Regulation of straw-derived DOM and clay mineral complexation on mercury accumulation in vegetables

Straw return-to-field releases substantial dissolved organic matter (DOM), which can interact with clay minerals and influence mercury (Hg) dynamics in soil-plant systems. However, its detailed mechanisms remain poorly understood. In this study, DOM-montmorillonite (DOM-M) complexes were synthesized using DOM extracted from composted rice straw (DOM_rice) and rape straw (DOM_rape). The objective of this study was to investigate their impacts on Hg methylation in soil and the accumulation of total Hg (THg) and methylmercury (MeHg) in vegetables. The results demonstrated that straw-derived DOM significantly increased MeHg levels in the soil and water spinach. However, humified straw-derived DOM effectively suppressed this elevation by 29.0–64.5%. Specifically, humified DOM_rice resulted in lower MeHg concentrations in the soil and reduced THg and MeHg levels in water spinach compared to humified DOM_rape. Natural montmorillonite reduced Hg methylation in the soil but increased the accumulation of THg and MeHg in water spinach. In contrast, the humified DOM_rape-M complex significantly mitigated the MeHg accumulation in water spinach that was enhanced by montmorillonite, with a reduction percentage of 25.8–52.0%, while the humified DOM_rice-M complex did not demonstrate a similar advantage. This discrepancy could be attributed to certain molecular components in DOM_rape, such as higher thiol-rich protein-like fractions and oxidized S species, which could promote Hg retention within mineral layers. The reduced adsorption capacity of humified DOM_rice-M for Hg²⁺ also emphasized the unique role of humified DOM_rape-M. Overall, this study highlights the importance of humified straw-derived DOM and its interaction with soil minerals in shaping Hg dynamics within the plant-soil system.