Connecting plant–soil feedbacks to long-term stability in a desert grassland

Temporal fluctuations in plant species coexistence are key to understanding ecosystem state transitions and long-term maintenance of species diversity. Although plant microbiomes can alter plant competition in short-term experiments, their relevance to natural temporal patterns in plant communities is unresolved. In a semiarid grassland, the frequency and magnitude of change in plant species composition through time varied from relatively static to highly dynamic among patches across the landscape. We field tested whether these alternative successional trajectories correlated with alternative plant–soil interactions. In temporally stable patches, we found negative plant–soil feedbacks, where plants grew worse with conspecific than heterospecific soil biota—a mechanism that maintains stability in mathematical models. In contrast, feedbacks in temporally dynamic patches were neutral to positive. Importantly, the magnitude of feedbacks depended on plant frequency, enabling plant species to increase in cover when rare, which theory predicts will promote long-term, stable coexistence. Although our study does not determine the direction of causality, our results reveal a novel link between plant—microbe interactions and temporal stability of plant species coexistence and help to explain 20+ yr of plant abundance dynamics at the patch-to-landscape scales.