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

Abstract

Organic acids could improve the phosphorus (P) availability through enhancing the release of inorganic phosphorus (Pi) in the soil. However, the effects of organic acids on the Pi 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 Pi fractions (such as H2O-Pi, NaHCO3-Pi, NaOH-Pi, HCl-Pi, 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-Pi, NaOH-Pi, and HCl-Pi fractions also showed a decreasing trend with an increasing content of soil organic matter, and more NaOH-Pi than the other Pi 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 Pi, respectively. The path analysis indicated that the NaOH-Pi release had the highest direct and indirect effects on the total inorganic P (TPi) release. NaOH-Pi was, therefore, the most effective source of Pi in the Mollisols.

Keywords

• available phosphorus
• black soil
• correlation analysis
• path analysis
• phosphorus fractions