The chemical composition of dust transported in red rains—its contribution to the biogeochemical cycle of a holm oak forest in Catalonia (Spain)

The chemistry of North African dust reaching NE Spain with red rains is here described to (1) characterize the red dust elemental composition, (2) analyze the relative contribution of dissolved and particulate forms to the total element inputs for the period 1983–1994, and (3) study the role of the particulate and dissolved inputs in red rains to the forest nutrient cycle. Five dust samples, obtained from the filtration of five red rain events, have been considered as representative of the dust reaching NE Spain as they include dust from the main source regions in North Africa. Enrichment factors were lower than four for all elements and samples, indicating the crustal character of the red dusts and their scarce mixing with anthropogenic pollutants. Back trajectory analysis of the red rain air masses computed at various isentropic surfaces showed northward fluxes at all altitudes in four out of five events. The remaining event, which presented a lower layer from European origin contacting upper layers of North African provenance, had higher trace metal concentrations in the dust (still lower than reported values for the circum-Mediterranean area) and higher S and N concentrations in dissolved form. The occurrence of red rains introduced high interannual variability in the input fluxes for the major elements. Phosphorus inputs occurred mostly in particulate form linked to red dust deposition. Red dust particulate inputs were also important for K+ and Mg2+. For Ca2+, dissolved inputs in red rains equalled and sometimes overuled particulate inputs in red dust, due to the calcite dissolution. Sodium and S inputs in red rains were mostly in dissolved form. The amounts of base cations delivered by red rains are important contributors to the holm oak forest needs at Montseny, by providing 27% of K+, 45% of Ca2+ and 84% of Mg2+ fluxes needed for the above ground biomass annual increment. For Ca2+ this is specially relevant because of the calcium-poor lithology of the site