In Situ Efficiency of Ion Exchange Resins in Studies of Nitrogen Transformation

Mixed‐bed ion‐exchange resin bags have previously been used in studies of soil N transformation rates with NH 4 ‐N and NO 3 ‐N being adsorbed from the solution percolating through the incubated soil core. An evaluation of the in situ adsorption efficiency of mixed‐bed resin bags was performed by comparing dissolved inorganic nitrogen (DIN, sum of NH 4 ‐N + NO 3 ‐N) accumulated in resins with DIN fluxes in throughfall (TF) and with DIN concentrations in soil water. A significant correlation was found between DIN fluxes in TF and accumulated DIN in resins placed at the soil surface ( r 2 = 0.92 for NO 3 ‐N, r 2 = 0.86 for NH 4 ‐N, P < 0.001). The ratio of accumulated DIN in resins to DIN flux in TF was significantly affected by season. A low but significant correlation was found between NO 3 ‐N concentrations in soil water and NO 3 ‐N contents in resins deployed in the bottom of soil cylinders ( r 2 = 0.34, P < 0.01), however, when only the winter periods were taken into account, the correlation improved ( r 2 = 0.72, P < 0.001). As little water and few nutrients are removed from the soil water by the vegetation during the dormant season, the conditions inside and outside the core were more comparable. For NH 4 ‐N there was no correlation between accumulated amounts in the resins and concentrations in soil water sampled at depths of 13 cm and 20 cm, respectively, probably due to the strong depth gradient in the NH 4 ‐N concentrations of the soil. Although the resin bags were unable to adsorb all the incoming DIN, they gave valuable information on small‐scale inpnt of N and on small‐scale differences in NO 3 ‐N leaching.