Effect of climate and vegetation on soil organic carbon, humus fractions, allophanes, imogolite, kaolinite, and oxyhydroxides in volcanic soils of Etna (Sicily)

A soil sequence along an elevational gradient ranging from to subalpine climate zones in the Etna region (Sicily, southern Italy) investigated with respect to organic C, kaolinite, and crystalline noncrystalline Al and Fe phases. Special emphasis was given to stabilization of soil organic carbon (SOC) and its interaction with inorganic phases. The soils were variations of Vitric Andosols developed on a trachy-basaltic lava flow with an age of 15,000 years. main vegetation systems dominated the sites: at the lower sites, it mainly maquis vegetation and, at the higher elevated sites, predominantly coniferous forest. The concentration of SOC in the topsoil, SOC stocks in the profiles, the humus fractions such as humic and acids, functional groups and substances of organic matter, type materials (ITM), and oxyhydroxides were found to be related to elevation and, thus, climate (precipitation and temperature) and vegetation. The C/N ratio in the topsoil was especially indicative the vegetation type. The amount of SOC, ITM, and crystalline oxyhydroxides decreased with increasing altitude. Weathering, as to the proportion of crystalline Fe-oxyhydroxides or the kaolinite centration in the clay fraction, seemed to be greater at the lower sites. At these sites, maquis vegetation led to a higher accumulation SOC as compared with the coniferous trees at the higher sites. activity, as indicated by aromatic compounds in the humic acids and the presence of charcoal in the soil, has most probably influenced important soil processes. The identification and radiocarbon dating charcoal revealed evidence that repeated bush fires had played a significant role in soil formation. The better stabilization of SOC at altitudes might be due to the specific climatic conditions with a pronounced change in periods of humidity alternating with periods droughts and resultant fire activity. The positive correlation mean annual temperature and SOC content supports such a hypothesis. The climate- and vegetation-dependent stabilization of organic matter the soil can be ascribed to the proportion of aromatics in the humic acids, to the presence of noncrystalline Al and Fe phases, to the kaolinite concentration, to the amount of clay, and to a lesser extent to the fraction.