TRANSFER OF DISSOLVED AL, FE AND SI IN 2 AMAZONIAN FOREST ENVIRONMENTS IN BRAZIL

The balance of Si, Fe and Al in the soil solution determines more or l ess the course of soil formation in the tropics. We have tried to impr ove understanding of the processes by studying fluxes of dissolved Si, Fe and Al from the atmosphere, through the canopy and the soil, and t o the groundwater, in two distinct Amazonian ecosystems, one a typical rainforest, developed on a Ferralsol, the other a so-called 'Campinar ana' forest, developed on a Podzol. The Si, Fe and Al in the rain and the throughfall and the stemflow were measured throughout a year, and the leaching of Si, Fe and Al through the upper soil and at the ground water level were estimated. The annual balance showed that stemflow in puts were negligible compared with the contributions from throughfall and rain. The inputs of Fe, Al and Si to the topsoil from the rain and from dust and biological release in the canopy were small but not neg ligible. These sources contributed more aluminium in the Campinarana t han in the rainforest. The rainfall constituted the main input of diss olved Fe and Al to the topsoil in both ecosystems. The element balance s in the soil horizons confirmed that the present functioning of the F erralsol results in aluminization and desilicification. We also found that the elements are transported in micropore flow and on translocate d particles, as well as in freely percolating water.