Spatial Characterization and Operational Fractionation of Residual Soil Phosphorus in Arable Lands of Hot-Arid Regions: Comparison of Conventional and Customized Extraction Procedures

One of the vital secondary resources for sustainable agriculture is residual soil phosphorus (RSP), which is accumulated from the continuous deposition of unutilized phosphate fertilizers. This study employed a tripartite approach comprising the spatial characterization of dominant phosphorus (P) fractions, assessment of their variation among land-use and nutrient-management categories, and comparison of conventional and customized P extraction procedures. The investigated soils were deep, coarse-textured, alkaline, and poor in fertility, representing Entisol and Aridisol soil orders. Total soil P ranged from 151–717 mg kg−1 comprising inorganic (~67%) and organic (~33%) phosphates. Calcium phosphates and phytates were the dominant inorganic and organic P fractions, respectively, and their concentrations differed significantly among the surveyed land-use and nutrient-management categories. - These differences indicate that the operationally extracted fractions were sensitive to the management categories represented in the survey, but do not by themselves demonstrate plant availability or reversible transformation. Based on their chemical extractability, the RSP was operationally grouped into three conceptual P pools (CPPs): highly reversible (HRV), moderately reversible (MRV), and irreversible (IRV). The largest proportion of RSP was recovered in MRV (57.8–62.0%), followed by IRV (27.6–34.0%) and HRV (8.2–10.4%). The conventional and customized extraction procedures were subsequently compared to evaluate differences in the operational recovery and allocation of P among these pools. The two procedures showed a strong linear association for total RSP (R2 = 0.92, p < .001), whereas the relationships among individual pools differed substantially, being strongest for MRV (R2 = 0.926), moderate for HRV (R2 = 0.5904), and poor for IRV (R2 = 0.0926). Differences in pool recovery were therefore interpreted as differences in operational extractability and allocation resulting from the extraction procedures, rather than as evidence of transformation or transfer of P between pools. The results provide a soil-specific operational framework for characterizing residual soil P in hot-arid agricultural soils and highlight the importance of interpreting chemically extracted P pools separately from direct measures of crop availability or biological reversibility.