identity of native soil amendments (from P. amarum, Uniola paniculata, or unvegetated areas), the presence of soil microbes in the native soil amendments (live or sterile), and the presence of the commercial AM fungi (present or absent). Native soils from vegetated areas contained 149% more AM fungal spores than unvegetated areas. The commercial AM fungi, when combined with previously vegetated native soils, increased aboveground biomass of P. amarum by 26%. Effects on belowground biomass were weaker, although the addition of any microbes decreased the root:shoot ratio. The origin of native soil amendments can influence restoration outcomes. In this case soil from areas with vegetation outperformed soil from areas without vegetation. Combining native soils with commercial AM fungi may provide a strategy for increasing plant performance while also maintaining other ecosystem functions provided by native microbes.