Soil type determines the impact of simulated rainfall on desert microbiota

Despite the aridity and low organic content of desert soils, they support diverse microbial communities that influence many biogeochemical processes. However, a better understanding of the microbial response to simulated rainfall is required to predict the impact of future changes in precipitation patterns. Here, we performed and in-situ experiment to investigate the effect of simulated rainfall on the bacterial communities in soils associated with shrubs of Rhazya stricta, a widespread perennial plant of medicinal and ecological ?importance?. The bacterial community composition was distinct between three different soil compartments: surface biological crusts, root-attached, and within the rhizosphere. Simulated rainfall had minor effects on the overall bacterial community composition, but some population-level responses were observed, especially in soil crusts where Betaproteobacteria, Sphingobacteria and Bacilli became more abundant after water addition. Within crusts (that had significantly higher organic carbon and nitrogen than the rhizosphere samples), there was a three-fold increase in bacterial biomass one week after watering; whereas bacterial biomass in the rhizosphere remained unchanged, despite the fact that the rhizosphere retained soil moisture for much longer. These findings suggest that crust microbiota were primarily water-limited, whereas the rhizosphere microbiota were co-limited by nutrients and water. Nevertheless, water-induced stimulation of microbial activities such as nitrification may be inferred from changes in bacterial populations and soil chemistry in the rhizosphere. These results indicate that between rainfall events, desert-soil microbial communities enter into stasis, with limited species turnover, and reactivate rapidly and relatively uniformly when water becomes available, rather than communities reassembling from preferentially-surviving inocula.