Citric Acid Promotes the Mobilization of Phosphorus under the Lower Concentration of Low Molecular Weight Organic Acids in Acidic Forest Soil

Low molecular weight organic acids (LMWOAs) secreted by plant roots enhanced the release of inorganic P (Pi) and organic P (Po) into the soil solution and thereby increased plant-available Pi in soils. Not the effect of LMWOAs on inducing organic P (Po) released into soil solution through soil microorganisms at different temperatures was poorly understood, but the transform mechanism for P fraction was also not well explained. This study used three experiments to determine the release of P and the transformation mechanism of P fractions induced by oxalic acid, citric acid, and malic acid in acidic forest soils. The results showed that LMWOAs, as carbon sources for microorganisms, mobilize Po more effectively than glucose. Inorganic P and organic P were released by LMWOAs followed by oxalic acid > citric acid > malic acid if the substrates of P and LMWOAs were enough. There may be a critical threshold for the concentration of citric acid and oxalic acid between 10 mM and 25 mM to require for the solution of adsorbed and precipitated P, respectively. In all, LMWOAs increased the concentration of labile P by decreasing the concentration of stable P. The results indicated that LMWOAs can significantly promote P availability in acidic forests soils, and the effect of microorganisms on soil available P was more inclined to use LMWOAs than glucose.