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BackgroundRecent evidence suggests that soil microbial communities can regulate plant community dynamics. In addition, the drought tolerance of plants can be enhanced by soil microbes. So far, few studies have assessed the variation in the microbiome of specific plant species along environmental gradients. Yet understanding these dynamics is essential to improve predictions of plant-soil feedbacks and the consequences of ongoing climate changes. Here we characterized the soil microbiome of two co-occurring Mediterranean oaks along a precipitation gradient, using amplicon sequencing of phylogenetic marker genes for prokaryotes and fungi. Additionally, we identified tree-specific and locally-specific microbes potentially responsible for tree community dynamics.ResultsWe show that two co-occurring, evergreen Mediterranean oak species harbor distinct microbiomes along a precipitation gradient. The soil microbial diversity increased along the precipitation gradient, for prokaryotic α and β diversity and for fungal β diversity. Quercus ilex harbored richer fungal communities than Quercus suber, and host-specific taxa more often belonged to fungi than to prokaryotes. Notably, the microbial communities at the dry end of the precipitation gradient harbored more locally-specific prokaryotic and fungal taxa than the microbial communities with a higher diversity, at the wet end of the gradient, suggesting higher specialization in drier areas.ConclusionsEven congeneric tree species, belonging to the same functional group, can harbor distinct and specific soil microbiomes. These microbiomes become more similar and consist of more specialized taxa under drier compared with wetter conditions. With this, our study offers a step towards a better understanding of the context-dependency of plant-soil feedbacks by going beyond α and β diversities and focusing on specialized taxa potentially driving community changes along environmental gradients. We hope that our study will stimulate future research assessing the importance of context-dependency of interactions between plants and soil communities in a changing world.

With ongoing biodiversity loss, it is important to understand how the mechanisms that promote coexistence relate to those that increase functioning in diverse communities. Both coexistence and biodiversity functioning research have unified their mechanisms into two classes. However, despite seeming similarities, theory suggests that coexistence and biodiversity mechanisms do not necessarily map onto each other, yet direct empirical evidence for this prediction is lacking. We coupled field-parameterized models of competition between 10 plants with a biodiversity-functioning experiment measuring biomass production, litter decomposition, and soil nutrient content under contrasting environmental conditions. We related biodiversity mechanisms (complementarity and selection effects), to coexistence mechanisms (niche and fitness differences). As predicted by theory, complementarity effects were positively correlated with niche differences and differences in selection effects were correlated with fitness differences. However, we also found that niche differences contributed to selection effects and fitness differences to complementarity effects. Despite this complexity more stably coexisting communities (i.e. those in which niche differences offset fitness differences) produced more biomass, particularly under drought. This relationship was weaker for litter decomposition rates and soil nutrient acquisition, showing that the mechanisms promoting plant coexistence may differ from those promoting high levels of functions that are less directly related to plant performance. We provide the first empirical evidence that the mechanisms promoting stable coexistence correlate with those driving high biomass production. These findings establish a link between stable coexistence and functioning, which could allow better predictions of how diversity loss induced by global change translates to changes in ecosystem function.