Neutral Additivity Dominates the Outcome of Soil Microbial Community Coalescence

Microbial community coalescence is a major mode of microbial dispersal and assembly, yet the ecological mechanisms underlying this process remain largely unexplored. Here, we addressed this knowledge gap through several microcosm experiments involving soil mixing, sterilization, as well as microbial extraction and inoculation, using soils collected from six paired croplands and adjacent ecosystems in southwestern China. We found that coalesced soils generally exhibited intermediate properties relative to their source soils. Regarding soil microbial community coalescence with an equal ratio, neutral additivity, defined as the direct summation of source communities based on absolute abundance, emerged as the primary mechanism, as indicated by high Bray-Curtis similarities between observed and expected communities (58.2% ± 0.7% for prokaryotes and 62.9% ± 2.2% for fungi). In contrast, the estimated relative importance of environmental selection, quantified by the unbiased centroid distance, was markedly lower (18.0% ± 0.4% for prokaryotes and 19.5% ± 1.9% for fungi). Similar patterns were also observed in unequal soil mixing scenarios. When microbial extracts and sterilized soils were mixed separately, the similarity proxy for neutral additivity decreased to 54%-55%, while the unbiased centroid distance for prokaryotic and fungal community coalescence increased to 29.6% ± 2.7% and 33.0% ± 3.5%, respectively. Notably, the combined proxies of abiotic and biotic selection were substantially higher than the total environmental selection indicator. This discrepancy indicates that the antagonistic interactions between these two selective forces largely offset their individual impacts. Overall, our findings suggest that neutral additivity is the dominant mechanism driving soil microbial community coalescence, underscoring the predictability of post-coalescence soil microbial community assembly.