Effects of Low Molecular Weight Organic Acid Extraction on the Release of Metal Cations and Chemical Alteration in Purple Soil

[Objective] The specific mechanism of influence of low molecular weight organic acids in root exudates on metal cation release in purple soil in Yuanmou dry and hot river valley of Yunnan Province was explored to fill the current research gap and provide scientific basis for understanding soil weathering and plant-soil interaction. [Methods] The organic acid leaching test was employed to investigate the structural disintegration characteristics of purple soil under various low molecular weight organic acids and the release kinetics of ions during weathering. The quantitative analysis of the relationship between release rate and time was conducted, and the influence of different organic acids on CIA (chemical index of alteration) was discussed. [Results] Application of organic acids leads to a substantial increase in CIA value for purple soil weathering products ranging from 7.25% to 15.42%. Citric acid, at equivalent concentrations, demonstrates greater efficacy in accelerating weathering compared to oxalic acid. Scanning electron microscopy (SEM) results reveal that organic acids induce structural damage in purple soil, while MATLAB software analysis indicates that leaching by organic acids promotes the development of surface pores in soil minerals and reduces particle uniformity. The release rates of metal cations from soil vary with concentrations of low molecular weight organic acids (LMWOAs), decreasing gradually over leaching time. The metal ion release rates increase with increasing organic acid concentrations, and the total ion release pattern follows Fe3+ > Mg2+ > Mn2+ > K+ > Ca2+ > Na+. A quantitative analysis model for the metal cation release rate under changing acidity conditions is established using multivariate nonlinear regression. There exists a significant exponential function relationship between the release rate of metal cations and time. [Conclusion] The leaching of low molecular weight organic acids promoted metal cation release and weathering of purple soil, and citric acid was more significant than oxalic acid.