Pasture management systems are, in general, based on the grazing intensity and pressure, dry forage cutting, soil fertility improvement or vegetation burning. In view of the influence of soil management systems on soil organic matter (SOM) dynamics, this study aimed to evaluate soil carbon and nitrogen distribution, and to investigate humic substances (HS) and their interactions with Fe and Al ions in a subtropical Oxisol in the following environments: native pasture without burning for the last 41 years, native pasture without burning for 8 years, native pasture amended by liming and fertilization, and native forest. In samples from the layers 0-5, 0-20, 20-40 and 40-60 cm, chemical fractionation was performed the humic substances characterized by infrared spectroscopy and the contents of co-extracted Fe and Al determined. Soil fertility amendment resulted in an increase of 24 % of the C content and stimulated humin formation in the upper 5 cm in comparison to the native pasture without burning for 8 years. In this latter environment, HS were found mostly as fulvic acids (9.4 g kg-1) while in the native pasture without burning for 41 years humic acids prevailed (11.2 g kg-1). Ionic Fe was mostly associated with humic acids and Al with fulvic acids in all environments. Soil fertility amendment represents a sustainable alternative to the practice of vegetation burning in the management of native pastures.
burning; soil fertility amendment; iron oxides; infrared; chemical composition of humic substances
