Abstract: We investigated the effects of exogenous calcium (Ca) on the rhizosphere microbial community structure and soil phosphorus cycling of ectomycorrhiza (ECM)associated Pinus massoniana seedlings, aiming to provide reference for improving the productivity of ECM-dominated forest plantations. There were four treatments, including control, ECM fungal inoculation, Ca addition, and ECM fungal inoculation + Ca addition. The effects of Ca addition on rhizosphere microbial community composition of ECM-associated seedlings were analyzed by metagenomics. The structural equation model (SEM) was used to analyze the relationship between phosphorus cycle functional genes, microbial biomass (MBC, MBP), extracellular enzymes (CBH, ACP, POD), and root parameters. The main factors affecting phosphorus nutrition of ECM-associated seedlings were analyzed. The results showed that: (1) Ca additions significantly increased the relative abundance of Actinobacteria and Bacillota in the rhizosphere of ECM-associated seedlings, mainly including species from Arthrobacter, Mycolicibacterium, Streptomycetaceae, Bacillus, and Rhodococcus; (2) These enriched beneficial bacteria cooperated with ECM fungi to significantly increase ACP secretion in rhizosphere. After the addition of Ca, the relative abundance of microbial phosphorus absorption and transport genes (pstB) and inorganic phosphorus dissolution genes (ppa and pqqC) increased in rhizosphere, which thereby significantly increased the rhizosphere MBP and phosphorus content of ECM-associated seedlings by 86.21% and 47.07%, respectively; (3) Ca additions increased the number of absorptive roots with larger diameter (root diameter of 0.5-2.0 mm) of ECM-associated seedlings. These seedlings adopted dual strategies of altering root architecture and enhancing mycorrhizal symbiosis to acquire soil phosphorus. Under the condition of phosphorus limitation, adding a certain amount of calcium can improve soil phosphorus cycling and promote the uptake of soil phosphorus by ECM-associated seedlings.

Key words: exogenous calcium, ECM-associated seedling, microbial community composition, soil phosphorus cycling, plant phosphorus nutrition