Reducing phosphorus alters the soil microbial community structure and function, promoting cotton root growth

Microorganisms participate directly in the phosphorus cycle and affect crop growth and development. Studying the changes in microbial community diversity in the cotton rhizosphere under different phosphorus levels has important theoretical and practical significance for the rational application of phosphorus fertilizers. In this study, based on P₂O₅ application at the test field location over nine years, three phosphorus levels were established: low phosphorus (LP), medium phosphorus (MP), and high phosphorus (HP). Changes and correlation analyses of soil physical and chemical properties, cotton root morphology, rhizospheric phosphorus cycling, microbial diversity, and key functional genes were studied under different phosphorus levels. The results showed that inorganic, organic, and total phosphorus contents increased with an increase in the soil available phosphorus content. Compared with the HP treatment, the soil organic matter and total nitrogen contents significantly increased by 22.76% and 35.48% in the LP treatment, the alkaline hydrolyzed nitrogen content significantly increased by 17.61% in the MP treatment. Compared with the HP treatment, the phosphorus content and P₂O₅ uptake in cotton leaves, stalks, and seed cotton were significantly decreased in the LP treatment, while the phosphorus fertilizer use efficiency was significantly increased in the MP treatment. The root length and root surface area in the 0−20 cm soil layer under LP treatment were significantly increased compared with those under MP and HP treatments, while the root volume was significantly decreased compared with that under MP and HP treatments. The total root length, root-shoot ratio and specific root length in the 0−60 cm soil layer decreased significantly with the increase of phosphorus level. Compared with the HP treatment, the MP treatment showed no significant difference in seed cotton yield, whereas the LP treatment resulted in a significant 7.54% reduction in yield. With an increase in phosphorus levels, the number of rhizospheric microorganisms in each taxon decreased. Compared with HP treatment, there was no significant change in the Simpson index of microbial genes in MP treatment, while the Simpson index of microbial genes in LP treatment increased significantly. At the genus level, compared with the HP treatment, the MP treatment showed consistent ranking of the top 10 microbial taxa in relative abundance except for Lysobacter and Variovorax. In contrast, the LP treatment exhibited increased relative abundances of Mesorhizobium, Sphingomonas, Pseudomonas, and Rhizobium, while decreasing the abundances of Steroidobacter, Bradyrhizobium, Lysobacter, Variovorax, Streptomyces and Phenylobacterium. Notably, the abundance of Variovorax was significantly reduced by 52.31% in LP treatment. Compared with the HP treatment, the LP treatment showed increased relative abundances of phosphate-solubilizing microorganisms including Bacillus and Arthrobacter, while decreasing the abundances of Flavobacterium, Salmonella, Alcaligenes, and Rhizopus. In contrast, the MP treatment exhibited higher abundance of Flavobacterium but lower abundance of Alcaligenes. Compared with the HP treatment, the gene abundance of phosphorus nutrient activation process in LP treatment increased, the gene abundance of phosphorus nutrient absorption process in MP treatment increased. Therefore, reducing phosphorus application can increase the genetic diversity of rhizospheric soil microorganisms, enhance the activation and uptake of phosphorus nutrients, and promote root growth. In cotton production, the P₂O₅ application rate can be appropriately reduced. The recommended amount of P₂O₅ for cotton in Hebei Province is 90 kg·hm⁻².