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We studied Nitrogen (N) transformations in Pinus sylvestris forest stands in the foothills of the SE Pre-Pyrenees (NE Spain). Plots were selected in two contrasting aspects (two plots per aspect) and N supply rate was measured by the resin-core incubation technique once every three months. N leaching through litter layers (L and F horizons) was evaluated by 5 zero-tension lysimeters in each plot. NH +-N, NO 3 -N and soluble organic-N were determined in all solutions. N supply rate showed a clear seasonal pattern. Ammonification and nitrification were segregated in space and in time. While ammonification showed a peak in spring, nitrification was higher in summer. There was evidence suggesting that nitrification occurs mostly in A1 horizon. Nitrification rates differed significantly among plots. N supply rate was 12.7-23.5 kg N.ha-~ -yr-l but it did not differ between aspects or plots. Inorganic-N leached through litter layers was 14-17 kg N.ha -1.yr -1, and represented a high proportion of N supply rate. Organic-N leached through litter layers (27.8-37.0 kg N.ha-1.yr-1 ) was higher than leached inorganic-N. However, in most cases organic-N did not represent a high proportion of changes in soluble organic-N pools in H and A1 horizons (about 240 kg N.ha-l.yr-1). This large decrease in soluble organic-N was much greater than the increase in inorganic-N. The possible fate of these large amounts of organic-N is discussed.

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