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 subtropical to subalpine climate zones in the Etna region (Sicily, southern Italy) was investigated with respect to organic C, kaolinite, and crystalline to noncrystalline Al and Fe phases. Special emphasis was given to the stabilization of soil organic carbon (SOC) and its interaction with the inorganic phases. The soils were variations of Vitric Andosols that developed on a trachy-basaltic lava flow with an age of 15,000 years. Two main vegetation systems dominated the sites: at the lower sites, it was mainly maquis vegetation and, at the higher elevated sites, predominantly coniferous forest. The concentration of SOC in the topsoil, the SOC stocks in the profiles, the humus fractions such as humic and fulvic acids, functional groups and substances of organic matter, imogolitetype materials (ITM), and oxyhydroxides were found to be strongly related to elevation and, thus, climate (precipitation and temperature) and vegetation. The C/N ratio in the topsoil was especially indicative of the vegetation type. The amount of SOC, ITM, and crystalline Fe oxyhydroxides decreased with increasing altitude. Weathering, as related to the proportion of crystalline Fe-oxyhydroxides or the kaolinite concentration in the clay fraction, seemed to be greater at the lower elevated sites. At these sites, maquis vegetation led to a higher accumulation of SOC as compared with the coniferous trees at the higher sites. Fire activity, as indicated by aromatic compounds in the humic acids and by the presence of charcoal in the soil, has most probably influenced several important soil processes. The identification and radiocarbon dating of charcoal revealed evidence that repeated bush fires had played a significant role in soil formation. The better stabilization of SOC at lower altitudes might be due to the specific climatic conditions with a more pronounced change in periods of humidity alternating with periods of droughts and resultant fire activity. The positive correlation between mean annual temperature and SOC content supports such a hypothesis. The climate- and vegetation-dependent stabilization of organic matter in 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 silt fraction. (Soil Science 2007;172:673–691) Key words: Soil organic matter, kaolinite, fulvic and humic acids, climate change, charcoal, IR-spectroscopy, 14C, 13C CPMAS NMR.