Bioavailability Index for Phosphorus Using Ion Exchange Resin Impregnated Membranes

Abstract

Accurate measures of nutrient bioavailability are important in understanding nutrient cycles. Bioavailability is related to both the diffusion rate of the nutrient through the soil and nutrient solution concentration at any given time. The purpose of this work was to investigate the use of ion exchange resin impregnated membranes as a potential bioavailability measure. We propose that the ion sink membranes (IS) will integrate soil nutrient concentration and effective nutrient diffusion rates, resulting in a more complete measure of nutrient bioavailability. The IS are applied directly to the soil and recovered after fixed time intervals. The rate at which ions are adsorbed by the IS is directly related to c²Dₑ, where c is the labile P concentration and Dₑ is the effective diffusion coefficient. We refer to this value as the IS bioavailability index of a given nutrient. This index provides an integrated measure of ion diffusion, sorption-desorption rates, and solution concentration. The resulting bioavailability index can then be correlated with plant nutrient uptake. The IS bioavailability index for P and Fe correlated well with P extracted by ammonium bicarbonate-diethylenetriamine pentaacetic acid (AB-DTPA) in saturated soils across a range of soil Fe and P concentrations. In addition, plant P uptake under saturated conditions correlated with the IS bioavailability index across a range of soil P concentrations.