Effects of Low-Molecular-Weight Organic Acids on Selenium Speciation Transformation and Dissolved Organic Matter Component Changes in Selenium-Rich Red Soil

Abstract Ganzhou City in Jiangxi Province is a core production area for navel oranges in China and represents a typical selenium-rich specialty agricultural region. However, the selenium-rich red soils in southern Jiangxi are strongly acidic with high iron-manganese oxide content, which strongly immobilizes soil selenium, severely restricting the development of the local selenium-enriched navel orange industry. Low-molecular-weight organic acids (LMWOAs), as root exudates and microbial metabolites, can activate soil selenium and synergistically promote plant growth. However, the regulatory mechanisms of LMWOAs on soil selenium speciation remain unclear. This study investigated how low-molecular-weight organic acids (LMWOAs) affect selenium transformation and availability in selenium-enriched red soils. Six LMWOAs at concentrations of 0.1–100 mmol kg⁻¹ were tested to examine their influence on selenium speciation and dissolved organic matter (DOM) composition. Soil selenium speciation and DOM fluorescent components were analyzed following LMWOAs application to assess the relationship between DOM changes and selenium transformation. LMWOAs significantly increased soluble selenium (SOL-Se) content, especially under 100 mmol kg⁻¹ citric acid (CA). Low-concentration treatments (≤10 mmol kg⁻¹) promoted the release of exchangeable (EXC-Se) and Fe-Mn oxide-bound (FMO-Se) selenium, while high-concentration treatments (100 mmol kg⁻¹) of CA, acetic acid (AA), and n-butyric acid (N-BA) inhibited their release. Only oxalic acid (OA) increased organically bound selenium (OM-Se). DOM components were negatively correlated with bioavailable selenium under CA treatment, but positively correlated with bound selenium under 10 mmol kg⁻¹ OA. LMWOAs notably alter soil selenium speciation. Short-term application enhances selenium bioavailability, promoting plant uptake. Prolonged use may increase humification, immobilizing selenium and reducing its availability. In contaminated areas, long-term LMWOAs application can mitigate selenium toxicity through immobilization. As natural rhizosphere exudates, LMWOAs are biodegradable and environmentally safe, posing minimal risk to soil ecosystems.