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

摘要

In the eastern Amazon basin, four neighbouring clayey Oxisols with similar particle-size distributions were selected, one under rain forest and three under pasture for 7, 12 and 17 years, respectively. These soils were sampled at depth intervals of 0.1 m down to 1 m. Although no clear effect of pasture establishment on aggregate stability was assessed, 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 evidence of a great change of the nature of organic materials was also suggested, particularly with (i) an increase in both negative surface charges and clay dispersibility, attributed to an increase in organic functional groups, and (ii) a decrease in clay fabric porosity, attributed to an increase in the hydrophilic-hydrophobic balance on organic surfaces (short-range adhesion forces during drying processes).