A role for soil microbial communities in plant-plant facilitation

Background/Question/Methods Plant-plant facilitation is an important driver of plant diversity, which in turn maintains ecosystem multifunctionality. Plant facilitation is classically attributed to the amelioration of environmental extremes and resource availability. Integrating soil microbial communities into the positive plant interactions framework should provide a more realistic understanding of this process. We asked whether the soil microbial community growing under a well-known benefactor shrub species, Retama sphaerocarpa L., plays a role in plant facilitation and which is its relative contribution compared to other mechanisms such as the improvement of microclimatic conditions and soil properties. We included three different field and greenhouse studies. Soils from under Retama shrubs and from gaps among them were sampled and soil characteristics and microbial communities were characterized. Using soil extracts from these microhabitats we further tested the effect of each soil microbiota on the performance of a community of beneficiary plants growing under controlled conditions, as well as the relative contribution of microhabitat, soil physicochemical properties and soil microbiota on plant performance under field conditions. Seed germination and plant performance were monitored and we characterized initial and final soil microbial community composition via pyrosequencing of 16S rDNA in all studies, and biomass and enzymatic activity under field conditions.

Results/Conclusions Under greenhouse conditions, soil biota from Retama shrubs had a significant positive effect on the abundance, growth, functional traits and reproductive output of beneficiary plant species through processes that were independent of the direct influence of the benefactor species. This was also the case under field conditions, although some results were plant species-specific, i.e., Retama soil biota mainly affected seed germination either positively or negatively depending on plant identity. Preliminary analysis of the composition of soil communities across all samples and experiments reveals that, compared to those from gaps, soils from under Retama shrubs had greater relative abundance of Bacteroidetes, Acidobacteria and main Proteobacteria classes (Alpha-, Beta- and Gamma-proteobacteria); members of these groups include species that may promote plant growth and organic matter decomposition. Instead, gap soils had greater abundance of Firmicutes and Actinobacteria. Overall, our results showed that soil characteristics and microorganisms played a fundamental role in positive interactions between plant species, and were more decisive than canopy effects on plant growth. Plant-soil interactions are major drivers of facilitation contributing to the preservation of diversity of plant communities.