Macroaggregates dominate colloidal phosphorus sequestration in Ultisol paddies: Evidence from size‐fractionation experiments

Abstract Soil aggregates serve as critical reservoirs for colloidal phosphorus (P coll ), governing its sequestration and release, and thereby influencing nutrient cycling and mitigating P loss in agricultural systems. However, the mechanisms controlling P coll dynamics across aggregate fractions, particularly macroaggregates (MAC, 2–0.25 mm) and microaggregates (MIC, <0.25 mm) remain poorly understood in Ultisols. This study systematically investigated P coll release via water‐dispersible colloids (C rq ), P coll content, colloidal surface morphology, and stability across varying slope positions, pedogenic horizons, and cultivation histories. Key findings reveal that C rq and P coll concentrations increased significantly with soil depth, by approximately 200%–1400% and 25%–67%, respectively, with MAC exhibiting consistently higher P coll retention than MIC. Scanning electron microscopy highlights distinct colloidal structures: MAC contain larger, loosely bound particles, whereas MIC display compact, layered arrangements. Colloidal aggregation intensified at lower slope position and in deeper soil horizons, forming denser clusters. In dynamic KH 2 PO 4 dispersion experiments, both MAC and MIC demonstrated strong colloidal mobility, but MAC demonstrate a greater capacity to immobilize P coll . These findings underscore the dominant role of MAC in P coll sequestration. To enhance MAC abundance in practice, management strategies such as applying organic amendments, reducing tillage intensity, and optimizing water management can be employed, thereby providing a practical pathway for sustainable phosphorus management in rice paddies.