Co-inoculation of nitrogen-fixing cyanobacteria promotes phosphorus activation and shifts phoD-harboring microbial communities in coastal saline-alkaline soils

Low phosphorus (P) availability constrains the nutrient supply in saline-alkaline soils. Nitrogen-fixing cyanobacteria have potential as biological amendments, yet their effects on soil P activation and phoD-harboring microbial communities remain insufficiently understood. In this study, a 90-day incubation experiment was conducted to evaluate the effects of single inoculation and co-inoculation with Anabaena azotica and Tolypothrix tenuis on soil physicochemical properties, P fractions, dissolved organic matter (DOM), alkaline phosphatase (ALP) activity, and phoD-harboring microbial communities in coastal saline-alkaline soil. Cyanobacterial inoculation improved the soil microenvironment, with the co-inoculation treatment (DT) producing the most pronounced effects. In the DT treatment, soil pH decreased by 0.42 units, while available P increased from the initial value of 16.53 to 23.84 mg/kg, representing a 44.2% increase. Sequential P fractionation and 31P-NMR analyses indicated a redistribution of soil P toward more labile pools and a more pronounced orthophosphate signal, while XRD revealed treatment-associated differences in relative mineral composition. Cyanobacterial inoculation also increased humic-like DOM components and ALP activity, with DT showing 49.07% higher ALP activity than the CK at 90 days. Redundancy analysis (RDA) showed that the first two axes explained 87.59% of the variation in the phoD-harboring community, with soil pH, total salt content, and nutrient availability as the main associated environmental variables. These results indicate that co-inoculated nitrogen-fixing cyanobacteria enhance P availability and phosphatase-related microbial potential in saline-alkaline soil, highlighting their potential role in improving soil ecological functions.