Soil microbes and community coalescence
Introduction
Community coalescence is a recently coined term (Rillig et al., 2015) describing situations where two or more entire communities (and their environments) interact because pieces of the environment that are large relative to the size of the organisms they contain can be translocated by a variety of forces. While such interactions of whole communities are hard to envisage in the normal context of plant and animal ecology, community coalescence among microbes, especially in the soil, is likely an ever-present feature. Community coalescence is only partially encompassed by existing metacommunity theory (Rillig et al., 2015; Fig. 1; also see there for a pertinent discussion of microbial biogeography), which captures the idea of connectedness, but not of wholescale exchange of environments and communities. To understand the latter in a soil context where community coalescence is likely to be common, thus necessitates a fresh look at features of such exchanges in order to develop suitable theory and experimental approaches. The purpose of this contribution is to more explicitly place the concept of community coalescence in a soil microbial context.
Many examples of wholesale exchanges between microbial communities come from the aquatic literature (Livingston et al., 2013, Adams et al., 2014, Souffreau et al., 2014), where flows and confluence of water bodies are the natural force driving such mixing. Here we highlight such coalescence events in soils, and explore how they may help explain the large microbial biodiversity and its spatial and temporal organization. Soils are uniquely suited for thinking about community coalescence, because coalescent phenomena are likely to be commonplace there. Soil microbial soil communities are likely to provide the major systems in which coalescent processes are both functionally important and where they can be empirically investigated. This is in part because soil microbes are at the base of the soil food web, and play key roles for ecosystem processes including interactions regulating plant communities (Bever et al., 2010).
In the following, we differentiate between intrinsic (naturally occurring via ecological interactions) and extrinsic (as a result of external influences and disturbance) sources of coalescent events in soil. We separate between these events to illustrate how commonly occurring soil processes can be understood in the light of community coalescence. Both cases have in common the initial development of separate communities, with different abiotic conditions and community composition, which are abruptly mixed by those events.
Section snippets
Soil-intrinsic coalescence events
Here, we focus on soil-intrinsic coalescence events, and consider external drivers of such events in the next section (see also Fig. 1). First we also need to ask: where (and what) are the microbial communities in soil? Assemblages of organisms can be described at various spatial and temporal scales, and for our purposes we explicitly take a microbial vantage point. Beare et al. (1995), for example, designated various arenas of activities in soil, acknowledging the large physiochemical and
Human-mediated or externally driven community coalescence events
Many external influences and disturbances would be expected to trigger fine-scaled coalescence events in the soil (Fig. 1). Examples include tillage (Fig. 1h), flooding, litter-fall, outplanting, and the addition of materials containing microbial communities, e.g., compost, stored biochar, or manure (Fig. 1d). While the net effects of all these events have been well studied, the microbial community coalescence aspect remains unquantified and poorly understood; for example, the consequences of
Functional consequences
Given that community coalescence may be a common feature in soils, what does this mean for soil microbial community composition, biodiversity and ecosystem function?
Microbial communities in close proximity, such as in root-associated habitats or those inhabiting interiors or exteriors of soil aggregates, will have contrasting trait distributions. Depending on the frequency and mixing ratios of community coalescence events, community encounters will result in very different average trait
The way forward —how to study this phenomenon?
A long term research goal is to identify the contribution of community coalescence to the composition and function of the microbial soil biota. This is now possible because of high-throughput sequencing, but even with the advances in such methods, there are huge challenges, given the fine-scale heterogeneity and complexity of the soil environment, not to mention the huge diversity of microbial taxa. Hence it will be necessary to initially focus on clearly delineated, experimentally tractable
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