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

Soil aggregates serve as critical reservoirs for colloidal phosphorus (Pcoll), governing its sequestration and release, and thereby influencing nutrient cycling and mitigating P loss in agricultural systems. However, the mechanisms controlling Pcoll 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 Pcoll release via water-dispersible colloids (Crq), Pcoll content, colloidal surface morphology, and stability across varying slope positions, pedogenic horizons, and cultivation histories. Key findings reveal that Crq and Pcoll concentrations increased significantly with soil depth, by approximately 200%-1400% and 25%-67%, respectively, with MAC exhibiting consistently higher Pcoll 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 Pcoll. These findings underscore the dominant role of MAC in Pcoll 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.