Microbial community assembly is governed by the interplay among historical contingency, environmental selection, and dispersal, yet their relative importance and temporal dynamics remain poorly resolved, particularly in freshwater ecosystems. Most previous studies have examined these drivers in isolation, limiting our ability to predict microbial community trajectories under changing environmental and dispersal regimes. Here, we conducted a 60-day full-factorial reciprocal transplant microcosm experiment using freshwater bacterial communities, manipulating historical source communities, environmental media, and immigration rates. Bacterial community dynamics were tracked at early (day 13) and late (day 60) successional stages using 16S rRNA gene amplicon sequencing. We found that dispersal limitation strongly influenced community assembly during early succession, with higher immigration rates increasing alpha diversity and altering community composition. However, this effect weakened over time, indicating a transition toward dispersal saturation. In contrast, the influence of historical contingency persisted throughout the experiment and became the dominant driver of community composition at the late successional stage, exceeding the explanatory power of environmental selection. Communities sharing the same historical source consistently followed distinct assembly trajectories, regardless of environmental medium or immigration rate. Together, our results demonstrate a time-dependent shift in the mechanisms governing freshwater microbial community assembly, from early dispersal limitation to late-stage historical dominance driven by priority effects. These findings highlight the critical and lasting role of historical contingency in shaping microbial community structure and suggest that community assembly follows predictable trajectories once niche space becomes saturated, with important implications for understanding and managing freshwater ecosystems under environmental change.

To predict suitable growing regions for Leonurus japonicus and to provide scientific sopport for the habitat conservation and the exploitation and utilization of germplasm resources under climate change conditions, this study combined niche and priority conservation models to assess the future potential distribution of L. japonicus in China. To this end, distribution points and samples of L. japonicus were gathered through online and field surveys. The Maxent model with optimized parameters was used for predicting the suitable habitats of L. japonicus at different stages, and the Marxan model was used to determine the priority of protected areas. The results showed that the highest temperature in the hottest month, the lowest temperature in the coldest month, the precipitation in the wettest month, the precipitation in the driest month, and altitude were the main environmental factors influencing the distribution of L. japonicus. Under the three climate change scenarios, the centroid of the suitable area of L. japonicus migrated northward, and the migration position tended to expand further northwest. In the future, there would be no significant niche differentiation of L. japonicus; the Marxan results showed that priority protected areas for L. japonicus were in southwestern central China, Lingnan, southern east China, and Guizhou. Overall, the results of this research can provide a strategy for the determination of priority protection areas for Leonurus japonicus in China.