Phase partitioning of ammonium in Australian and European forest soils

An understanding of phase partitioning of ammonium (NH,) 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 f or predicting NH4 uptake by roots, However, very few data are available on solid-liquid partition coefficients (K-d) for NH4. The main aims of this st udy were to determine and compare the K-d values for NH4 in a variety of so ils and horizons from Australia and Europe. A Freundlich function was fitte d to the sorption or desorption data for each sample and the K-d value calc ulated 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 concent rations, or depth and plot effects at any concentration. Depth effects were evident at one of the European sites. Despite samples of contrasting origi n and different methods of measurement, there was similarity in the K-d val ues at 0.1 mM for most mineral soils. Organic horizons had higher K-d value s 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, NH4 concentrations were well buffered a t 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.