Phase Partinitioning of Ammonium in Australian and European Forest Soils

An understanding of phase partitioning of ammonium (NH 4 ) in soils is one of a number of parameters needed to predict the consequences of nitrogen (N) inputs from atmospheric deposition or the application of fertilizers, and for predicting NH 4 uptake by roots. However, very few data are available on solid-liquid partition coefficients (K(d)) for NH 4 . The main aims of this study were to determine and compare the K(d) values for NH 4 in a variety of soils and horizons from Australia and Europe. A Freundlich function was fitted to the sorption or desorption data for each sample and the K(d) value calculated at three concentrations (0.001, 0.1, and 1 mM). The Australian study indicated that, although site differences in K(d) values were evident at 0.1 mM, variability was too high to distinguish site effects at other concentrations, or depth and plot effects at any concentration. Depth effects were evident at one of the European sites. Despite samples of contrasting origin and different methods of measurement, there was similarity in the K(d) values at 0.1 mM for most mineral soils. Organic horizons had higher K(d) values at 0.1 mM (8.7-26.2 mL g -1 ) than most mineral soils (0.3-7.4 mL g - 1 ), but mineral soil at one site had a particularly high K(d) value (36.9 mL g -1 ). In common with other solutes, NH 4 concentrations were well buffered at low concentrations ( < 0.005 mM), but improved precision and accuracy of NH 4 measurements at low concentrations will be needed to further improve our knowledge of partitioning at these concentrations.