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 (NO3- and SO42-) 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 almost-equal-to 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.