An anion resin membrane technique to overcome detection limits of isotopically exchanged P in P‐sorbing soils

Summary Isotopically exchanged phosphorus is difficult to determine in soils that strongly sorb P (so that there is little P in solution) and in soils with large concentrations of colloidal P in soil suspensions. A method is proposed in which anion exchange membranes (AEM) are added to the soil suspension after an initial period of isotopic exchange with 32 P‐labelled phosphate ions. Isotopically exchanged P, termed E AEM , is calculated from the ratio of labelled phosphate ions to the total phosphate ions on the membrane. The E AEM was compared with the E value measured in an aqueous soil extract ( E Water extract ) for 14 soils with different degrees of P sorption. The two methods gave similar results in soils with large P concentrations in an aqueous soil extract. However, E Water extract values significantly exceeded the E AEM values by up to 18‐fold when soluble P was near the determination limit (0.008 mg P l −1 ). In a second experiment, two Ferralsols received further P from inorganic and plant sources and were incubated for 7 days. Treatment effects on labile P were erroneous as detected by the E Water extract but were significant as detected with the AEM method. Third, E AEM values were followed in a Lixisol and a Ferralsol which received labelled phosphate ions with carrier just before the beginning of a 23‐day incubation. The approximate recovery of added inorganic P in the E AEM value suggested that this method adequately samples labile P in P‐sorbing soils. All these results showed that errors in the determination of E values for soils with very small concentrations of P in the soil solution are reduced using the proposed method.