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40 páginas, 4 gráficos, 3 tablas .-- The definitive version is available at http://www.elsevier.com, Soil quality is strongly affected by microbial biomass that is involved in organic matter mineralization and the supply of nutrients to plants. The effects of trace elements on soil microbial biomass and activity are still controversial, and the contents of the elements in different forms, more than the total amounts, may affect soil microbial community. Volcanic soils are peculiar environments because of their chemical characteristics. Therefore, the aims of this research were to evaluate in volcanic soils: i) the elemental composition; ii) the elemental availability; and iii) the effects of elemental fractions on soil microbial biomass and activity. In order to reach the aims, the BCR sequential extraction method was applied in order to separate 22 elements in different soil fractions: acid soluble, reducible, oxidisable and residual. The studied biological parameters were: microbial and fungal biomasses, soil respiration, metabolic quotient, coefficient of endogenous mineralization, dehydrogenase and hydrolase activities, and phytotoxicity. Among the investigated elements, Al, B, Ba, Ca, Cr, Fe, K, Mg, Mn, Na, Ni, Ti, V and Zn were scarcely available; Cd appeared to be the most ready available element; Zn was mainly present in the acid soluble and in the residual fractions. Microbial biomass and activity appeared to be mainly affected by the reducible and oxidasable fractions of the investigated elements more than the acid soluble or residual ones. With the exception of La and V, the elemental content in the various fractions would seem to stimulate the microbial biomass and activity. Finally, the investigated volcanic soils showed phytotoxic properties., This research activity has been realised in collaboration of the Biology Department of University Federico II of Napleas and the Vesuvius National Park within the “Azione di Sistema - Impatto antropico da pressione turistica nelle aree protette: interferenze su territorio e biodiversità” funded by “Ministero dell'Ambiente e della Tutela del Territorio e del Mare” (CUP: E65J13000030001), Direttiva Conservazione della Biodiversità. Chemical analysis has been economically supported by Ministry of Economy and Competitiveness of Spain (CTM2013-47874-C2-2-R)