Co-variation among DOM components, Fe fractions, and Cd / As mobility under fulvic acid amendment in flooded paddy soils

Both dissolved organic matter (DOM) and Fe oxides are key factors associated with cadmium and arsenic (Cd / As) behavior in paddy soils. However, the co-variation of DOM and Fe fractions induced by humic acid-like substances and their effects on Cd / As availability are not fully evaluated under flooded conditions. Hence, a 60-day incubation experiment was conducted to elucidate the effects of fulvic acid on Cd / As behavior under reduction conditions using a Cd / As co-contaminated paddy soil. Results showed that FA addition resulted in a significant decrease in CaCl 2 -Cd (14.1–28.5%) and an increase in KH 2 PO 4 -As (9.5–28.1%). These were accompanied by a redistribution of Cd from the exchangeable fraction (EX-Cd) toward more stable forms (CA-Cd, WOM-Cd, and OX-Cd), and by a reduction in Fe-As together with increases in AE-As and Al-As fractions. Total DOM content increased markedly upon FA addition, with substantial increases in humic-like (C1 and C2, 0.9–1.5-fold) and protein-like (C3, 1.3–3.7-fold) components. Furthermore, decreases in Fed (6.2–11.5%) and increases in Feo (6.2–25.9%) under reduction conditions were accelerated by FA amendment. Random Forest (RF) and Structural Equation Modeling (SEM) analysis consistently identified DOM components and Fe fractions as the dominant predictors of Cd / As availability. The relationships among DOM components, Fe oxide fractions, and metal availability suggest that the co-variation of DOM components and Fe fraction changes may contribute to reduced Cd availability. In contrast, the increased As mobility may be associated with Fe oxide reduction, DOM competitive interaction, and possible As valence transformation. These findings provide association-based evidence linking DOM composition, Fe fraction changes, and Cd / As behavior in flooded paddy soil. • FA amendment substantially increased total DOM and fluorescence components (C1-C3). • Reactive DOM components may facilitate Fe fraction changes under flooded conditions. • Co-variation of DOM and Fe fractions were associated with reduced Cd availability. • Fe oxides reduction and DOM competition may contribute to increased As mobility.