Intensive agricultural management-induced subsurface accumulation of water extractable colloidal P in lime concretion black soil

Abstract. Long-term excessive application of mineral fertilizer leads to accumulation of phosphorus (P) in lime concretion black soil, which increases the risk of P migration and loss from soil profile. The colloids in the profile are important carriers for P migration due to high P adsorption and transport capacity. In this study, water extractable colloids (WECs) were obtained from 0 ~ 120 cm soil profile by soil fractionation method, and their physicochemical properties were analyzed. Solution 31P nuclear magnetic resonance (NMR) and P K-edge XANES were used to characterize the species and distribution of colloidal P in fertilized farmland soil profile. Total and available P in bulk soil and colloids decreased with soil depth. The organic P (OP) contained ~97 to 344 mg kg-1 per bulk soil and 110–630 mg/kg per WECs in soil profile with composition of orthophosphate monoesters and diesters according to NMR results. It suggested that OP in WECs from subsoils might be affected by the translocation of colloidal P (CP) from surface soils probably due to soil acidification and preferential flow caused by swelling-shrinkage clays including montmorillonite and nontronite. Additionally, the more negative zeta potential of surface soil colloids suggests the high mobility of colloidal P to the subsoils. The CP concentrations for <2 µm was about 38–93 mg/kg per bulk soil, which is 6–37 folds of dissolved reactive P (DRP) concentrations, suggesting that the role of CP for P transport in the soil profile is dominated. This study showed that inorganic and organic P migrated from surface to deeper layer along the lime concretion black soil profile, with soil colloids having a significant effect on P migration from both surface and subsurface layers. The results have an important significance for soil P migration evaluation and agricultural non-point source pollution control in lime concretion black soil.