Microbial-mediated structural changes in humic acid increase phosphorus availability in the fertosphere

• First study on HA’s structural dynamics in phosphate fertilizer fertosphere (PFF). • HA increases the phosphorus supply capacity within the PFF. • Elevated P availability in PFF from HA’s expanded ion adsorption sites. • HA in PFF: Decrease in carboxyl groups and increase in microbial residues. • HA’s structural dynamics in PFF redefine HAP efficiency mechanisms. It is widely accepted that humic acid-enhanced phosphate fertilizers (HAEP) improve crop yield and phosphorus (P) use efficiency, largely because the carboxyl-rich structure of humic acid (HA) reduces P fixation and enhances soil P availability. However, this prevailing view overlooks the potential structural transformations of HA in soil and its localized regulation of P availability—restricted to the phosphate fertilizer fertosphere (PFF) rather than the bulk soil. In this study, we simulated the PFF to examine how HA influences the migration, transformation, and availability of P within the PFF. Using multispectral analysis, we characterized structural changes of HA and its corresponding ability to inhibit P immobilization within the PFF. We also identified the key microbial drivers, thereby clarifying the mechanisms through which HA regulates P availability in this microzone. Compared with that of the control, HA amendment resulted in greater migration of PFF-derived P over 90 days: cumulative migration increased by 10.7%, the migration distance extended by 0.40 mm, and the effective radius of the available P-enriched zones around the fertosphere expanded by 0.84 mm. The increased P availability might be attributed to two main mechanisms. First, the increase in HA-specific surface area and the accumulation of fungal necromass provided additional sites for ion adsorption, thereby inhibiting the transformation of P into insoluble forms. Second, HA slowed the luxury uptake of P by microorganisms, effectively reducing its biological fixation. Notably, the carboxyl functional groups of HA contributed minimally to this improvement. Instead, HA within the PFF underwent progressive depletion of carboxyl groups, mediated jointly by soil bacteria and fungi. HA enhances both the P supply area and the intensity of PFF primarily through microbial-driven structural modifications that maintain its ion-adsorption capacity. These findings offer new perspectives for re-evaluating the efficiency-enhancing mechanisms of HAEP.