Evaluation of anion exchange membranes to estimate bioavailable phosphorus in native grasslands of semi‐arid Northwestern Australia

Anion exchange membranes (AEMs) were used to assess the P status of semi‐arid sub‐tropical soils of high P sorption capacity from the Pilbara region in northwestern Australia. We determined the most appropriate procedure for using AEMs in these soils using a factorial of extraction ratios and shaking times and compared the method with extraction by water. Significantly more inorganic P (Pi) was extracted by the membranes (AEM‐Pi) than by water, and the amount extracted increased with extraction time but was generally independent of the extraction ratio. Maximum AEM‐Pi was 3.61 μg g‐1 after eight hour extraction. The AEM procedure was compared with traditional extraction procedures using 0.5 M sodium bicarbonate (NaHCO3) and 0.1 M sodium hydroxide (NaOH) to assess ability to detect spatial heterogeneity. The amount of Pi extracted decreased in the order: AEM>NaOH>NaHCO3* The AEM method detected a significant effect of depth on Pi (P=0.0001), while the NaOH method detected both site and treatment effects (P<0.05). Inorganic P extracted by NaHCO3 did not vary by site, treatment, or depth. Coefficients of variation were generally least using the AEM method. We recommend that studies of spatial and temporal dynamics of P on highly‐weathered soils in semi‐arid regions include measurement of both AEM‐Pi and NaOH‐extractable Pi.