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

Effects of Etnean broom (Genista aetnensis (Biv.) DC.) and Corsican pine (P inus nigra Am. ssp. laricio Maire) on the morphological, mineralogical and chemical properties of volcanic soils from Mt. Etna (Italy) were compared a nd contrasted. For this purpose, we studied the rhizosphere and the bulk so ils under adjacent 30 years old pure plantations of both species. Morpholog y of the soil under broom differs from that under pine for (i) a higher acc umulation 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 ma de of plagioclases, pyroxenes, magnetite and glass. The yellowish colour of the collars is attributed to a root effect that results in a confined alte ration of primary volcanic glass and also iron-bearing minerals, leading to the precipitation of amorphous Fe-oxides. Under pine, we observed a more w idespread weathering of primary minerals throughout the profile, and a depl etion of base cations and a release of Al in the topsoil. On the whole, the refore, Corsican pine-commonly planted in the last decades on the pyroclast ic deposits and lava flows of the Etna volcano-seems to play a detrimental role on these soils. (C) 2001 Elsevier Science B.V. All rights reserved.