Macroaggregates dominate colloidal phosphorus sequestration in Ultisol paddies: Evidence from size-fractionation...

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 KH₂PO₄ 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.