Combined application of phosphorus fertilizers enhances phosphorus uptake and use efficiency in cotton by regulating soil phosphorus transformation

Background Phosphorus (P) availability is a major constraint on cotton production in the calcareous soils of arid Xinjiang because fertilizer-derived P is readily converted into less available forms. Whether combining water-soluble P fertilizers can improve soil P availability and P-use efficiency by modifying soil P transformation under drip-irrigated cotton production remains unclear. Results We conducted a two-year field experiment (2022–2023) comparing a no-P control, single applications of monoammonium phosphate (MAP), ammonium polyphosphate (APP), and urea phosphate (UP), and MAP–APP or MAP–UP combinations at P₂O₅ ratios of 7:3 and 8:2 under drip fertigation. Compared with single P sources, the MAP–APP and MAP–UP combinations generally increased soil available P and promoted the accumulation of labile and moderately labile inorganic P fractions. Exploratory structural equation modeling (SEM) further indicated that Ca₂-P, Ca₈-P, and Fe-P were positively associated with soil available P, which in turn was positively associated with seed cotton yield. Among the four combinations, the 7:3 MAP–APP treatment (M7A3) achieved the highest P recovery efficiency (PRE, 15.1%) and the highest two-year mean seed cotton yield (7114.5 kg ha⁻¹), representing an increase of 2418.2 kg ha⁻¹ over the no-P control. Conclusions The overall advantage of MAP–APP and MAP–UP co-application was associated with a redistribution of soil inorganic P toward more labile and moderately labile pools, together with greater soil P availability and plant P utilization. Among the tested combinations, M7A3 showed the strongest integrated soil–plant P response, suggesting that it may represent a promising P-management option for drip-irrigated cotton grown on calcareous soils under the conditions tested.