A comparative study of the soil solution chemistry of two Amazonian forest soils (Central Amazonia, Brazil)

Analyses of the chemical composition of rapidly percolating soil water were used to study the genesis of a shallow podzol in a Campinarana forest and a clayey ferralsol from a typical rainforest located in North Manaus (Amazonia, Brazil). The samples were collected in lysimeters and analysed for Ca2+, Na+, K+, NH4+, SO42-, NO3-, Fe, Si, and Al. A large percentage of the nutrients was recycled in the upper 40 centimetres of both soils. The soil water concentrations in nutrients were very similar for both environments but levels of Si, Fe and Al were higher in the podzol than in the ferralsol. In the podzolic environment, the waters were enriched in Si, Fe and Al when passing through the organic layer and the top 10 cm of the soil. The concentrations decreased between 10 and 40 cm depth due to variations in mineralogy of this soil. In the ferralsol, the Si concentrations increased considerably on reaching the soil top-horizons while small increases occurred for Al and Fe. Thermodynamic equilibrium calculations indicate that most of the dissolved Al and Fe in both soil environments were in the form of organometallic complexes and that the waters were under-saturated in respect to kaolinite and gibbsite.