ELECTRON PARAMAGNETIC RESONANCE AND THERMO-GRAVIMETRIC CHARACTERIZATION OF HUMIC ACIDS IN NUTRIENT-RICH SOILS FROM THE RAPOSA-SERRA DO SOL INDIAN RESERVE, RORAIMA, BRAZIL

Little is known about the organic matter status in nutrient-rich cultivated soil in Amazonia since most studies dealt with slash-and-burn nutrient-poor soil. In Roraima (north Amazonia) Macuxi Indians have cultivated food crops (maize, manihoc and beans) on nutrient-rich Alfisols for many years in the Flexal Indian community. The attributes of humic acids fraction from Chernosols under different land uses in the Raposa Serra do Sol Indian reserve were characterized. The humic substances were extracted according to procedures recommended by the International Humic Substances Society and characterized by means of elemental analysis, thermogravimetry, and electron paramagnetic resonance (EPR). The cultivated Chernosol, under long-term burning practices, showed increasing production of oligomers of humic acids, indicated by the largest concentration of semiquinone-type free radicals. High fertility uncultivated soils in fallows showed less concentration of semiquinone-type free radicals compared with cultivated soils and less fertile soils from the same area. Positive correlations were obtained between the values of semiquinone-type free radicals concentration and the thermogravimetry indexes for temperature >350 degree C. The most weathered soils and those under more intense land use showed an overall higher degree of humification.