The effects of atmospheric nitrogen deposition on the soil chemistry of coniferous forests in The Netherlands

Nitrogen fluxes, particularly those of ammonium, are extremely high in Dutch forests. In soils exposed to high ammonium deposition, acidification, eutrophication or a combination of both processes may occur. In addition to the amounts of ammonium deposited, the rate of soil nitrification determines which process takes place. A nation-wide investigation, in which three coniferous tree species were involved, was carried out to study the relation between deposition fluxes, measured by means of throughfall and bulk samplers, and the chemical composition of the soil. The ammonium deposition accounted directly for the high ammonium content and the high ammonium/cation ratios in the soil. In the top layer of most of the forest soils which were investigated nitrification rates were low. In these stands ammonium/cation ratios in the soil often reflected ammonium/cation ratios in throughfall water. Even in soils with relatively high nitrification rates, ammonium concentrations exceeded those of nitrate in the top layer of the mineral soil, indicating that ammonium deposition was more important than nitrification rate in determining the predominant form of nitrogen.