Anaerobic fermentation of straw with sulfate addition: A suitable approach for straw utilization in mercury-contaminated areas.

Returning raw straw to the soil can significantly elevate soil methylmercury (MeHg) and crop mercury (Hg) levels, underscoring the need to investigate safer approaches to straw utilization in mercury-contaminated regions. In this study, rice straw underwent anaerobic fermentation with the addition of sulfate, and the resulting fermentation products were utilized in a pot experiment involving water spinach to assess the impact of anaerobically fermented straw return on soil Hg methylation and its bioaccumulation. Findings revealed that the addition of sulfate during straw fermentation markedly increased the fermentation degree of the products, and sulfate was converted into organic sulfur-containing ligands that can functionalize the fermentation residuals. These changes enhanced adsorption or complexation of the fermentation products with Hg. Consequently, compared with raw straw returning to the soil, adding co-fermentation products of straw and sulfate to the soil can significantly reduce the bioavailable Hg and MeHg in the soil, the total mercury (THg) and MeHg in plants, with the maximum reduction rates being 68%, 92%, 66% and 78%, respectively. Therefore, returning the straw that has been anaerobically co-fermented with sulfate to the soil can effectively mitigate Hg methylation and bioaccumulation, while simultaneously increasing biomass, offering a suitable straw utilization method in Hg-contaminated cultivation areas.