Land application of biosolids (SS) can cause a buildup of phosphorus (P) in the top soil. The changes in the soil P characteristics may be assessed by the sorption isotherm and the sequential fractionation techniques. Samples of Haplorthox were collected from a field experiment where maize was cultivated for two years, after two applications of SS originated from two cities of S？o Paulo State, Brazil. SS applications added a total of 125, 250, 500, 1000 and 2000？kg？ha？1 of P in the area. To perform the sorption isotherms and obtain P maximum sorption capacity ( ) and the binding energy, soil samples were submitted to increasing P concentration solutions until equilibrium was reached. Sequential fractionation was done by a sequential extraction with CaCl2, NaHCO3, NaOH, HCl, and HNO3 + HClO4 (residual). Addition of biosolids from both cities to the soil decreased and the binding energy obtained by the Langmuir equation. SS additions changed the P fractions distribution in the soil by increasing the labile fractions (P-CaCl2 and P-NaHCO3) and the moderately labile fraction (P-NaOH) by 11.2% and 20.3%, respectively, in detriment of the most resistant P fraction. 1. Introduction A sustainable option to dispose biosolids (SS) originated from sewage treatment plants is the use as soil conditioner or as a source of nutrients to plants. Nevertheless, high SS rates might add to soil excessive amounts of phosphorus (P). One of the consequences is the increase on the risk of environment contamination by P transport to superficial water reservoirs, since P can be weakly sorbed in the superficial soil layer and carried over by rainfall runoff. Furthermore, topsoil P concentrations have been related to P movement below the plough layer and tile flow [1]. Recently, Gebrim et al. [2] observed that P leaching through the profile can be significant in highly weathered soils, where residual P is accumulated by successive crops, particularly when poultry litter is applied as fertilizer. Among the useful parameters to monitor the increase in soil P availability is the P sorption capacity and P distribution among the main soil fractions. The application of organic residues to soils might implicate in change in soil P sorption capacity. According to Reddy et al. [3], application of organic wastes increases soluble P and decreases the P sorption capacity of the soil, and these parameters were directly related to the loading rates of animal wastes. The positive effects of manure on the solubility of P are due to the addition of soluble inorganic P and mineralization of organic
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