Phosphorus transformations in alfisols and ultisols under different land uses in the atlantic forest region of Brazil

This study examined the impact of land use changes on phosphorus (P) transformations in soils of different pedogenesis with little or no addition of P in the humid tropical region. The sequential extraction method was used to determine P fractions, and structural equations modeling was employed to investigate the P cycle in soils under plantations of rubber tree (Hevea brasiliensis), rubber tree + cocoa (Theobroma cacao), rubber tree + acai palm (Euterpe oleracea), rubber tree + cupuassu (Theobroma grandiflorum) and cocoa + erythrina (Erythrina glauca) as well as pastures (Brachiaria decumbens) and natural forests. The distribution of P fractions in soil appears to be affected by land use in all soil orders. Agroforestry systems of rubber tree + acai palm and rubber tree + cocoa showed high capacity to increase the concentrations of all the P fractions in soil compared to natural forest in soils of advanced stage of weathering. Pastures and rubber plantations provided higher concentrations of inorganic P fractions. The P fractionation method revealed the potential of labile fractions (resin-Pi + NaHCO3-Pi + Po) to supply appropriately the demand of all vegetation types evaluated in all soil orders. The structural model enabled to identify functional pools of P in soil, and to identify transformation processes of P in soil, in which organic P pool was the main P source for the available P pool and which part of the occluded P pool (recalcitrant) can be available for the plants.