Kinetics of inorganic and organic phosphorus release influenced by low molecular weight organic acids in calcareous, neutral and acidic soils

Abstract

AbstractLow molecular weight organic acids exuded by plants roots enhance inorganic Pi release into soil solution and thereby increases plant‐available Pi in soils. Low molecular weight organic acids may also induce organic P (Po) release into soil solution, but kinetics of both Pi and Po displacement from the soil matrix into soil solution of agricultural soils is poorly understood, and the mechanism for Po release is not well explained. This study used kinetic experiments to determine the concentrations and release rates of Pi and Po induced by oxalic acid, citric acid, and malic acid in calcareous, neutral and acidic soils. Kinetic data were well described by Elovich (r2 = 0.801–0.993, P < 0.001) and power functions models (r2 = 0.721–0.977, P < 0.001). Low molecular weight organic acids at 10 mmol kg−1 soil induced the exponential release of both Pi and Po, which reached a plateau approx. 480– 2,880 min after the start of the experiment. Cumulative Po release induced by low molecular weight organic acids was ranked as oxalic acid (0.63–3.17 mg kg−1) > citric acid (0.61–2.82 mg kg−1) > malic acid (0.52–1.76 mg kg−1) and mainly resulted from the release of labile Po (NaHCO3‐Po) regardless of soil type. By contrast, oxalic acid was most effective in enhancing Pi release from the HCl‐Pi (Ca‐Pi) fraction of the calcareous soil, and citric acid was most effective in releasing Pi from the NaOH‐Pi (Fe/Al‐Pi) fraction of the neutral and acidic soils. Therefore, the mechanism for the kinetics of Po release induced by low molecular weight organic acids is ascribed to their ability to mobilize the labile Po (NaHCO3‐Po) rather than their ability to chelate cations (i.e., Fe3+, Al3+) bound to Po in soil.