Communities at any trophic level along the food chain are determined by simultaneous top-down (predators) and bottom-up (nutrients) effects; however, we still lack an understanding of this concept in the soil ecosystem. Here, we aimed to reveal the contributions of the top-down and bottom-up factors on the formation of paddy field bacterial communities. The position of an indigenous bacterial community at the trophic level was centred between soil nutrients (chemical and organic fertilisers [CF and OF, respectively], i.e., bottom-up factors) and bacterial predators (phagotrophic protists, i.e., top-down factors) in a paddy field soil. A 16S rRNA gene amplicon sequencing method was used to evaluate the top-down and bottom-up effects on the bacterial community composition. The results showed that the top-down effects of protists were greater than the bottom-up effects of the applied fertilisers on the formation of bacterial communities. The presence of protists caused the formation of a distinct bacterial community by affecting several bacterial species mainly belonging to Proteobacteria and Bacteroidetes. Among the bottom-up factors, OF significantly affected the bacterial beta diversity, while CF did not. The numbers of bioindicator genera that were associated with the top-down factors were 4.4 and 3.7 times higher than those associated with the bottom effects of CF and OF, respectively. Overall, we provided unique information on the importance of protists in regulating bacterial communities in paddy field soil, which is likely to affect bacterial activities and agricultural productivity.

食物链上任何营养级的社区都由同时的自上而下的（捕食者）和自下而上的（营养素）效应决定。但是，我们仍然对土壤生态系统中的这一概念缺乏了解。在这里，我们旨在揭示自上而下和自下而上的因素对稻田细菌群落形成的贡献。在营养水平上，本地细菌群落的位置集中在土壤养分（分别为化学肥料和有机肥料[CF和OF]，即自下而上的因子）和细菌捕食者（吞噬营养的原生生物，即自上而下的因子）之间在稻田土壤中。16S rRNA基因扩增子测序方法用于评估自上而下和自下而上对细菌群落组成的影响。结果表明，原生生物的自上而下的作用大于施用的肥料对细菌群落形成的自下而上的作用。生物体的存在通过影响几种主要属于细菌的细菌种类而导致形成了独特的细菌群落。变形杆菌和拟杆菌。在自下而上的因素中，OF显着影响细菌的β多样性，而CF则没有。与自上而下的因素相关的生物指示物种类的数量分别比与CF和OF的底效应相关的生物指示物种类的数量高4.4倍和3.7倍。总体而言，我们提供了有关原生生物在调节稻田土壤中细菌群落的重要性方面的独特信息，这很可能会影响细菌的活动和农业生产力。