Joining empirical and modelling approaches to estimate dry deposition of nitrogen in Mediterranean forests

In Mediterranean areas, dry deposition is a major component of the total atmospheric N input to natural habitats, particularly to forest ecosystems. An innovative approach, combining the empirical inferential method (EIM) for surface deposition of NO₃⁻ and NH₄⁺ with stomatal uptake of NH₃, HNO₃ and NO₂ derived from the DO₃SE (Deposition of Ozone and Stomatal Exchange) model, was used to estimate total dry deposition of inorganic N air pollutants in four holm oak forests under Mediterranean conditions in Spain. The estimated total deposition varied among the sites and matched the geographical patterns previously found in model estimates: higher deposition was determined at the northern site (28.9 kg N ha⁻¹ year⁻¹) and at the northeastern sites (17.8 and 12.5 kg N ha⁻¹ year⁻¹) than at the central-Spain site (9.4 kg N ha⁻¹ year⁻¹). On average, the estimated dry deposition of atmospheric N represented 77% ± 2% of the total deposition of N, of which surface deposition of gaseous and particulate atmospheric N averaged 10.0 ± 2.9 kg N ha⁻¹ year⁻¹ for the four sites (58% of the total deposition), and stomatal deposition of N gases averaged 3.3 ± 0.8 kg N ha⁻¹ year⁻¹ (19% of the total deposition). Deposition of atmospheric inorganic N was dominated by the surface deposition of oxidized N in all the forests (means of 54% and 42% of the dry and total deposition, respectively). The relative contribution of NO₂ to dry deposition averaged from 19% in the peri-urban forests to 11% in the most natural site. During the monitoring period, the empirical critical loads provisionally proposed for ecosystem protection (10–20 kg N ha⁻¹ year⁻¹) was exceeded in three of the four studied forests.