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

• Published in: Journal of plant nutrition and soil science Vol. 178; no. 4; pp. 555 - 566
• Main Authors: Wang, Yongzhuang, Chen, Xin, Whalen, Joann K, Cao, Yanhong, Quan, Zhi, Lu, Caiyan, Shi, Yi
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY‐VCH Verlag 01.08.2015; WILEY-VCH Verlag; Wiley Subscription Services, Inc
• Subjects: acid soils; agricultural soils; aluminum; calcareous soils; cations; citric acid; iron; malic acid; molecular weight; organic acid ligands; oxalic acid; phosphorus; roots; soil phosphorus fractions; soil solution
• ISSN: 1436-8730; 1522-2624
• DOI: 10.1002/jpln.201500047

Low 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.