Theoretical chemical characterization of phosphate-metal-humic complexes and relationships with their effects on both phosphorus soil fixation and phosphorus availability for plants

BACKGROUND: Previous studies showed that phosphate can be complexed by humic acids (HA) through stable metal (M) bridges (PMHA). We studied the thermodynamic properties of PMHA and their relationships with the ability of PMHA to both decrease soil P fixation and increase P availability for plants. With this aim, we studied the theoretical stability of PFeHA, PAlHA and PCaHA by molecular modelling methods in relation to the degree and intensity of P absorption in soils and the ability of plants to take up complexed P. RESULTS: A density functional theory (DFT) quantum chemical study enabled us to obtain stable structures for the three PMHA complexes in water solution. The theoretical stabilities (Delta G(0)) were consistent with that for apparent stability obtained by Scatchard method, PFeHA >= PAlHA > PCaHA, though the differences were clearer by the DFT method. Also the reduction of soil P fixation and the release of P from PMHA in the presence of an anionic resin confirmed the stability order of the different PMHA. Plant studies confirmed the ability of diverse plant species to take up both P and metal complexed in PMHA. CONCLUSION: The results indicated the potential efficiency of PMHA-based fertilizers to optimize P fertilization for crops cultivated in soils with high P fixation ability. (C) 2012 Society of Chemical Industry