Factors and P‐status implications of size‐dependent colloidal stability in pedogenic horizons of reddish paddy soil

Colloidal stability plays a critical role in regulating nutrients transport and contaminant mobility in paddy soils. This study systematically investigated the mineral composition, stability characteristics, and controlling factors of water‐dispersible colloids across pedogenic horizons (Ap1, Ap2, Br, and Brs) in a representative Ultisol with over 200 years of cultivation history. Four colloidal size fractions (<0.1, 0.1–0.45, 0.45–1, and 1–5 µm) were characterized using scanning electron microscope and X‐ray diffraction. Colloidal stability was quantified through critical coagulation concentration measurements in KH 2 PO 4 and NaCl solutions (0–100 mg L −1 ). Results show that: (1) smaller colloids (<0.1 µm) exhibited the highest stability due to its expandable minerals constituents and pronounced weathering features; (2) Colloidal stability showed significant positive correlations with surface‐area‐normalized carbon and nitrogen content, as well as total organic matter, while being negatively associated with amorphous aluminum oxide content; (3) Electrolyte effects followed concentrations‐dependent thresholds, with Na + (>80 mmol L −1 ) inducing flocculation and H 2 PO 4 − (20–80 mg L −1 ) enhancing colloidal dispersion. These results provide fundamental insights into pedogenically‐driven colloidal behavior and offer practical implications for optimizing nutrient management strategies in intensive paddy farming systems.