Abiotic processes controlling the effects of citrate on phosphorus availability: study of a fertilized ferralsol

Organic acids exuded by roots, especially citrate, can increase P availability in soils and thus facilitate P acquisition by plants. However, controlling processes are poorly known. In order to improve this understanding, we added different citrate concentrations (2, 10, 20, 50, 100, 250 and 500 μM as trisodium citrate) to water extractions performed with a fertilized ferralitic soil. We performed the experiments at T = 2°C in order to minimize the effects of biological activity. We measured dissolved inorganic P concentrations (available P) along with the concentrations of major solutes (Na, K, Ca, Al, Fe, Si and organic carbon) and pH. We also modeled aqueous speciation and determined mineral saturation indexes. Results showed that available P gradually increases with citrate concentration. Similar trends were observed with dissolved Al, Fe, Si, organic C, and Na. In contrast, pH decreased compared to control (i.e. 0 citrate) with addition of low citrate concentrations while it became more alkaline at higher citrate concentration. Dissolved K, Mg and Ca did not show monotonous variations as well. Dissolved K gradually decreased up to 250 μM of citrate. Dissolved Ca slightly increased with low citrate concentrations, vanished at intermediate concentrations (50 and 100 μM) and finally increased markedly at high citrate concentrations. Dissolved Mg exhibited intermediate variations. Multiple linear regression analysis showed that variations of P availability were significantly correlated with dissolved Si. This result along with these of the modelingsuggested that available P was mainly controlled by dissolution of P-sorbing minerals, especially kaolinit