The objective of this study was to evaluate alterations in the soil chemical attributes and in humic and fulvic acids at a clay extraction site with spontaneous grass vegetation [Brachiaria mutica (Forsk.) Stapf] and aforested with Acacia mangium Willd. Soil samples were collected at 0-10, 20-20 and 20-30 cm depths. A 33 % and 80 % increase in carbon stocks was observed under A. mangium as compared to the B. mutica cover at the 0-10 cm and 20-30 cm soil depths, respectively. The lowest carbon stock was observed in the free fulvic acids (FFA) fraction. An increased humification degree of the organic matter was further noticed under A. mangium cover. The increases varied from 38 to 280 % in the fulvic acid (FA) and from 26 to 217 % in the humic acid (HA) fraction, depending on the soil depth. The total acidity in the FA and HA fractions was high, varying from 810 to 920 cmol c kg-1. However, around 67 % of the H+ exchange capacity was due to the phenolic OH- groups characterized as weaker acid groups. The E4/E6 ratios were within the range normally found for FA (between 8.2 and 10.5) and HA (between 1.3 and 3.9). The values of E4/E6 in HA isolated from the site with A. mangium cover was higher, suggesting the presence of a less condensed fraction with lower molecular mass.
degraded land reclamation; legumes; humic substances
