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 fraction s and P sorption in a Vertisol (Typic Haplustert) were assessed in a 16 wee k 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 (P-i) and organic (P-o) P, and microbial P throughout the incubation period, except that the WR de creased labile P-i during the first two weeks due to P-i immobilization. In tegration of FP with SR had no added benefit compared to SR alone. while us e of FP + WR proved better in ensuring short-term P availability by offsett ing initial P immobilization associated with WR alone. Sequential fractiona tion of soil P at the end of 16 weeks showed that addition of SR and WR alo ne or in combination with FP favoured a build-up in labile P-i and P-o (NaH CO3-P-i and -P-o), and moderately labile P-o (NaOH-P-o) fractions at the ex pense of recalcitrant P (HCl-P). The P sorption capacity of soil and P requ ired to maintain optimum solution P concentration of 0.2 mg P l(-1) also de creased with addition of these crop residues. The implication of the result s of this study is that soybean and wheat residues can potentially improve soil P fertility by increasing labile P-i and P-o, and moderately labile P- o fractions, decreasing P sorption and concomitantly causing dissolution of recalcitrant P in soil. (C) 2001 Elsevier Science Ltd. All rights reserved .