Soil nitrogen (N) dynamics were studied in a dense, holm oak (Quercus ilex ssp. ilex) stand in the Montseny mountains to determine annual and seasonal patterns of N availability and uptake in an undisturbed Mediterranean forest on acidic soil. Soil mineral N content, net N mineralization (NNM), and net nitrification (NN) were determined by monthly sampling at two soil depths followed by in situ incubation in polyethylene bags. N N M per unit of soil mass was much higher at 0-5 cm than at 5-20 cm (annual means 24 and 2.5 mg N/kg, respectively) but on an area basis N N M was similar at both depths. A total of 80 kg N/ha/yr were mineralized from the first 20 cm of soil. NN amounted to only 9°~ o of the annual N N M (7.5 kg N/ha/yr) and it occurred only in the upper 5 cm. N N M was maximum in June and July, while the NN peaked in May. Despite favourable soil temperature and moisture, N N M was negative in autumn because of microbial immobilization. Seasonal and depth variations of N N M appeared to be controlled more by substrate quality than by organic matter quantity, temperature or moisture. NN was not limited by ammonium availability. Calculated N uptake amounted to 91 kg/ha yr, peaking in June and July. The investigated stand showed a moderately high N availability, but ammonium was the major form of mineral N supply for holm oak.
:' Organic matter. b Extractable P with the Bray-Kurtz method using ammonium fluoride as extractant. " Extracted with anllnOllium acetate at pH 7.