We studied Nitrogen (N) transformations in Pinus sylvestris forest sta
nds in the foothills of the SE Pre-Pyrenees (NE Spain). Plots were sel
ected in two contrasting aspects (two plots per aspect) and N supply r
ate was measured by the resin-core incubation technique once every thr
ee months. N leaching through litter layers (L and F horizons) was eva
luated by 5 zero-tension lysimeters in each plot. NH4+-N, NO3--N and s
oluble organic-N were determined in all solutions. N supply rate showe
d a clear seasonal pattern. Ammonification and nitrification were segr
egated in space and in time. While ammonification showed a peak in spr
ing, nitrification was higher in summer. There was evidence suggesting
that nitrification occurs mostly in Al horizon. Nitrification rates d
iffered 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 repr
esented 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 i
norganic-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 fat
e of these large amounts of organic-N is discussed.