Interplay of ecological processes modulates microbial community reassembly following coalescence.

Microbial community coalescence refers to the mixing of entire microbial communities and their environments. Despite conceptually analogous to a multispecies invasion, the ecological processes driving this phenomenon remain poorly understood. Here, we developed and implemented a beta-diversity-based statistical framework to quantify the contribution of distinct donor communities to community reassembly dynamics over time following coalescence. We conducted a microcosm experiment with soils manipulated at varying levels of community structure (via dilution-to-extinction) and subjected these to pairwise coalescence scenarios. Overall, our results revealed variable patterns of abiotic and biotic donor dominance across distinct treatment sets. First, we show the occasional presence of an upfront stringent abiotic filter to disproportionally favor a donor biotic dominance through a "home-field advantage" mechanism, with abiotic factors explaining >90% of the variance in community structure. Functional community metrics (i.e. carbon metabolism and extracellular enzymatic activities) were significantly linked to donor contributions in these cases. Second, in the absence of abiotic dominance, interspecific interactions gained importance, with abiotic variables explaining <40% of the variance. Here, functional redundancy in donor communities (e.g. lower dilution) led to nonsignificant relationships between donor contributions and functional metrics. Collectively, this study advances the integration of coalescence with well-established fundamentals of invasion biology theory, highlighting the interplay of abiotic and biotic factors structuring community reassembly following coalescence. Last, we propose that our beta-diversity-based framework is widely applicable across various microbial systems. We believe this approach will promote research advances by offering a unified method for analyzing and quantifying coalescence.