Understanding how diverse biological communities contribute to ecosystem functioning has become essential to ensure the retention of valuable ecosystem services, especially in the face of global change. Although the positive correlation between diversity and ecosystem function is well documented, much debate centers on the underlying mechanisms that cause this relationship. Here we present a study that tests the importance of soil microbes in driving diversity - ecosystem function relationships under ambient water and drought conditions in experimental native tallgrass prairie plant communities. Using 18 co-occurring grass and forb species we established a mesocosm experiment in which we manipulated plant species richness (monocultures, 3 or 6 species mixtures), phylogenetic relatedness (close or distantly related), the presence of live soil biota (yes/no) and water availability.
Our preliminary results show that not only plant species richness but also phylogenetic diversity is positively correlated with ecosystem functions. Community wide productivity as well as plant fecundity was higher in plant communities of higher phylogenetic diversity. While removing live soil biota did not significantly change overall community productivity, it drastically influenced biomass as well as fecundity of individual plant species. Drought minimizes the influence of soil biota on different ecosystem functions, such as soil carbon sequestration. Collectively, our preliminary results show that plant-soil microbial interactions have fundamental implications for the functioning of these ecosystems. Furthermore, evolutionary relationships among plants may fundamentally mediate plant-soil microbial interactions in a way that influences ecosystem function. However, under the face of a drying climate, the stabilizing forces of negative interactions formed between plants and soil microbes may weaken and have negative repercussions for long-term stability and ecosystem function.