Non-rhizobial endophyte recruitment and diversity in Pisum sativum are strongly shaped by phosphorus fertilizer form

Non-rhizobial endophytes (NREs) support plant health and nodule function by enhancing symbiotic interactions and nitrogen fixation. However, their recruitment dynamics under fertilizers of varying phosphorus solubility remain poorly understood. This study investigated how four P fertilization treatments-no phosphorus (P0), bone char (BC), surface-modified bone char plus (BC^plus), and triple superphosphate (TSP)-with increasing solubility influence microbial recruitment and diversity in Pisum sativum, leading to differences in plant-available phosphorus across bulk soil, rhizosphere, roots, and nodules.

Using 16S rRNA amplicon sequencing, we found that nodule-associated microbial communities were primarily recruited from unknown sources, likely seeds, followed by roots, especially under BC^plus. Phosphorus solubility of treatments significantly influenced recruitment patterns, with solubility further shaping microbial diversity. BC^plus recruited beneficial taxa like Beijerinckiaceae and Flavobacteriaceae, which are associated with nitrogen fixation and biocontrol. In contrast, the highly soluble TSP treatment expanded recruitment from the rhizosphere, reflecting less stringent environmental filtering and promoting taxa like Steroidobacteraceae and Blastocatellaceae, known for nutrient cycling and pathogen suppression. In the absence of P fertilization (P0), recruitment relied heavily on seeds and roots, with arbuscular mycorrhizal fungi colonization prioritized over nodulation. Notably, TSP supported significantly more nodules with greater microbial diversity, potentially enhanced by NREs.

Phosphorus solubility of the applied fertilizers strongly influences NRE recruitment dynamics in P. sativum. Seeds and roots act as primary reservoirs, while highly soluble fertilizers promote broader recruitment from the rhizosphere and increase microbial diversity in nodules. These results underscore the importance of the fertilization form in modulating NRE recruitment.