Inhibitory Effects of Crop Residue-Derived Organic Ligands on Phosphate Adsorption Kinetics

Abstract

This study was conducted to investigate the effects of dissolved organic matter isolated from field corn (Zea mays L.) and wheat (Triticum aestivium L.) residue on the kinetics of phosphorus (P) adsorption of an acidic soil. Phosphorus adsorption studies were conducted with the following experimental variables: P concentrations from 10 to 70 µM, pH from 4.51 to 5.38 and surface area of soil from 13.3 to 39.8 m2 L−1. Initial rate of reaction was determined through polynomial fitting of solution [P] at 0, 2, 4, 6, 8, and 10 rain after soil addition. Double-logarithmic plots were used to obtain reaction orders of 0.64 ± 0.05 for [P], −0.07 ± 0.04 for −log [H], and 1.04 ± 0.17 for surface area. Thus, the experimental rate equation was written as: d [P]/dt = kf s [P]0.64, where [P] is the P concentration in solution, kf is the forward rate constant in L m−2 s−1 units, and s is surface area in m2 L−1. The rate constant, kf, was (1.61 ± 0.16) × 10−6 L m−2 s−1. The rate constant declined linearly and Al release into solution increased linearly with increasing concentration of dissolved organic matter in solution. This suggests that P adsorption is occurring through reaction with soil Al. Reaction rates determined at 8.8, 15.5, 20.0, 26.0, and 33.5°C demonstrated that Al release from the soil was surface controlled, while P adsorption was under diffusion control. The results of this study suggests that management systems that return crop residues back into the soil may increase the availability of P by decreasing the adsorption of P onto soil surfaces.