CHANGES IN SOIL PORE-SPACE DISTRIBUTION FOLLOWING DEFORESTATION AND REVEGETATION - AN EXAMPLE FROM THE CENTRAL AMAZON BASIN, BRAZIL

Deforestation of the Central Amazon is leading to a dramatic modification not only of the chemical composition of the clay latosols, but also, and maybe more importantly, of the relative distribution of their pore size. The use of machines reduced the volume of micro- and mesovoids (1-mu-m < phi < 100-mu-m) and results in homogenization of the spaces between the clay particles (characterized as 'cryptovoids' <0.1-mu-m). This size is crucial for determining the amount of water available to plants. When plants such as Pueraria (Kudzu bean) are introduced, their roots further modify the relative pore distribution, this time reducing the volume of cryptovoids through increasing the volume of micro- and mesovoids, thus freeing for plants even more water than in the original soil. This study, using mercury porosity analysis in addition to usual methods, shows the importance of measuring variations in porosity and water availability before deciding on cultivation and reforestation. Water availability may vary to a large extent depending on how the soil is cleared and what root action is allowed to develop. (Certain types of deforestation - heavy machinery, no cover plant such as Pueraria - may result in a completely unproductive soil although the chemical composition remains unchanged).