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Different mechanisms of organic and inorganic phosphorus release from Mollisols induced by low molecular weight organic acids1

Abstract: Enhancement of inorganic phosphorus (Pi) release by low molecular weight organic acids (LMWOA) increases soil phosphorus (P) availability to plant, but the release of organic phosphorus (Po) and the associated kinetics of both Pi and Po release by LMWOA have not been well considered. The aim of this study was therefore to investigate the impacts of three common LMWOA (citric acid, oxalic acid, and malic acid) on the kinetic release of Pi and Po in a black clay soil in northeast China. All kinetic data were well described by the Elovich and power functions models (P < 0.01). The LMWOA at 10 mmol kg-1 soil increased the rate of the kinetic release of both Pi and Po. Organic phosphorus released by LMWOA derived from the soil-labile Po (NaHCO3-Po) fraction in the order of oxalic acid (3.58 mg kg-1) > citric acid (2.67 mg kg-1) > malic acid (1.76 mg kg-1). In contrast, Pi release by LMWOA resulted from mobilization of the moderately labile NaOH-Pi (Fe/Al-P) and HCl-Pi (Ca-Pi) fractions in the order of citric acid (4.83 mg kg-1) > oxalic acid (2.40 mg kg-1) > malic acid (2.04 mg kg-1). Therefore, the release of Po by LMWOA is mainly via the dissolution of soil-labile Po (NaHCO3-Po) rather than via chelation of organic acid ligands.