Effects of low-molecular-weight organic acids on soil phosphorus release

Effects of low-molecular-weight organic acids, citric acid and malic acid on soil phosphorus release were studied by a series of batch experiments, and the sequential phosphorus fractionation was analyzed in the soils before and after the extraction of citric acid or malic acid. The results show that the release rate of phosphorus from soil is increased with the increase additions of citric acid or malic acid, and that results in continuous release of phosphorus. Phosphorus release amount by citric acid is larger than that of malic acid. Low-molecular-weight organic acids can promote the release of the inorganic phosphorus fractions (H2O-P and NaHCO3-Pi), and mineralization of the organic phosphorus fractions (NaHCO3-Po and NaOH-Po). When citric acid and malic acid are at high concentrations (≧0.5 mmol/L), the releases of phosphorus fractions is in the order of Fe-oxide- and Al-oxide- associated (NaOH-Pi) Ca-associated (HCl-P) plant-available inorganic P (NaHCO3-Pi H2O-P). Large amounts of soluble iron and aluminum are also released with the additions of the low-molecular-weight organic acids, and the amounts are significantly correlated with the phosphorus releases (P0.05). It suggests that Fe-oxide- and Al-oxide- associated inorganic P (NaOH-Pi) plays an important role in phosphorus release.