Interaction of Phosphorus Compounds with Anion-Exchange Membranes: Implications for Soil Analysis

Anion exchange membranes are commonly used to measure readily exchangeable and microbial P in soil, yet there is little information on their interactions with organic and condensed inorganic P compounds, which can interfere with interpretation of the results. We addressed this by quantifying the sorption of a range of P compounds to a commonly used anion exchange membrane (551642S, BDH-Prolabo, VWR International, Lutterworth, UK). Sorption and recovery of orthophosphate by the membranes was complete up to 1.17 g P m(-2). The membranes also completely recovered sodium pyrophosphate, glucose 6-phosphate, and adenosine 5'-monophosphate, as well as significant levels (20-60%) of 2-aminoethylphosphonic acid, sodium hexametaphosphate, and sodium phytate. Only sodium pyrophosphate, sodium hexametaphosphate, and D-glucose 6-phosphate were detected subsequently as molybdate-reactive P after elution with 0.25 mol L-1 H2SO4, however, indicating their hydrolysis in the acid eluant. Solution P-31 nuclear magnetic resonance spectroscopy was used to confirm the stability of the tested compounds when exposed to the membrane and the absence of significant concentrations of orthophosphate as trace contaminants in the compound preparations. Finally, for a series of tropical wetland soils from the Republic of Panama, we found negligible difference in eluted P concentrations determined by molybdate colorimetry and inductively coupled plasma optical emission spectrometry for both unfumigated and hexanol-fumigated samples. We therefore conclude that although organic and condensed inorganic P compounds can be recovered by anion exchange membranes, this is likely to have limited impact on the analysis of soil samples.