Changes in P fractions and sorption in an Alfisol following crop residues application

With the emphasis on sustainable agriculture, attention has been increasingly turning to recycling of crop residues as a component of fertility management strategies for tropical soils. We assessed the effects of soybean residue (SR) and wheat residue (WR) applied either alone or in combination with fertilizer P (FP) on dynamics of labile P, distribution of P fractions, and P sorption in a semiarid tropical Alfisol by conducting a 16 w long incubation experiment. The amount of P added through crop residues, FP or their combinations was kept constant at 10 mg P (kg soil)(-1). Addition of SR or WR resulted in net increase of labile inorganic (P) and organic P (P.) and microbial P throughout the incubation period, except that the WR decreased labile P-i during first 2 w due to P, immobilization. The P immobilization associated with WR addition was, however, offset when fertilizer P was combined with WR. Generally, the increases in labile-P fractions were larger with the SR and SR+FP than with the WR and WR+FP. The sequential fractionation of soil P at the end of 16 w indicated that a major part of added fertilizer P transformed into moderately labile and stable P fractions as evident from the increased NaOH-Pi and HCl-P in the FP treatment. In contrast, the addition of SR and WR alone or in combination with FP favored a build-up in NaHCO3-P-i and -P-o and NaOH-P-o fractions while causing a decrease in NaOH-P-i and HCl-P fractions. The addition of these crop residues also effectively decreased the P-sorption capacity and hence reduced the standard P requirement of the soil (i.e., the amount of P required to maintain optimum solution P concentration of 0.2 mg P l(-1)) by 24%-43%. Results of the study, thus, imply that soybean and wheat crop residues have the potential to improve P fertility of Alfisols by decreasing P-sorption capacity and by redistributing soil P in favor of labile-P fractions and promoting accretion of organic P.