Soil fungi promote the positive diversity-productivity relationship under contrasting water availability

Background/Question/Methods

Plant diversity has been positively linked to increased ecosystem functions including plant productivity; however, interactions with soil microbes may play a role in apparent plant diversity-productivity relationships. Furthermore, the influence of resource availability on diversity-productivity relationships and their interactions with microbial communities is poorly understood. We used a biodiversity-ecosystem function experiment from the International Diversity Network with Trees exposed to high and low growing season water availability to determine if soil fungal communities mediate the diversity-productivity relationship and how these interactions vary with water availability. The experiment consisted of plots with monocultures and mixtures of two, four, or six species of trees creating gradients in species richness, functional diversity, and functional identity of the tree communities. The plots were randomly assigned to either high or low soil water availability via water exclusion gutters or weekly growing-season irrigation. Soils were sampled after seven years of tree growth and soil fungal communities were identified with high-throughput metabarcoding. Structural equation models were used to determine whether soil fungal communities play a role in diversity-productivity relationships in tree communities exposed to contrasting soil water regimes.

Results/Conclusions

We found that tree functional diversity was negatively associated with soil fungal richness, which was negatively associated with tree above-ground productivity. Therefore, fungal richness mediated a positive, indirect effect of tree functional diversity on productivity. Fungal community composition was also a significant driver of tree productivity and supported positive, indirect effects of tree species richness, functional diversity, and functional identity on productivity. Ectomycorrhizal richness was negatively associated with productivity and mediated a negative indirect effect of tree species richness on productivity. However, plant pathogen and ectomycorrhizal richness had an interactive effect on productivity that mediated both positive and negative indirect effects of tree diversity. Furthermore, fungal communities were important drivers of positive net diversity and complementarity effects on productivity. Finally, soil fungal richness and community composition mediated tree water relations by dampening the role of water stress in regulating tree productivity. Our study provides strong evidence that soil fungi play an important role in diversity-productivity relationships under variable water availability.