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

A previous study of this toposequence from semiarid northeastern Brazi l concluded that processes of P transformation in semiarid tropical la ndscapes with steep slopes are difficult to describe with concepts dev eloped in temperate regions. In these soils, derived from syenitic roc ks, particle-size fractions reflect increasing effects of weathering p rocesses 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 transformat ions 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 ph osphate, largely of primary origin, was most abundant in sands and dec reased downslope in all particle-size fractions. At the lower slope, a ll particle-size fractions were dominated by resistant inorganic P for ms that correlated with total Fe. Comparison of the proportion of Ca-a ssociated and resistant P between different profile depths and with th e underlying saprolite at the mid and lower slopes indicated increased transformations and P losses in deeper horizons and substantial weath ering and leaching extending through the solum into the saprolite unde rlying the soil. The interpretation of P transformations with soil dev elopment and landscape position is complicated by intense weathering, highly variable leaching, and colluvial mixing that results from the c omplex interactions of lithology, weathering, and colluvial action in this semiarid tropical environment of steep slopes, annual moisture de ficits, and intense rainfalls.