SOIL MICROBIOLOGY Relationship Between Soil Properties and Patterns of Bacterial β-diversity Across Reclaimed and Natural Boreal Forest Soils Pedro A. Dimitriu & Susan J. Grayston Received: 17 July 2009 / Accepted: 14 September 2009 / Published online: 15 October 2009 # Springer Science + Business Media, LLC 2009 Abstract Productivity gradients in the boreal forest are largely determined by regional-scale changes in soil conditions, and bacterial communities are likely to respond to these changes. Few studies, however, have examined how variation in specific edaphic properties influences the composition of soil bacterial communities along environ- mental gradients. We quantified bacterial compositional diversity patterns in ten boreal forest sites of contrasting fertility. Bulk soil (organic and mineral horizons) was sampled from sites representing two extremes of a natural moisture-nutrient gradient and two distinct disturbance types, one barren and the other vegetation-rich. We constructed 16S rRNA gene clone libraries to characterize the bacterial communities under phylogenetic- and species- based frameworks. Using a nucleotide analog to label DNA-synthesizing bacteria, we also assessed the composi- tion of active taxa in disturbed sites. Most sites were dominated by sequences related to the α-Proteobacteria, followed by acidobacterial and betaproteobacterial sequen- ces. Non-parametric multivariate regression indicated that pH, which was lowest in the natural sites, explained 34% and 16% of the variability in community structure as determined by phylogenetic-based (UniFrac distances) and species-based (Jaccard similarities) metrics, respectively. Soil pH was also a significant predictor of richness (Chao1) and diversity (Shannon) measures. Within the natural edaphic gradient, soil moisture accounted for 32% of the variance in phylogenetic (but not species) community structure. In the boreal system we studied, bacterial β- diversity patterns appear to be largely related to “master” variables (e.g., pH, moisture) rather than to observable attributes (e.g., plant cover) leading to regional-scale fertility gradients. Introduction Boreal ecosystems comprise about 25% of the world’ s closed-canopy forest as well as vast expanses of open transitional forest; therefore, changes in their productivity levels due to disturbance may have significant effects on the global climate [13]. In these systems, much research to date has focused on assessing the effects of anthropo- genic disturbances either on microbial populations driving relevant biogeochemical cycles (e.g., methane production: [40, 42]) or on fungal communities required for mycor- rhizal symbioses [2, 59]. Considering the critical roles the bacterial communities may play in boreal forest soils [58], studies on their composition and distribution and response to disturbance from a molecular–phylogenetic perspective are surprisingly scarce [44]. Although several studies have recognized the impor- tance of forest type as a driver of microbial community composition (e.g., [8, 22, 23]), we have a poor under- standing of how regional environmental gradients affect the abundance of specific bacterial taxa. In boreal forests, differences in slope position and soil parent material create moisture gradients along which upland vegetation is arranged [8, 28]. Across the gradients, plant species can influence forest floor microbial communities though organic matter quality (e.g., C/N ratio) effects [46], though non-specific changes in bacterial composition have also been attributed to other site factors such as pH and, indirectly, moisture [28, 45]. In mineral horizons, evidence P. A. Dimitriu (*) : S. J. Grayston Department of Forest Sciences, University of British Columbia, 2424 Main Mall, Vancouver, BC, Canada V6T 1Z4 e-mail: dimitriu@interchange.ubc.ca Microb Ecol (2010) 59:563–573 DOI 10.1007/s00248-009-9590-0