NITROGEN SUPPLY RATE IN SCOTS PINE (PINUS-SYLVESTRIS L) FORESTS OF CONTRASTING SLOPE ASPECT

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. NH4+-N, NO3--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 Al horizon. Nitrification rates differed significantly among plots. N supply rate was 12.7-23.5 kg N.ha(-1).yr(-1) 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 Al horizons (about 240 kg N.ha(-1).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.