Bulk deposition of atmospheric inorganic nitrogen in mountainous heathland ecosystems in North-Western Spain

Abstract

Nitrogen (N) deposition has been identified as one of the main traits of terrestrial ecosystems, affecting their structure and functioning. Among terrestrial ecosystems, heathlands are characterised by low nutrient status and highly affected by N deposition. However, few studies have been developed under natural field conditions to evaluate the amount of N deposition in these ecosystems. Therefore, a field experiment was carried out to investigate the inorganic N inputs in mountainous heathlands of North-Western Spain. Two study sites (La Majúa and San Isidro) were selected on the south side of the Cantabrian Mountains, as a representative monitoring N-sensitive ecosystems. Three replicated bulk collectors and one rain gauge were installed at each study site to collect monthly rainwater samples over three years. Wet bulk N deposition was different between the study areas (2.81 kg N ha-1 year-1 in La Majúa and 4.56 kg N ha-1 year-1 in San Isidro), but showed the same temporal pattern, with the highest N deposition rate observed in April and the lowest in August-September. In general, annual wet bulk NO3--N deposition was higher than NH4+-N, reflecting the prevailing NOX emission sources. Depositions of NO3--N were mostly originated by NOX emissions from northern highly industrialized and populated areas of North Spain. The lower NH4+/NO3- ratio in rainwater observed in the study area could be due to the decline of traditional land uses (livestock grazing, heath burning and croplands) associated with NHY emissions. Despite the low rates of N deposition observed in this study, N-sensitive heathlands in North-Western Spain could be threatened by N depositions.