Soil-dependent effects of climate-adapted phosphate-solubilizing bacteria and salmon-bone nanohydroxyapatite on early maize phosphorus acquisition under controlled conditions

Abstract

Phosphorus (P) availability remains a major constraint to crop productivity, and improving the efficiency of recycled P fertilizers requires biological strategies able to mobilize poorly soluble phosphate sources. Salmon-bone nanohydroxyapatites (SnHAs) represent a promising circular P fertilizer, but their agronomic effectiveness may depend on microbial mediation in the rhizosphere. The study tested, under controlled conditions, whether native phosphate-solubilizing bacterial (PSB) consortia isolated from long-term agroecosystems under contrasting climatic conditions improve early maize growth, P nutrition, and P-acquisition responses when applied alone or in combination with SnHAs. A total of 59 native PSB isolates were obtained from Mediterranean, Continental, and Arid soils and characterized for taxonomic identity, phosphate-solubilization ability, indole-3-acetic acid production, biological N₂ fixation, and quantitative P release from SnHAs. Selected strains were assembled into site-specific consortia and applied to maize seeds grown in their corresponding soils with or without SnHA fertilization. Quantitative screening revealed significant strain-dependent differences in P solubilization from SnHAs, with efficiencies ranging from 4.8% to 11.4% and a maximum release of 34.5 mg L⁻¹ soluble P. In vivo, PSB inoculation increased bacterial biomass in the substrate at all sites, with increases of 47%, 56%, and 155% in Mediterranean, Continental, and Arid soils, respectively. Maize responses were strongly soil-dependent: PSB inoculation and/or SnHA fertilization improved biomass accumulation mainly in Continental and Arid soils, while nutrient concentration responses varied among organs and elements. Root expression of ZmPHT1;1, ZmPHT1;2, and ZmPHT1;6 was significantly upregulated only in the Arid soil, where combined PSB inoculation and SnHA fertilization also increased P recovery efficiency. These findings show that native, site-specific PSB consortia can enhance the agronomic value of SnHAs, particularly under more limiting soil conditions, and highlight the potential of integrating locally adapted microbial resources with recycled phosphate nanofertilizers for sustainable P management in maize. However, field validation is required.