Nitrogen circulation in a Mediterranean holm oak forest, La Castanya, Montseny, northeastern Spain

Bulk deposition, wet-only deposition, throughfall and dry deposition inferred from washing foliage and surrogate surfaces were used to describe inorganic N inputs to a forested catchment in the Montseny Mountains (La Castanya, Catalonia, Spain). Bulk inputs of inorganic N were moderate, with a mean of 5.7 kg N ha-1 yr-1, ranging between 4 and 10 kg N ha-1 yr-1 for the period 1983 to 2000. Dry deposition fluxes estimated from washing branches added about 9 kg N ha-1 yr-1 to wet inputs and the total atmospheric deposition was estimated in 15 kg N ha-1 yr-1. Despite this substantial input flux, nearly all the inorganic nitrogen was retained within the forest ecosystem: NH4+ and NO3- concentrations decreased dramatically as water crossed the canopy and the soil profile. In the stream, at baseflow conditions, NH4+ and NO3- concentrations were always below the analytical detection limit (< 2 μeq L-1). Only briefly during peak flows did NO3- concentrations increase up to 100 μeq L-1. Averaged over 10 years (1984-1994), the export of N at the catchment outlet was 0.05 kg N ha-1 yr-1. This indicates a very tight N cycling allowing for an increase of N availability in these undisturbed forest ecosystems.