NITROGEN TRANSFORMATIONS IN 2 NITROGEN SATURATED FOREST ECOSYSTEMS SUBJECTED TO AN EXPERIMENTAL DECREASE IN NITROGEN DEPOSITION

Nitrogen transformations were studied in the forest floor and mineral soil (0-5 cm) of a Douglas fir forest (Pseudotsuga menziesii (Mirb.) Franco.) and a Scots pine forest (Pinus sylvestris L.) in the Netherlands. Current nitrogen depositions (40 and 56 kg N ha(-1) yr(-1), respectively) were reduced to natural background levels (1-2 kg N ha(-1) yr(-1)) by a roof construction. The study concentrated on rates and dynamic properties of nitrogen transformations and their link with the leaching pattern and nitrogen uptake of the vegetation under high and reduced nitrogen deposition levels. Results of an in situ field incubation experiment and laboratory incubations were compared. No effect of the reduced N deposition on nitrogen transformations was found in the Douglas fir forest. In the Scots pine forest, however, during some periods of the year nitrogen transformations were significantly decreased under the low nitrogen deposition level. At low nitrogen inputs a net immobilization occurred during most of the year leading to a very small net mineralization for the whole year. In laboratory and in individual field plots nitrogen transformations were negatively correlated with initial inorganic nitrogen concentrations. Nitrogen budget estimates showed that nitrogen transformations were probably underestimated by the in situ incubation technique. Nevertheless less nitrogen was available for plant uptake and leaching at the low deposition plots.