ORGANIC-MATTER DYNAMICS AND AGGREGATION IN SOILS UNDER RAIN-FOREST AND PASTURES OF INCREASING AGE IN THE EASTERN AMAZON BASIN

In the eastern Amazon basin, four neighbouring clayey Oxisols with sim ilar particle-size distributions were selected, one under rain forest and three under pasture for 7, 12 and 17 years, respectively. These so ils were sampled at depth intervals of 0.1 m down to 1 m. Although no clear effect of pasture establishment on aggregate stability was asses sed, significant negative effects of cattle trampling on porosity and water retention and of vegetation change on clay water-dispersion were observed in the organic-rich horizons (0-0.40 m layers). Indirect evi dence of a great change of the nature of organic materials was also su ggested, particularly with (i) an increase in both negative surface ch arges and clay dispersibility, attributed to an increase in organic fu nctional groups, and (ii) a decrease in clay fabric porosity, attribut ed to an increase in the hydrophilic-hydrophobic balance on organic su rfaces (short-range adhesion forces during drying processes). Studies of soil organic matter (SOM) changes along this forest-to-pasture sequ ence were based on total C and C-13 measurements, which allowed to cal culate the distribution of C derived from forest (Cdff) and from pastu re (Cdfp) throughout the profiles. The distribution of C and C-13 in t he whole soil, in water-stable (WSA), in not stable (NWSA) aggregates, and in particle fractions, was compared. Young organic residues deriv ed from pasture were trapped in WSA, from where they were released by dispersion. After 17 years, the decrease in forest-derived SOM and the input of about 25% of pasture-derived SOM were suggested to be more e ffective on clay dispersability than on aggregate stability.