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. $NH_4^ + $-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 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.