Humic acid (HA) and fulvic acid (FA) fractions isolated from a humic red-yellow Latosol (Typic Hapludox) under different land uses (forest, pasture, coffee) were studied by pyrolysis-gas chromatography–mass spectrometry (Py-GC/MS). Humic substances were extracted from soxhlet lipid free soil samples and the pyrolysis experiments consisted of a thermal desorption at sub-pyrolysis temperature (280°C) followed by a flash pyrolysis (600°C) on the same sample. Thermal desorption of HA and FA released mainly carbohydrate-derived compounds, N-compounds and series of alkanes. In surface samples most lignin- and carbohydrate-derived compounds are found weakly bounded to the HA structure indicating recent lignocellulosic contribution. In deeper dark soil layers the release of carbohydrate-derived moieties and N-compounds could indicate active biosynthesis by soil biota producing secondary biopolymers like chitin. Major compounds released after pyrolysis of HA were phenol and other benzene derivatives, whereas for FA aromatic and polyaromatic, N-containing and furan derivatives prevailed. In terms of land use the soils under pasture, FA became more aromatic and enriched in carbohydrate-derived compounds and the presence of p-vinylguaiacol in the HA could indicate the contribution of lignin from grasses. In contrast, the coffee culture seems to favor the formation of FA with a more polyaromatic and phenolic structure. Most N-bearing compounds in HA and FA extracted from forest soil samples were released by desorption, indicating comparatively weaker bonds in their structure. This could be explained as a more recent contribution and/or comparatively rapid cycling of these organic substances in natural environments. With respect to the general dynamic of organic matter (OM) in the studied Oxisol, our results are compatible with the occurrence of a high biological activity associated with fast humification processes and the migration of oxidized OM, probably with soil cations as vectors.

9 pages, 5 figures, 5 tables, 48 references.-- We also thank Trinidad Verdejo for her technical assistance in obtaining the pyrograms.

To FAPEMIG for the financial support and to CNPq for a scholarship to Cristiane Pereira de Assis.

Peer reviewed