Measuring In Situ Changes in Labile Soil Phosphorus with Anion‐Exchange Membranes

Conventional chemical extraction methods to measure labile soil P are often inadequate for detecting fine temporal‐ and spatial‐scale soil P dynamics in situ. We refined and calibrated methodology for anion‐exchange resin‐impregnated membranes (AEM), related AEM‐P to soil solution P for a high‐P‐retaining soil, and evaluated the method's viability under humid tropical field conditions. We determined: (i) AEM recyclability, (ii) AEM P sorption kinetics, (iii) the correlation between soil solution P and AEM‐P for an Andic Humitropept, and (iv) potential interference from other anions (NO⁻₃ and SO²⁻₄) on AEM P extraction. We used AEMs in a field decomposition study to evaluate plant residue and manure P release characteristics and concurrent fluxes in labile soil P. The AEM P sorption capacity was not altered significantly by repeated use. Nitrate solution concentrations in an aqueous medium of 50 and 100 mg NO₃‐N/L reduced AEM P sorption by 50 and 75%, respectively, regardless of P solution concentration; SO₄‐S at 500 and 1000 mg/L reduced AEM P sorption by ≈98%. The relationship between AEM‐P and soil solution P was curvilinear at both nonequilibrium and equilibrium soil solution P concentrations; it was essentially linear at soil solution concentrations ranging from 0 to 2 mg P/L. The AEM behaved as a dynamic exchanger rather than an infinite sink for P, particularly in the context of a low‐pH, high‐P‐retaining soil. The AEMs detected biologically relevant soil P pulses in the field decomposition study. The technique holds promise as an easy method for measuring soil P fluxes with minimal soil disturbance.