The aim of this work was to evaluate the effects of human intervention, i. e., jungle clearing and introduction of pasture (Brachiaria sp), on the total-N and organic-C contents and humic acids, fulvic acids and humina fractions. This research was carried out in a forest near Porto Velho - Rondônia (Amazo Forest) and Senador Canedo - Goiás ("cerrado"). The soil samples were collected in 1995, from areas of primary vegetation and planted pasture, obeying a line with 24 points every 10 meters (12 under natural vegetation and 12 under pasture). The results showed a reduction in fulvic acid, humina, total-N and organic-C when the soil under native vegetation was submitted to agricultural cultivation. The highest contents of organic compounds (fulvic acid, humic acid, humina, total-N and organic-C) were detected in the superficial horizon of the original vegetation in the Amazon Forest. The results confirm a certain soil degradation in cultivated soils by the introduction of pasture compared with non-cultivated soils in both Brazilian ecosystems (Amazon Forest and "cerrado").
organic fractions; total-N; organic-C; pasture; cerrado; Amazon forest