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The processes controlling community assembly are a central focus of community ecology (Clements, 1912; Gleason, 1939; Hubbell, 2001; Leibold et al., 2004). A hierarchical framework of dispersal limitation, followed by species sorting through environmental and biotic filtering is the main progression controlling niche-driven community assembly (Leibold and McPeek, 2006). Conversely, neutral theory dictates that community assembly is a random process of dispersal, births and deaths, and genetic drift, stochastically regulated by the regional abundance of taxa (Bell, 2001; Hubbell, 2001). The relative importance of these processes in the community assembly of macroorganisms has received much attention (eg, Funk et al., 2008; Leibold et al., 2010). However, how these filters affect microbial community assembly is largely unclear (Lekberg et al., 2007; Dumbrell et al., 2010a, b; Opik et al., 2010, 2013; Kivlin et al., 2011; Opik et al., 2013). In particular, the comparative influence of dispersal limitation versus environmental filtering is unknown for microorganisms. Determining community assembly rules is critical for microorganisms, as microbial taxa can alter ecosystem processes such as decomposition rates (Setala and McLean, 2004; Hattenschwiler et al., 2005) and aboveground productivity (Maherali and Klironomos, 2007). Comparing dispersal limitation versus environmental filtering for microbial community assembly has been historically challenging due to two main obstacles: large population sizes and microscopic individual sizes. Because of these challenges, direct tests of the relative importance of dispersal limitation versus …