Organic acids promote phosphorus release from Mollisols with different organic matter contents

Organic acids could improve the phosphorus (P) availability through enhancing the release of inorganic phosphorus (P_i) in the soil. However, the effects of organic acids on the P_i release are still poorly understood, especially from soils with different organic matter contents. Here, a biochemically produced humic acid and P fertiliser were added to the soil to modify the content of the soil organic matter (SOM) and soil P, respectively. And then the soil samples were incubated at 25 °C for 30 days. The release of P_i fractions (such as H₂O-P_i, NaHCO₃-P_i, NaOH-P_i, HCl-P_i, and Residual-P) from the soils with different organic matter contents in the presence of citric, oxalic, and malic acids was evaluated using a sequential chemical fractionation method. The results showed that the release of the NaHCO₃-P_i, NaOH-P_i, and HCl-P_i fractions also showed a decreasing trend with an increasing content of soil organic matter, and more NaOH-P_i than the other P_i fractions was generally released in the presence of organic acids. Considering the types of organic acids, oxalic acid and malic acid most effectively and least effectively released P_i, respectively. The path analysis indicated that the NaOH-P_i release had the highest direct and indirect effects on the total inorganic P (TP_i) release. NaOH-P_i was, therefore, the most effective source of P_i in the Mollisols.