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 subtropicalto subalpine climate zones in the Etna region (Sicily, southern Italy) wasinvestigated with respect to organic C, kaolinite, and crystalline tononcrystalline Al and Fe phases. Special emphasis was given to thestabilization of soil organic carbon (SOC) and its interaction with theinorganic phases. The soils were variations of Vitric Andosols thatdeveloped on a trachy-basaltic lava flow with an age of 15,000 years. Twomain vegetation systems dominated the sites: at the lower sites, it wasmainly maquis vegetation and, at the higher elevated sites, predominantlyconiferous forest. The concentration of SOC in the topsoil, theSOC stocks in the profiles, the humus fractions such as humic and fulvicacids, functional groups and substances of organic matter, imogolitetypematerials (ITM), and oxyhydroxides were found to be stronglyrelated to elevation and, thus, climate (precipitation and temperature)and vegetation. The C/N ratio in the topsoil was especially indicative ofthe vegetation type. The amount of SOC, ITM, and crystalline Feoxyhydroxides decreased with increasing altitude. Weathering, as relatedto the proportion of crystalline Fe-oxyhydroxides or the kaolinite concentrationin the clay fraction, seemed to be greater at the lower elevatedsites. At these sites, maquis vegetation led to a higher accumulation ofSOC as compared with the coniferous trees at the higher sites. Fireactivity, as indicated by aromatic compounds in the humic acids and bythe presence of charcoal in the soil, has most probably influenced severalimportant soil processes. The identification and radiocarbon dating ofcharcoal revealed evidence that repeated bush fires had played a significantrole in soil formation. The better stabilization of SOC at loweraltitudes might be due to the specific climatic conditions with a morepronounced change in periods of humidity alternating with periods ofdroughts and resultant fire activity. The positive correlation betweenmean annual temperature and SOC content supports such a hypothesis.The climate- and vegetation-dependent stabilization of organic matter inthe soil can be ascribed to the proportion of aromatics in the humic acids,to the presence of noncrystalline Al and Fe phases, to the kaoliniteconcentration, to the amount of clay, and to a lesser extent to the siltfraction. (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.