Mechanisms of community assembly act at both local scales, through environmental filtering of taxa that function best in defined conditions, and at regional scales, through dispersal of taxa from other environments. For soil microorganisms, filtering may act at sub-centimeter scales, while dispersal limitation may come into play at ranges of centimeters to meters. Grassland ecosystem drivers, such as fire or large ungulate grazing, are often manipulated and managed at plot or watershed scales to understand plant community and ecosystem function responses. However, the ecosystem functional responses driven by microorganisms, such as decomposition, may respond to large-scale drivers via mechanisms that manifest at much smaller scales. We predicted that watershed-scale bison grazing and annual prescribed fire would differentially affect the centimeter-scale heterogeneity of the soil chemical environment, thereby influencing both the community composition and decomposition potential of soil microbiota primarily at local scales. To evaluate this prediction, we collected surface soil samples from watersheds that have been burned annually or every 20 years, and bison-grazed or ungrazed for decades, along replicate log-distance transects to evaluate spatial heterogeneity from 10-cm to 1-km scales. Microbial community composition (MCC), extracellular enzyme activity (EEA), and the soil chemical environment was measured at each sample location.
Results/Conclusions
Both watershed fire and grazing history significantly affected the community composition of soil bacteria and archaea, as measured by 16S rRNA gene sequencing (permutational ANOVA: Burn effect P=0.01, R2=0.048; Grazing effect P<0.0001, R2=0.108), and the suite of carbon, nitrogen and phosphorus acquiring extracellular enzyme activity potentials, as measured using fluorometric substrate degradation assays. EEA potential reflected expected differences in C and N availability driven by grazer presence (higher N availability via consumer-driven nutrient recycling) and annual fire (lower N availability via plant litter volatilization), and was predictable based on soil chemical characteristics, as expected. However, while MCC did vary significantly by watershed treatment history, local-scale factors did not predict MCC; instead bison grazing increased heterogeneity and decreased distance-dissimilarity of MCC, implying low barriers to microbial dispersal. Even more surprisingly, annual fire also produced a low decreased distance-dissimilarity pattern in MCC, combined with a high alpha diversity, implying barriers to stochastic microbial dispersal. Understanding the mechanisms that drive soil microbial diversity and function is central to predicting how management impacts microbially-mediated ecosystem services including decomposition and N mineralization.