Phosphate-Solubilizing Bacteria Mobilize Nano-Hydroxyapatite In Vitro with Limited Transferability to Maize (Zea mays L.) Phosphorus Nutrition in Soil

Abstract

Limited phosphate rock resources and inefficient phosphorus (P) fertilization challenge sustainable crop production. Nano-hydroxyapatite (nHA) serves as a model for poorly soluble calcium phosphates relevant to recycled P fertilizers. We tested whether phosphate-solubilizing bacteria (PSB) enhance P mobilization from nHA and maize (Zea mays L.) nutrition under moderate-temperature conditions. In vitro assays evaluated six bacterial strains and organic acid (OA)-mediated P mobilization; a climate-chamber pot experiment tested nHA, PSB, and their combinations under low and moderate P supply. Several strains increased supernatant P concentrations in vitro; Pseudomonas rhodesiae showed the strongest response, corresponding to 90% of the P introduced with nHA. OA-mediated P mobilization was greater at pH 3.5 than at pH 5.0, although different OA concentrations were used to reach the respective target pH values. The descriptive differences among OAs may reflect additional OA-specific interactions with Ca, including complexation. In the pot experiment, shoot P uptake did not differ significantly among treatments under low P supply. Under moderate P supply, the treatment with nHA depot fertilization without added medium or bacterial inoculation showed the numerically highest shoot P uptake and dry matter production. No consistent additional PSB-mediated benefit relative to the corresponding sterile-medium controls was demonstrated. Strong in vitro nHA mobilization showed limited transferability to buffered soil systems.