Phosphorus sorption, desorption and resorption by soils of the Brazilian Cerrado supporting eucalypt

Oxisols of the Brazilian Cerrado are highly weathered phosphorus deficient soils, on which eucalypt is increasingly being grown as a source of carbon and energy for steel manufacturing. Phosphorus (P) fertilization is a necessary practice to assure adequate eucalypt production; therefore, an understanding of cycling by inorganic soil P should lead to efficient P management and more accurate modeling of P bioavailability. Since sorption and desorption reactions control inorganic P bioavailability, the purpose of this study was to contrast P sorption, desorption and subsequent resorption for a range of Cerrado soils. Its specific objectives were to determine (i) if desorption and resorption show the same hysteresis shown by sorption and desorption, (ii) if Kd values of resorption and desorption for Cerrado soils are dependent on the soil's clay content and (iii) if resorption and desorption Kd values are a function of the amount of labile P on the soil surface. Three levels of P were sorbed onto four Cerrado soils with clay contents between 13% and 81%. Phosphorus desorption was measured using anion-exchange membranes. Sorption was a function of soil clay content, and a pedotransfer function for the soil partition coefficient was calculated with an r2=0.99. Desorption and resorption were dependent on both the clay content of the soil (r2=0.59-0.99) and the amount of sorbed labile P. Pedotransfer functions for each of these processes depend on accurate measurement of the inorganic P that responds to disequilibria exchange. Desorption and resorption were not hysteretic; yet desorption was hysteretic with the original sorption isotherm. This suggests the question: how useful are commonly produced sorption isotherms?