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

• Bulk inorganic nitrogen depositions range between 2.81 and 4.56 kg N ha− 1 yr− 1 in heathlands of North-Western Spain.
• The highest bulk NO3− and NH4+ deposition rates were displayed in the rainiest months.
• Annual bulk NO3− deposition was comparable to bulk NH4+ deposition in La Majúa and was higher in San Isidro.
• NH4+/NO3− ratio was characteristic of industrialized areas in San Isidro and of undefined areas in La Majúa.
• N-sensitive heathlands of North-Western Spain are susceptible to elevated N deposition rates.

Nitrogen (N) deposition has been identified as one of the main traits of terrestrial ecosystems, affecting their structure and functioning. However, few studies have been developed under natural field conditions to evaluate the amount of N deposition in low nutrient status heathland ecosystems. Therefore, a field experiment was carried out to investigate the bulk 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 of N-sensitive ecosystems. Three replicated bulk collectors and one rain gauge were installed at each study site to collect monthly bulk deposition samples over three-year period (2011–2014). Bulk inorganic N deposition was different between the study sites (2.81 kg N ha− 1 yr− 1 in La Majúa and 4.56 kg N ha− 1 yr− 1 in San Isidro), but showed the same seasonal dynamic, with higher N deposition rate in the wet period (October to April) compared to the dry period (May to September). Annual bulk NO3− deposition was comparable to annual bulk NH4+ deposition in La Majúa (1.42 vs. 1.39 kg N ha− 1 yr− 1), and higher in San Isidro (2.89 vs. 1.67 kg N ha− 1 yr− 1). San Isidro displayed a characteristic bulk NH4+/NO3− deposition ratio below 1 of industrialized areas (0.58), while La Majúa displayed a bulk NH4+/NO3− deposition ratio close to 1 (0.98), distinctive of an intermediate situation between industrialized and agricultural areas. Total bulk inorganic N depositions observed in the present field study are consistent with the modelled estimation of N depositions for North-Western Spain, but only San Isidro was consistent with the estimated dominance of oxidized N over reduced N.