Nitrogen retention and export in an alpine catchment in Valtellina: an example from the Italian Alps

Since 1994, throughfall and open field depositions were monitored in a lateral valley of Valtellina, Val Masino, characterised by a high sensitivity to acidic deposition as attested by the national maps on the critical loads of the acidity. The atmospheric flux of nitrogen, based on five years period, resulted exceeding the maximum critical load of nutrient nitrogen obtained by the EMEP maps. To investigate the role of nitrogen as nutrient, the absorption-release capacity of the ecosystem was assessed using a mass balance method. The main input of nitrogen is represented by the atmospheric deposition while the output is the load of the River Masino at the watershed outlet of the study area. Results of two years monitoring indicate that ammonia was almost completely retained within the basin, while nitrate was leached for the 90%. The total nitrogen mass balance (including the organic N) reveals that only the 60% of nitrogen input is retained by this ecosystem. The approach proposed by Traeen and Stoddard (1995) was applied to evaluate the nitrogen saturation status of the Masino catchment. It was classified as stage 2, in which the seasonal period of N limitation during the growing season is delayed and reduced with the effect that biological request does not control N concentrations for a too longer period.