Abstract

One mechanism by which plants can mobilize organic and inorganic forms of phosphorus (P) in soils is by exudation of low-molecular-weight organic acids and anions. However, little is known about their effects on the transformation of P fractions in soils. We used a modified Hedley fraction method to examine the changes in P fractions in a sandy loam soil induced by a range of low-molecular-weight organic acids and anions. Generally, the effectiveness of organic acids to mobilize P occurred in the order of tricarboxylic (tricarboxylate)>dicarboxylic (dicarboxylate)> monocarboxylic acid (monocarboxylate). Addition of organic acids and anions, especially citric acid and citrate significantly increased resin-P content. As the stability of P fractions increased, the effectiveness of organic acids and anions to mobilize the P fractions decreased, with no significant effects on Conc. HCl-P and Residual-P fractions. Both organic acids and H₂SO₄ appeared to be less effective on the solubilization of organic-P because of the relatively low organic-P content of the soil (30% of total-P). The pattern of P mobilization induced by organic acids was highly controlled by the intrinsic P status such as amounts and distributions of P fractions. Exudation of organic acids and organic anions increased the resin-P content which suggests that organic anions rather than rhizosphere acidification are more important for P solubility and availability for plant growth. These results indicate that root released organic acids and anions can increase the solubility of some of the more stable pools of soil P, which can be important for soil P availability and P losses from soil to water.

Key words: Low-molecular-weight organic acids, organic anions, phosphorus (P) transformation, Hedley fractionation, sandy loam soil, microbiological interactions