Path analysis of phosphorus activation capacity as induced by low-molecular-weight organic acids in a black soil of Northeast China

Purpose

The present study was designed to assess the potential effects of low-molecular-weight organic acids on the activation of inorganic phosphorus, obtain exact information on the acidity effect of proton and complex effect of organic anion in P availability, and determine the components among phosphorus fractions that contributed the most to inorganic phosphorus activation in black soil.

Materials and methods

Both oxalic and citric acids treated with different concentrations and pH values were carefully selected. The activating amounts of total inorganic phosphorus and individual phosphorus fractions were estimated. Path analysis was used to analyze the direct and indirect effects on inorganic phosphorus activation.

Results and discussion

The amount of total activating-P_i increased as the concentrations of oxalic and citric acids increased. When the concentrations were ≤?1.0 mmol L^?1, oxalic acid exhibited a lower capability than citric acid in total activating-P_i, but when the concentrations were ≥?1.5 mmol L^?1, oxalic acid exhibited a higher capability. The amount of total activating-P_i decreased as pH of LMWOAs increased. LMWOAs-induced P_i activation might be attributed to combine acidity and complex effects.

Conclusions

Correlation analysis showed that the activation of total P_i was significantly correlated with the activation of H₂O-P_i, NaHCO₃-P_i, NaOH-P_i, and HCl-P_i (p?<?0.05). Path analysis revealed that soil activating-H₂O-P_i mainly affected P_i activation through an indirect path. The contribution of activating-NaHCO₃-P_i alone was maximal on the total activating-P_i by both the direct and indirect effects, followed by activating-NaOH-P_i and activating-HCl-P_i.