A review of ion exchange membrane and ion exchange resin beads for ecological research

Background/Question/Methods

Since the 1990’s, when the use of ion exchange resins in ecological research moved from novel to more common, two forms of ion exchange resins have been used. Both forms of ion exchange resin measure the bioavailability of soil nutrients as influenced by the soil environment and can be used in the field. The resins exchange ions for nutrient ions in the soil solution and, similar to plant roots, they create a sink for soil nutrients. Although their mode of action is similar, ion exchange membranes (IEMs) and ion exchange resin beads differ in several key areas. The objective of this poster is to review those differences as a means to help researchers better interpret ion exchange resin measurements and move toward a universal method.
Results/Conclusions

IEMs have a flat, essentially 2-D surface which, if placed properly, ensures a quantifiable surface area for ion exchange thus allowing simple calculation of an ion supply rate per surface area. In a bag of resin beads, the total surface area in contact with the soil is difficult to quantify and the innermost beads may rely more on secondary ion diffusion than direct diffusion from soil particle surface to resin surface. McLaughlin et al. (1994) and Fernandes and Warren (1996) found that cation IEMs required fewer extractions with 0.1M HCl than resin beads in order to remove all cations. Perhaps most importantly is the variation in data with the use of each tool. Although multiple examples of the reproducibility and repeatability of each method is generally lacking, McLaughlin et al. (1994) found that the IEMs had slightly greater coefficients of variation than resin beads in loose form. Finally, resins in membrane form create less soil disturbance than resin bags, are easier to remove from soil than loose resin beads, and require less effort to rinse soil thoroughly than from the resin. Although, IEMs and resin beads are valuable tools for measuring the labile ions available to plants, the accurately quantifiable surface area of the IEM combined with its ease of use and extraction make this tool preferable as a standard ion exchange resin method.