Influence of phenolic acids on phosphorus mobilisation in acidic and calcareous soils

Phenolic acids and phenols are abundant in soils. However, little information is available on the role of these compounds in mobilisation of soil phosphorus (P). The present study examined the effects of three phenolics on P mobilisation in comparison with citric acid in three soils differing in chemical properties. The soils were incubated with organic compounds at concentrations of 0 to 100 mol g^–1 soil for 30 min. While the addition of phenolic acid anions and phenol decreased soil pH, citrate either increased or did not affect soil pH depending on the soil type. All the organic compounds increased the amounts of easily-mobilised P fractions in the order of catechol citric acid proto-catechuic acid caffeic acid for the acid soils and citric acid > catechol = proto-catechuic acid > caffeic acid for the calcareous soil. Phosphorus mobilisation did not correspond to the amounts of Ca, Fe or Al ions released from the soils. These results suggest that organic acids and phenols altered the P fractions from stable, sparingly-soluble forms to easily dissolvable forms, and that chelation or precipitation of cations with organic ligands occur before biodegradation of the ligands.