Investigating chemical constraints to the measurement of phosphorus in soils using diffusive gradients in thin films (DGT) and resin methods

The effect of potential chemical constraints on the performance of two relatively new soil P testing methods, anion exchange membrane (AEM) and diffusive gradients in thin films (DGT), were evaluated. Exposures to ranges of anion (Cl−, NO3−, SO42− and HCO3−) concentrations relevant to agricultural soils had minimal effect on P recoveries using DGT. It has also been shown previously that DGT P recoveries are unaffected by varying pH (3–9). In contrast, increasing NO3− and SO42− concentrations in solution reduced the recovery of P using the resin method (anion exchange membrane, AEM) by 24% at 50 mg L−1 NO3− and by 47% at 12 mg L−1 SO42− when the P concentration of the test solution was 2 mg L−1. Phosphorus sorption by the resin decreased with increasing Cl− concentrations until there was a 100% decrease at 300 mg L−1 Cl− when the P concentration of the test solution was 2 mg L−1 and a 92% reduction at 700 mg L−1 Cl− when the P concentration of the test solution was 0.2 mg L−1. There was also a small but significant effect of carbonate species on P sorption to the resin at carbonate concentrations that can occur in agricultural soils. Changing the pH of the solution had minimal effects on the resin P measurements in solutions above pH 4, but below pH 4, resin P measurements decreased dramatically. A poor coefficient of determination for the regression fit between DGT and resin measurements on eight agricultural soils suggested that these two methods are measuring different amounts of P for different soils. Resin P measurements increased significantly, but non-uniformly across soils, when the soil:water ratio was decreased but this did not result in an improved relationship with DGT P. There was a minimal effect on measured P using either Cl− or HCO3− as counter ions on the resin.