Profile Distributions of Dissolved and Colloidal Phosphorus as Affected by Degree of Phosphorus Saturation in Paddy Soil

Soil dissolved phosphorus (P) and colloidal P mobilization could be closely related to the degree of phosphorus saturation (DPS). Effects of a wide range of DPS on the distributions of dissolved P and colloidal P in a paddy soil profile were investigated in this study. Dissolved P and colloidal P in water-dispersible soil colloid suspension increased obviously with increasing DPS. The change point of DPS was at 0.12 by using a split-line model. Above the value, dissolved P (3.1 mg P kg −1 ) in soil profile would increase sharply and then transfer downward. Compared with dissolved P, colloidal P was the dominant fraction (78%–91%) of P in soil colloid suspension, and positively related to DPS without a significant change point. The high release of colloids in subsoils with low DPS was attributed to the low ionic strength and high pH value in subsoils. The DPS also had a significant and positive correlation with electrical conductivity (EC), but it showed a negative correlation with pH value. However, the concentration of colloidal P was not greatly correlated to the pH value, EC and optical density of the soil colloid suspension. The results indicated that DPS was an important factor that may affect the accumulation and mobilization of water-extractable colloidal P and dissolved P.