PHOSPHORUS FARMS IN PARTICLE-SIZE FRACTIONS OF A TOPOSEQUENCE FROM NORTHEAST BRAZIL

A previous study of this toposequence from semiarid northeastern Brazil concluded that processes of P transformation in semiarid tropical landscapes with steep slopes are difficult to describe with concepts developed in temperate regions. In these soils, derived from syenitic rocks, particle-size fractions reflect increasing effects of weathering processes from sands to clays. We therefore document the association of P and different sequentially extracted P fractions with particle-size fractions along the toposequence to further investigate P transformations during pedogenesis in the tropics. Total P was exceptionally high in these soils and in all particle-size fractions. Total P increased with decreasing particle size and decreased downslope. Total P in sand and silt correlated with total Ca in these fractions. The Ca-bound phosphate, largely of primary origin, was most abundant in sands and decreased downslope in all particle-size fractions. At the lower slope, all particle-size fractions were dominated by resistant inorganic P forms that correlated with total Fe. Comparison of the proportion of Ca-associated and resistant P between different profile depths and with the underlying saprolite at the mid and lower slopes indicated increased transformations and P losses in deeper horizons and substantial weathering and leaching extending through the solum into the saprolite underlying the soil. The interpretation of P transformations with soil development and landscape position is complicated by intense weathering, highly variable leaching, and colluvial mixing that results from the complex interactions of lithology, weathering, and colluvial action in this semiarid tropical environment of steep slopes, annual moisture deficits, and intense rainfalls.