Clay formation and pedogenetic processes in tephra-derived soils and buried soils from Central-Southern Apennines (Italy)

Abstract Volcanic activity in Central-Southern Italy produced pyroclastic deposits that covered large areas of the Apennine Mountains. The major volcanic centres include Roccamonfina, Campi Flegrei (Phlegraean Fields), and Somma-Vesuvius. The properties of six soils formed in volcanic tephras in the Matese Massif were analysed to shed light on processes that lead to different expressions of andic properties in a mountain environment. Andosols at 1600 m a.s.l. at Campitello Matese are strongly acidic and rich in organic carbon, not only in the subsurface horizons but also in buried horizons. These pedons have horizons that meet all the criteria for non-allophanic Andosols and have modest contents of gibbsite and short-range ordered Fe minerals such as ferrihydrite. Pedons at lower altitudes of 830 m a.s.l at Colle Santa Croce developed on different pyroclastic materials and contain well-crystallized secondary minerals and have clay contents up to 30%. The general sequence of pyroclastic deposits and distribution of upland Andosols in the Matese Massif suggests that volcanic ash, and possibly Neapolitan Yellow Tuff, are the main soil parent materials in the upper Mountains. By contrast, the Andosols developed at lower-altitude at Colle Santa Croce were most likely influenced by geomorphologic deformation of a sequence of relict of lacustrine deposits related to old volcanic activity at Roccamonfina that occurred 437 ± 1.9 ka. Buried horizons of these pedons lack andic properties and contain sepiolite that originated from weathering in an alkaline lacustrine environment. Mineralogical and chemical data for the Colle Santa Croce pedons suggest that Andosols developed as non-allophanic aluandic Andosols and as a result contain mainly Al and Fe humus complexes, with poorly crystalline active Al and Fe phases. Buried soil horizons at Campitello Matese contain gibbsite, which suggests that these horizons weathered at the soil surface for relatively long periods before being buried by subsequent ash falls over the past 39 ka. Soils formed in volcanic ash may be more widespread in the Apennine Mountains than currently believed, and they may be more widespread in other mountainous areas of Italy as well.