The contrasting effect of broom and pine on pedogenic processes in volcanic soils (Mt. Etna)

Effects of Etnean broom (Genista aetnensis (Biv.) DC.) and Corsican pine (Pinus nigra Am. ssp. laricio Maire) on the morphological, mineralogical and chemical properties of volcanic soils from Mt. Etna (Italy) were compared and contrasted. For this purpose, we studied the rhizosphere and the bulk soils under adjacent 30 years old pure plantations of both species. Morphology of the soil under broom differs from that under pine for (i) a higher accumulation of organic matter in the topsoil, (ii) an incipient formation of E material around the base of the stem, and (iii) the presence of yellowish collars around the primary roots. Mineral horizons of the two soils are made of plagioclases, pyroxenes, magnetite and glass. The yellowish colour of the collars is attributed to a root effect that results in a confined alteration of primary volcanic glass and also iron-bearing minerals, leading to the precipitation of amorphous Fe-oxides. Under pine, we observed a more widespread weathering of primary minerals throughout the profile, and a depletion of base cations and a release of Al in the topsoil. On the whole, therefore, Corsican pine-commonly planted in the last decades on the pyroclastic deposits and lava flows of the Etna volcano-seems to play a detrimental role on these soils.