Fe-mediated DOM components drive effective arsenic immobilization in tea plantation soils: Mechanisms and quantitative insights

In iron (Fe)-rich tea plantation environments, the composition and behaviour of dissolved organic matter (DOM) differ markedly from those observed in other land use systems. Despite its importance, research on DOM in tea plantation soils remains limited, particularly with regard to its role in the transport and transformation of heavy metals, component-specific interactions and the quantification of environmental thresholds. This study examined the composition and fluorescence characteristics of DOM in surface soils, and quantified its Fe-mediated interactions with arsenic (As) under real-world tea plantation conditions. The results revealed that soil DOM in these environments exhibits low humification, high bioavailability and a predominantly terrestrial origin. Five distinct DOM components were identified, falling into two categories: humic acid-like and protein-like substances. A significant negative correlation was observed between soil As and Fe concentrations. Notably, in soils with high Fe levels (33,000 mg/kg), the presence of DOM reduced As concentrations by up to 53.6 %. This reduction is attributed to the rich array of adsorption sites in DOM, which influence Fe redox processes by facilitating the reduction of As(V) and promoting the formation of insoluble Fe-As precipitates through complexation. These findings enhance our understanding of DOM composition in tea plantation soils and shed new light on the environmental interactions among DOM, Fe and As. Such insights are crucial for assessing As mobility, enhancing soil quality monitoring in tea-growing regions and promoting the sustainable growth of the tea industry.