Although much recent work has identified how abiotic factors affect microbial community structure and assembly, the contribution of biotic forces like species interactions has been much more elusive. Furthermore how the importance of abiotic and biotic assembly processes change across environmental gradients has rarely been explored. Here we test how drivers of microbial community assembly in a high alpine landscape change across a successional gradient created by variability in topography and harsh environmental conditions. We use hierarchical Bayesian joint distribution modeling to investigate how abiotic factors (fixed effects) contribute to the structure of microbial communities consisting of bacteria, fungi, small (single-celled) eukaryotes, and soil mesofauna. We then extract the residual species-to-species variance-covariance matrix to examine species interaction networks after removing the influence of the fixed environmental covariates. We hypothesize that abiotic drivers will be stronger in early successional stages and that species interaction networks will be more important and complex later in succession.
Results/Conclusions
We find that abiotic variables (pH, soil moisture, total carbon, total nitrogen, snow depth) explain an average of 15% of the variation in microbial community structure across all microbial groups and were equally important in affecting community assembly across the successional gradient. Species interactions for the majority of microbial groups were more important in early succession: interactions networks for bacteria, fungi, and small eukaryotes were about three times more complex in early succession compared to late succession plots. However, species interactions for soil mesofauna were more complex in late succession. Additionally, the vast majority of interactions for all taxonomic groups were positive, not negative, in both early and late successional stages. Overall, results suggests the importance of taxon-specific syntrophic relationships and facilitative interactions in early succession, but that interactions and food webs consisting of higher trophic levels (mesofauna) take longer to develop