Community diversity shapes persisting soil legacy effects of individual plant species on subsequent plant performance

Positive plant diversity–productivity relationships have partly been attributed to decreasing abundances of belowground specialist plant antagonists with increasing plant diversity, resulting in less negative plant–soil feedbacks (PSFs) on establishing conspecific plant individuals. Yet, generalist plant antagonists also contribute to negative PSFs among conspecific and heterospecific plants. Possibly, contributions of generalist plant antagonists to PSF outcomes may become more important in high-diversity communities due to diversity-driven specialist plant–antagonist dilution, but this has remained untested. We set up a three-phase pot experiment to examine how plant diversity affects the differences between conspecific and heterospecific PSFs. In the first (‘pre-conditioning') phase, we conditioned soils with communities of one, two, four or eight species. By mixing soils within each diversity level, we created soil mixtures with legacies of all eight species, which only differed in the diversity level at which the plants conditioned the soils. We then established monocultures of all eight species in each mixture (‘conditioning phase') and finally examined feedback responses of all individual species (‘feedback phase'). We tested the hypothesis that compared to heterospecific PSFs, conspecific PSFs are most negative in soil mixtures with a legacy of the lowest plant diversity, but become less negative in soil mixtures with legacies of increasing plant diversity. Diversity legacy and pre-conditioning-phase conspecific biomass positively or negatively affected plant performance in the feedback phase, depending on species identity. Plants experienced more negative conspecific than heterospecific feedback, but this effect did not depend on soil legacies established in the pre-conditioning phase. Our study shows that diverse plant communities leave persistent soil legacies that differ from combined legacies of lower-diversity communities, indicating that plant–soil interactions of individual plants are affected by the community context. However, our study does not provide evidence that community diversity predictably affects differences between conspecific and heterospecific PSF interactions.