Effects of low-molecular-weight organic acids on phosphorus adsorption-desorption characteristics and phosphorus fractions in coastal saline soil

In the coastal saline-alkali soils of the Bohai Rim region, the abundance of calcareous components (e.g., calcium carbonate) and high pH (>8) lead to strong phosphorus adsorption and rapid conversion into insoluble calcium phosphate salts. Low-molecular-weight organic acids (LMWOAs), particularly oxalic and citric acids, are known for their superior phosphorus-solubilizing capacity and ubiquitous presence in soils, and play a crucial role in phosphorus mobilization for plants and microorganisms. However, their mechanisms in coastal saline-alkali soils are not yet fully understood. Consequently, this study used batch experiments and a modified Hedley phosphorus classification method to investigate the effects of different concentrations of oxalic acid (C5: 5 mmol∙kg⁻¹, C10: 10 mmol∙kg⁻¹, C50: 50 mmol∙kg⁻¹) and citric acid (N5: 5 mmol∙kg−1, N10: 10 mmol∙kg⁻¹, N50: 50 mmol∙kg⁻¹) on the adsorption-desorption characteristics and transformation of phosphorus fractions in coastal saline soils. Oxalic acid demonstrated a stronger ability to inhibit phosphorus adsorption and activate phosphorus than citric acid. Both oxalic and citric acids promoted phosphorus desorption and activation by competing for binding sites with phosphate ions via —COO⁻. Oxalic acid primarily functions through anion effects, while citric acid mainly acts via hydrogen ion (the “proton”) effects. C50 promoted the transformation of stable inorganic phosphorus (Pi) to labile Pi in the soil within 30 days. However, the activation mechanism of N50 differed from those reported in previous studies. During short-term incubation (3 days), N50 promoted the transformation of labile Pi and stable Pi into moderately labile Pi, resulting in a significant increase of 1.33% in moderately labile Pi. All the treatments promoted the transformation of moderately labile Po to labile Po. The C10, C50, and N50 treatments increased the available phosphorus in the soil. However, the effectiveness of the low-molecular-weight organic acids showed a time-dependent pattern. In saline-alkali soils, the effects began to diminish after 3 days, with citric acid losing its effectiveness completely by day 10, whereas oxalic acid remained effective until day 30. Soil adsorption and desorption capacities of phosphorus significantly correlated with diluted HCl-extracted Pi (D. HCl-Pi). On phosphorus fractions, NaHCO3-extracted Pi (NaHCO3-Pi) negatively correlated with D. HCl-Pi. D. HCl-Pi is the key phosphorus fraction governing phosphorus availability in coastal saline soils. According to the study results, applying 10–50 mmol∙kg⁻¹ oxalic acid is recommended for Bohai Rim coastal saline-alkali soils, which can significantly and persistently enhance soil phosphorus availability.