From andic non-allophanic to non-andic allophanic Inceptisols on alkaline basalt in Mediterranean climate. A toposequence study in the Marghine district (Sardinia, Italy)

The properties of a non-andic, allophanic Inceptisol (POM1) and two andic, non-allophanic Inceptisols (FLM5 and SPM8), representative of a toposequence on basanite rocks of Mt. St. Antonio (central-western Sardinia, Marghine sub-region), were analyzed to better elucidate factors and processes that lead to a different expression of andic properties and allophane formation in xeric Mediterranean environment. A series of laboratory techniques including physical, chemical, micromorphological, X-ray diffraction (XRD), scanning electron microscopy (SEM) analyses were used. All soils are usually rich in organic carbon, dystric at the Great Group level, with saturation base from 72 to 10%, and with pH from subalcaline to acidic. Allophanic Inceptisol appears more intensely weathered than non-allophanic Inceptisols. Volcanic glass content is, in any case, negligible. POM1, formed on the plateau, shows a shallow solum (45 cm) with A2 horizon that meets all the criteria for andic soil properties; the amounts acid oxalate-extractable Alo and Sio, together with pyrophosphate-extractable Alp, typifies such a pedon as allophanic Inceptisol (0.19 < Alp/Alo < 0.57; 3% < allophane < 8%). FLM5 and SPM8, developed on a slope, are both characterized by a low content of short-order secondary minerals (allophane content < 3%) and the lack of andic properties in any horizon. Despite similar climatic and geological conditions, the allophanic and non-allophanic andic Inceptisols coexist in a small area (6000 m2). The allophanic Inceptisol (POM1) is located at the plateau of the toposequence, where lower surface runoff, greater water infiltration rate, the absence of significant erosion and colluvial movement may have increased the degree of weathering processes. In these conditions, the prolonged soil moistness prevents the transformation of short-order components. On the whole, the results emphasize the role of topography and the importance of Al-, Fe-amorphous compounds in expressing soil andic properties and allophane content versus the volcanic origin of parent material and the insufficiency of volcanic glass. The results also suggest that the occurrence of soil with andic properties and significant allophane content in hot and dry environments, like the Mediterranean climate in Sardinia, may be more widespread than expected.