Effects of N-deposition on ion trapping by B-horizons of Danish heathlands

Coastal and inland heaths constitute a dominating fraction of the relatively few semi-natural ecosystems in Denmark. At the inland heath, the soil forming factors have been active with the same vegetation type during the last 2000 years, whereas at the coastal heath, these soil forming factors have only been active over a couple of hundred years, due to the activity of windblown sand. Effects on ion-balance and soil processes of nitrogen deposition on a young dune heath and an old inland heath have been investigated. Different levels of NH₄NO₃have been applied to the two heathlands, and rain, throughfall water and soil water chemistry below the experimental plots have been analysed. Results show that the ability to retain added, as well as deposited, nitrogen requires the presence and the integrity of a humifyed H-subhorizon. When the mor-layer for some reason breaks down, the nutrients will be retained by the B-horizons as a kind of back-up, and most probably be reused by the new vegetation. One year after destructive heather beetles attack on the inland heath, the mor-layer started to decompose. A large translocation of organic compounds, complexed associated cations and nitrogen-species, started from the mor-layer to the top of the B-horizon where they precipitate. The podzolic B-horizons can be seen as nutrient trap to the mor-layer.