Community assembly processes in soil: Mixing it up

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Jaimie West, Soil Science, University of Wisconsin-Madison, Madison, WI and Thea Whitman, University of Wisconsin

Background/Question/Methods
Soil is comprised of heterogeneous microhabitats, intermittently connected through pores, or isolated within aggregates. This spatial heterogeneity, along with vast microbial genetic diversity, enable us to manipulate and study ecological patterns and community assembly processes in the context of community coalescence, which describes not just the dispersal or interactions of organisms, but the wholesale exchange and restructuring of communities and their environments. Community coalescence is of particular relevance in soil, as well as aquatic ecosystems and gut microbiomes. By combining, mixing, and re-dividing small amounts of soil (50 mg; about the size of a lentil) at various rates over the course of a 16-week lab incubation, we sought to quantify the relative contributions of community assembly processes—namely dispersal, selection, and drift—and to assess changes in bacterial community composition. We hypothesized that well-mixed soil would harbor a less diverse microbial community dominated by homogenizing dispersal. Statistical models were applied to 16S rRNA sequences to infer ecological processes. Our findings may be applied to managed agroecosystems (e.g., tillage) as well as natural soil processes (e.g., frost heaving or bioturbation via earthworms and growing roots).
Results/Conclusions
Well-mixed soil demonstrates lower richness, and community assembly dominated by both homogeneous selection and homogenizing dispersal. While more soil mixing resulted in communities that were increasingly dissimilar from those of unmixed soil (Bray-Curtis dissimilarity), these well-mixed soil communities were increasingly similar to those samples with which they were mixed. Interestingly, concurrent soil samples that were similarly mixed by vortex, but never combined with any other soil, harbored bacterial communities that were dominated by dispersal limitation and variable selection, the non-homogenizing counterparts to the processes inferred for the mixed soil treatments. Our results support the idea that vast soil microbial diversity may be attributed to the generally unmixed and spatially heterogeneous nature of soil. By isolating and better understanding the effects that spatial heterogeneity can have on community assembly processes in soil, we are better able to extrapolate the effects that anthropogenic processes, such as climate change or land use change, may have on broad functions of soil communities. This empirical and experimentally manipulative approach to investigating soil microbial community assembly complements the more model-based approaches typically applied to these questions.