Effects of different phosphorus levels on root morphology, phosphorus cycling microorganisms and key functional genes of cotton

Microorganisms directly participate in the phosphorus cycle and affect the growth and development of crops. To study the changes of microbial community diversity in cotton rhizosphere under different phosphorus levels has important theoretical and practical significance for the rational application of phosphorus fertilizer. In this study, based on the phosphorus fertilizer application location test field for 9 years, three different phosphorus application treatments were set up, including low phosphorus (LP), medium phosphorus (MP) and high phosphorus (HP). The changes and correlation analysis of soil physicochemical properties, root morphology, rhizosphere phosphorus cycling microbial diversity and key functional genes were studied under different phosphorus levels. The results showed that the contents of inorganic phosphorus, organic phosphorus and total phosphorus increased with the increase of soil available phosphorus content, and the contents of organic matter and total nitrogen under LP treatment were significantly increased compared with those under MP and HP treatment. The utilization rate of phosphorus fertilizer in MP treatment was 128.50% higher than that in HP treatment. Cotton roots of LP treatment were mainly concentrated in 0~20cm shallow soil layer, and the root length and surface area increased. With the increase of soil phosphorus levels, cotton root-shoot ratio and specific root weight decreased. The root surface area and root volume of 20-40cm and 0-60cm soil layers in MP treatment were increased compared with LP and HP treatments. With the increase of phosphorus levels, the number of rhizosphere microorganisms in each taxa decreased. Soil phosphorus levels had no significant effect on the α diversity of cotton rhizosphere microbial community. The Simpsom index of microbial gene in LP treatment was significantly higher than that in MP and HP treatments, and the microbial community structure was more similar between MP and HP treatments. At genus level, compared with MP treatment, the relative abundance of Sphingomonas, Bradyrhizobium and Rhizobium increased under LP treatment, the relative abundance of Lysobacter, Variovorax, Streptomyces, Pseudomonas and Phenylobacterium decreased, and the relative abundance of Variovorax, which is closely related to root growth and development, decreased by 59.12%. Compared with MP treatment, the relative abundance of phosphorus solubilizing microorganisms Bacillus and Arthrobacter in LP treatment were up-regulated, while the relative abundance of Flavobacterium, Salmonella, Escherichia and Aspergillus were down-regulated. The relative abundance of Alcaligenes and Flavobacterium treated by HP treatment was up-regulated, while the relative abundance of flavobacterium was down-regulated. Compared with MP treatment, the abundance of phosphorus activation genes and phosphorus absorption genes increased in LP treatment. The abundance of phosphorus uptake genes increased and the abundance of phosphorus activation genes decreased in HP treatment. The changes of soil physical and chemical properties had certain effects on the microbial community structure and the relative abundance of phosphorus cycling functional genes, among which pH had the greatest effect. The root configuration was closely related to the phosphorus solubilizers, and the root volume was more closely related to phosphorus solubilizers. Therefore, low phosphorus level can induce the increase of microbial functional gene diversity to activate soil phosphorus nutrients and promote root growth. The amount of phosphorus fertilizer applied in cotton production should be reduced appropriately. The recommended amount of phosphorus fertilizer (P₂O₅) for cotton in Hebei Province is 90kg·hm^-2.