Ecology, 96(1), 2015, pp. 134–142 Ó 2015 by the Ecological Society of America Plant community richness and microbial interactions structure bacterial communities in soil DANIEL C. SCHLATTER, 1 MATTHEW G. BAKKER, 2 JAMES M. BRADEEN, AND LINDA L. KINKEL Department of Plant Pathology, University of Minnesota, 1991 Upper Buford Circle, Saint Paul, Minnesota 55108 USA Abstract. Plant species, plant community diversity and microbial interactions can significantly impact soil microbial communities, yet there are few data on the interactive effects of plant species and plant community diversity on soil bacterial communities. We hypothesized that plant species and plant community diversity affect soil bacterial communities by setting the context in which bacterial interactions occur. Specifically, we examined soil bacterial community composition and diversity in relation to plant ‘‘host’’ species, plant community richness, bacterial antagonists, and soil edaphic characteristics. Soil bacterial communities associated with four different prairie plant species (Andropogon gerardii, Schizachyrium scoparium, Lespedeza capitata, and Lupinus perennis) grown in plant communities of increasing species richness (1, 4, 8, and 16 species) were sequenced. Additionally, soils were evaluated for populations of antagonistic bacteria and edaphic characteristics. Plant species effects on soil bacterial community composition were small and depended on plant community richness. In contrast, increasing plant community richness significantly altered soil bacterial community composition and was negatively correlated with bacterial diversity. Concentrations of soil carbon, organic matter, nitrogen, phosphorus, and potassium were similarly negatively correlated with bacterial diversity, whereas the proportion of antagonistic bacteria was positively correlated with soil bacterial diversity. Results suggest that plant species influences on soil bacterial communities depend on plant community diversity and are mediated through the effects of plant-derived resources on antagonistic soil microbes. Key words: bacterial diversity; microbial antagonism; plant diversity; plant richness; plant–soil interactions; soil bacteria. INTRODUCTION Plant–soil feedbacks have been widely studied and are hypothesized to play important roles in the ecological and evolutionary dynamics of both plants and microbes (van der Putten et al. 2013, Bakker et al. 2014, Schweitzer et al. 2014). Soil microbial communities can influence plant fitness in diverse ways, such as via decomposition, nutrient cycling, nutrient acquisition, plant disease, and plant disease suppression (Garbeva et al. 2004, Singh et al. 2004, Berg and Smalla 2009, Latz et al. 2012). However, the majority of studies exploring plant–soil interactions are plant-centric, with a strong focus on measures of plant survival, productivity, and fitness, and generally give limited attention to the dynamics of soil microbial communities. As a result, we have relatively little understanding of the specific ways in which plants may impact soil microbial community composition, diversity, or function. Plants are broadly perceived to influence soil bacterial taxa directly through the provision of carbon com- pounds (Bardgett and Wardle 2010). Such hypothesized plant species-specific effects may occur through varia- tion in root exudates, mucilage, plant litter, or plant secondary metabolites, and through plant-induced changes to the abiotic soil environment (e.g., pH, soil moisture, N, P, K [Wardle et al. 2004, Bezemer et al. 2006, Boyle et al. 2008, Badri and Vivanco 2009, Herold et al. 2014]). Similarly, greater plant productivity, and thus correspondingly more carbon in soil, is suggested to have significant impacts on soil community composition (Zak et al. 2003, De Deyn et al. 2010). However, a clear predictive framework is lacking for understanding plant species, plant diversity, and species by diversity interac- tions on soil bacterial community composition, diversi- ty, or function. Most studies presume that plants are the primary selective factors for microbial community composition in soil (Garbeva et al. 2004, Marschner et al. 2004, Costa et al. 2005, Badri and Vivanco 2009). However, the assumption that plant–microbe interactions are the central force in determining microbial fitness ignores the well-established and significant role of microbial species interactions in soil community dynamics. Mi- crobial species interactions, including competition, antagonism, syntrophy, and signaling, are critical to bacterial fitness and the assembly and dynamics of soil bacterial communities (Ryan and Dow 2008, Hibbing et Manuscript received 27 August 2013; revised 5 June 2014; accepted 25 June 2014. Corresponding Editor: P. H. Templer. 1 E-mail: schl0453@umn.edu 2 Present address: USDA-ARS National Laboratory for Agriculture and the Environment, Ames, Iowa 50011-3120 USA. 134