The Role of Cotric Acid as a Phosphorus Mobilization Mechanism in Highly P-Fixing Soils

Exudation of low molecular weight carboxylates is an important phosphorus (P) acquisition strategy for plants in strongly P-fixing soils. The effect of citrate exudation on P availability was studied in five acid soils with a large organic matter content and a low available P pool. The amount of molybdate reactive P extracted by citric acid 10 mM (0.1-2 mg kg -1 ) was larger than the molybdate reactive P (MRP) extracted by CaCl 2 10 -3 M (0.007-0.07 mg kg -1 ). Both extractants were adjusted to soil pH prior to extraction. Also, the total P content of citric acid extracts (2-44 mg kg -1 ) far exceeded the total P content of CaCl 2 (0.2-1.5 mg kg -1 ). However, ratios of MRP to total dissolved P (TDP) remained constant. When citric acid was added to the CaCl 2 -extracts (the pH of citric acid being adjusted to the pH of CaCl 2 , and thus eliminating release of P through acidification) a significant pH increase was noted. Simultaneously, MRP increased significantly in all but one soil. This suggests that the P availability enhancing properties of citric acid are not only due to acidification of the plant rhizosphere, but also to its Al and Fe complexing capacity.