Crop residue addition effects on myriad forms and sorption of phosphorus in a Vertisol

Crop residues are a vital organic resource and their extensive use in soil management for sustainable agriculture is widely advocated. The effects of soybean residue (SR) and wheat residue (WR) applied alone or in combination with fertilizer P (FP) on dynamics of labile P, distribution of P fractions and P sorption in a Vertisol (Typic Haplustert) were assessed in a 16 week long incubation study. The amount of P added through crop residues, FP or their combination was kept constant at 10 mg P kg(-1) soil. Addition of SR or WR resulted in net increase of labile inorganic (Pi) and organic (Po) P, and microbial P throughout the incubation period, except that the WR decreased labile Pi during the first two weeks due to Pi immobilization. Integration of FP with SR had no added benefit compared to SR alone, while use of FP + WR proved better in ensuring short-term P availability by offsetting initial P immobilization associated with WR alone. Sequential fractionation of soil P at the end of 16 weeks showed that addition of SR and WR alone or in combination with FP favoured a build-up in labile Pi and Po (NaHCO3-Pi and Po), and moderately labile Po (NaOH-Po) fractions at the expense of recalcitrant P (HCl-P). The P sorption capacity of soil and P required to maintain optimum solution P concentration of 0.2 mg P 1(-1) also decreased with addition of these crop residues. The implication of the results of this study is that soybean and wheat residues can potentially improve soil P fertility by increasing labile Pi and Po, and moderately labile Po fractions, decreasing P sorption and concomitantly causing dissolution of recalcitrant P in soil.