Soil microbial richness exhibited significant increases in the coalesced soils (Fig. 1b), which can be elucidated through the following explanations. Initially, the original soils showed significant differences in microbial community profiles, indicating the introduction of many new microbial species through community coalescence (Fig. S2o). Additionally, community coalescence could also largely diversify soil habitats, indirectly leading to the increases in microbial diversity. Furthermore, a recent study showed that the abundance of some rare microbial taxa, previously undetectable, became observable after community coalescence [13]. Accordingly, the emergence of rare microbial taxa can also contribute to the microbial richness increase in the coalesced soils [13]. Finally, as a disturbance, coalescence may also stimulate the growth of dormant microbes, which is another possible cause. In fact, the phenomenon of increased microbial diversity following community coalescence has been widely documented, underscoring the crucial role of community coalescence in maintaining microbial diversity [12, 57, 58].
We also discovered that most properties (e.g., abundance, community profiles, and functional gene copies) exhibited an intermediate status in the coalesced soil microbial communities compared to their original counterparts (Fig. 1, Table S2, Fig. S7, Fig. S8). These findings are in line with our hypothesis, and can be primarily attributed to two reasons. First, as observed in this study, most physicochemical properties of the mixed soils were at the intermediate level between those of the original soils (Fig. S7). The intermediate soil conditions tended to shape microbial communities with intermediate characteristics [16, 59]. Second, most microbes are adsorbed or even wrapped by the soil particles [60, 61]. In mixed soils, most soil particles remain relatively independent, and thus many microbial properties are simply the arithmetic means of the original soils. This provides another plausible explanation for why coalesced soil microbial communities exhibited an intermediate status compared to their original counterparts. Our findings are consistent with several recent studies, but differ from the coalescence outcomes observed in freshwater and marine water microbial communities [13, 62]. These divergent results may be ascribed to the greater differences between freshwater and marine habitats, when compared to those between cropland and noncropland soils included in this study [13, 14]. Additionally, inherent differences in soil and water properties may also contribute to these distinct coalescent outcomes.
In accordance with our hypothesis, weighted species additive effects were recognized as the dominant mechanism of soil microbial community coalescence, which can be explained as follows (Fig. 2). Initially, dormant microbes can account for more than 95% of soil microbial community and are inherently insensitive to environmental selection [19, 63]. Thus, after soil mixing, dormant microbial taxa are probably coalesced simply in a mathematically additive manner. Additionally, as mentioned above, soil particles serve as the fundamental unit of soil mixing. Since their diameters generally far exceed those of microbial cells, abiotic selection primarily occurs at the scale of soil particles, and establishing new microbial interactions among soil particles also becomes challenging. Consequently, the physical barrier effects of soil particles can be another critical reason for the prominent role played by weighted species additive effects in shaping coalesced soil microbial communities. This perspective was supported by two lines of evidence. First, fungi typically have larger cellular sizes and interaction radii compared to prokaryotes [64]. Accordingly, the weighted species additive effects on the fungal community were much weaker than that on the prokaryotic community (Fig. 2c and d, Fig. 3c and d, Fig. S9). Second, when the soils and microbes were mixed separately to eliminate physical barrier effects posed by soil particles, the weighted species additive effects were largely attenuated, whereas the effects of abiotic and biotic environmental selection were considerably enhanced (Fig. 2c and d, Fig. 3c and d, Fig. S9). Moreover, the interaction between abiotic and biotic environmental selection substantially offset their main effects (Fig. 3c and d), which can also partially explain the dominant role of weighted species additive effects. Collectively, these findings indicate that soil microbial community coalescence can be a highly predicable process, thereby substantially advancing our understanding regarding soil microbial community assembly.
A potential limitation of this study is that non-equivalent coalescence of soil microbial communities occurs far more frequently than equivalent community coalescence, yet our most examinations were based on equivalent community coalescence. Therefore, we also investigated the effects of soil mixing ratios on the outcomes and mechanisms of community coalescence. Our findings indicate that microbial richness and community profiles differed significantly with varying soil mixing ratios (Fig. 4c–f, Fig. S11c–f). However, the underlying ecological mechanisms governing microbial community coalescence with different mixing ratios were generally similar (Fig. 4g and h). Thus, the effects of soil mixing ratios on microbial community coalescence were mainly caused by the ratio differences of weighted species addition and the environmental differences elicited by mixing ratios. The similar mechanisms governing microbial community coalescence largely support the reliability of our conclusions. Moreover, we observed that original soil differences did not significantly affect microbial coalescence mechanisms when soils were directly mixed (Table S3). Overall, these findings suggest that the mechanisms revealed in this study are applicable to a wide range of naturally occurring soil microbial community coalescences.
The consequences of soil microbial community coalescence were also significantly influenced by priority effects, with the initial colonizers largely dominated the coalesced microbial communities (Fig. 5a and b, Table S2). Although the priority effects of soil microbes are seldom studied, the findings presented in this study are corroborated by multiple observations in water, nectar, and even human intestine [14, 65, 66]. Similar to many previous studies, the priority effects observed in this study can be elicited through niche preemption and modification [24, 25]. Niche preemption refers to the phenomenon that early-arriving microbial taxa consume resources such as nutrients and space, thereby constraining the colonization of late-arriving microbial groups that rely on these resources for survival and reproduction [25]. We observed that the microbial taxa exhibiting significant differences among the priority treatments possessed considerably higher niche overlaps, indicating that niche preemption could be a crucial mechanism underlying priority effects in the coalesced microbial communities (Fig. 5c and d). Niche modification refers to the alteration in locally available niches by early-arriving species, resulting in the changes to the identities of late-arriving taxa that can establish themselves within the community [25]. In this study, the soil physicochemical properties exhibited significant variations among the priority treatments and displayed strong correlations with the microbial community composition (Fig. 5e and f). Therefore, niche modification could also constitute a pivotal mechanism underlying priority effects in the coalesced soil microbial communities. The strength of priority effects relied on the intervals of dispersal [29, 67], and thus our findings actually highlight the crucial roles of stable priority effects in determining the consequences of soil microbial community coalescence. Additionally, this study also represents the first observation of the predominant influences of priority effects on soil microbial community assembly. The findings also suggest that techniques such as microbiome inoculation must overcome the priority effects to achieve more effective manipulation of the microbiome.