Fire history and plant community composition outweigh decadal multi-factor global change as drivers of microbial composition in an annual grassland

1. Soil microbial communities regulate and respond to key biogeochemical cycles and influence plant community patterns. However, microbial communities also respond to disturbance events, motivating an assessment of the relative roles of decadal multi-factor global change, disturbance and plant community structure on microbial community responses. 2. We used high-throughput amplicon sequencing to characterize the diversity and composition of bacterial and fungal communities in bulk soil (0-7 cm) collected in 2014 from the Jasper Ridge Global Change Experiment, a full-factorial field experiment in which ambient and elevated levels of nitrogen deposition (+7 g N m(-2) yr(-1) calcium nitrate), CO2 concentration (+275 ppm), temperature (+1-2 degrees C) and precipitation (+50% volume with +3 weeks duration) were applied to a California annual grassland from 1998 to 2014. We used linear mixed-effects modelling to test for the effects of global change on microbial diversity (observed richness, Shannon index). We also used generalized dissimilarity modelling (GDM) to study controls on compositional dissimilarity in fungal and bacterial communities. 3. Bacterial community composition was best explained by exposure to fires in 2003 and 2011, whereas fungal community composition was best explained by plant community composition. The richness of fungi increased under elevated nitrogen deposition; bacterial diversity metrics decreased under warmer temperatures. Interactions between global change factors were statistically insignificant or weak. 4. Synthesis. Our results indicate that even on decadal time-scales, the effects of fire history and plant community composition on bacterial and fungal community composition, respectively, outweigh the effects of multi-factor global change. Furthermore, global change factors have mostly additive effects on microbial diversity patterns. Our results show that highly variable mediators such as fire history and plant community composition limit the generalizability of soil microbial responses to long-term global change.