Effects of macromolecular organic acids on reducing inorganic phosphorus fixation in soil

To improve the availability of inorganic phosphorus (P) in soil, we investigated the role of three macromolecular organic acids (MOAs), including fulvic acid (FA), polyaspartic acid (PA), and tannic acid (TA), in reducing the fixation of inorganic P fertilizer in the soil. AlPO₄, FePO₄, and Ca₈H₂(PO₄)₆·5H₂O crystals were chosen as insoluble phosphate representatives in the soil to simulate the solubilization process of inorganic P by MOAs. The microstructural and physicochemical properties of AlPO₄, FePO₄, and Ca₈H₂(PO₄)₆·5H₂O were determined by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS) before and after treatment of MOAs. In addition, the amounts of leached P and fixed inorganic P in Inceptisols and Alfisols affected by MOAs combined with superphosphate (SP) fertilizer were determined by soil leaching experiments. The presence of the three MOAs significantly increased the concentration of leached P and reduced the contents of insoluble inorganic phosphate formed with iron, aluminum, and calcium fixed in the soil, in which PA combined with SP had the most significant effect. Furthermore, the less inorganic P fixation in the combination treatment of MOAs and SP resulted in a greater wheat yield and P uptake. Therefore, MOAs could be a synergistic material for increasing P fertilizer utilization.