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

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 ³²P‐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.