Nutrient sorption dynamics of resin membranes and resin bags in a tropical forest

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Plant root simulator (PRS) probes, a resin membrane technology, are increasingly used to measure soil nutrient availability in agricultural and nonagricultural systems. Like other resin technologies, the charged surface is meant to sorb nutrients until saturation; however, there is evidence that it acts as a dynamic exchanger in soils with low nutrient availability. This study compared the nutrient sorption dynamics of PRS probes during an 8-wk period in a young Andisol forest soil on the island of Hawaii with and without P fertilization. Resin bags, a more established technology, were used to measure nutrient availability as a comparison. The PRS probe sorption dynamics differed for the six nutrients measured. Except for NO3, nutrient sorption appeared to reach equilibrium with the soil labile pools after 4 wk. Resin bags acted as an infinite sink for NH4+, NO3–, and PO43– and sorbed more than the PRS probes. Both technologies detected an increase in P availability due to fertilization. A comparison of nutrient availability from PRS probes and resin bags correlated poorly. Isolation of the PRS probes and resin bags from root activity did not improve measurements of labile nutrients. More research is required to understand the processes behind the apparent desorption of nutrients from the PRS probes in acidic forest soils. If used for a minimum of 4 wk, however, the PRS probes appear to provide an equilibrium measure of labile soil nutrients with high sensitivity to treatment differences.

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