Soil Bacteria to Regulate Phoebe bournei Seedling Growth and Sustainable Soil Utilization under NPK Fertilization

Soil bacteria play a key role in the plant–soil system and can regulate the growth of Phoebe bournei seedlings under fertilization. However, there are few reports on how soil bacteria respond to fertilization and regulate seedling growth. This study adopted the “3414” field fertilization experiment, combined with soil microbial sequencing, nutrient contents, and biomass measurement, to explore the changes of soil chemical properties and bacterial structure under different NPK fertilization conditions and to establish the coupling relationship between soil bacteria, soil nutrients, and plant growth. The results showed that NPK fertilization decreased soil pH; increased soil N, P, and K content; reduced bacterial diversity and abundance; promoted the growth of dominant bacterial species; and enhanced Phoebe bournei seedlings’ soil N, P, and K elements. NPK fertilization promoted Proteobacteria growth, especially of three genera (Methylobacterium, Sphingobium, and Acinetobacter) and Actinobacteria, while it decreased Acidobacteria and Chloroflexi. By reducing the ratio of N to K and increasing P, NPK fertilization can slow soil acidification, promote bacterial reproduction, maintain P. bournei seedlings’ soil ecological stability, and balance the seedlings’ growth and sustainable soil utilization. AD3, Pseudomonas, and Rhodanobacter can be used as the marker species for N, P, and K fertilization, respectively, while Methylobacterium, Brevundimonas, Acinetobacter, and Sphingobium can be used as indicator species for soil pH and soil N, P, and K content changes, respectively. These results provided a theoretical basis and technical guidance for the effective fertilization and cultivation of robust P. bournei seedlings.