The purpose of this research was to evaluate the changes in the organic matter of a Hydromorphic Podzol after the introduction of Brachiaria decumbens Stapf. Prain., in a non-flooding savannah area in the Pantanal, MS, Brazil. Soil sampling was performed on ten- and twenty-year old pastures cultivated in Savannah areas. In each area, trenches were dug to 1.5 m depth, and soil samples were collected down to 1.0 m, for the study of the C and N distribution on the soil profile. Soil samples were also collected to a depth of 40 cm along two transects to calculate the C and N reserves in each area. All samples were analysed for C and N. Chemical fractionation of soil organic matter was performed on a composite sample from the 0-20 cm layer. After 20 years of introduced pasture the soil organic matter decreased by 28% in the 0-40 cm layer. The relative proportion of the humic fractions also changed with the pasture introduction. In the native savannah the fulvic acid fraction predominated; in the 10-year-old pasture, the humine fraction predominated and in the 20-year-old pasture, fulvic acids predominated again. No statistical difference for the nitrogen content among the three areas was observed.
organic carbon; nitrogen; fractionation; humic fractions
