Chemical properties of humic substances in soils of an Italian volcanic system

Humic acids were extracted from the different horizons of three soil profiles sampled in the volcanic apparatus of Vico, Italy, above vitreous parent material. Two soils developed andic properties in a udic environment with a short dry season under a beech forest, whereas the third soil resulted in a well-structured Bw horizon in a xeric pedo-environment under an oak forest. The structural characteristics of humic substances showed differences between the andic and non-andic soils. In particular, the andic soils progressively accumulated more unsaturated carbon than the non-andic soil as it was revealed by both their H/C ratios and CPMAS-NMR spectra. The andic humic samples were also relatively rich in carboxyl functions, possibly involved in stable complexes with the aluminum of allophane materials, and in methoxyl groups from poorly degraded lignin deposited from the above vegetation. The temperature and moisture regimes of the soils were also suggested to influence the molecular composition of their relative humic associations which increased in molecular size passing from the less hydrated non-andic conditions to the more hydrated andic ones. In general, the humic molecular properties reflected the different pedogenetic processes of the three soil profiles and those of humic matter from andic soils generally showed a lower degree of organic matter transformation.