Organic foliar spraying: A method that synchronously reduces mercury methylation in soil and accumulation in vegetable

Although the use of foliar spraying with organic matter has been extensively studied and applied to reduce heavy metals in plants, research on its application for reducing mercury (Hg) accumulation in plants, particularly the more toxic methylmercury (MeHg), remains scarce. Furthermore, previous researches on the barrier mechanisms of foliar spraying primarily concentrated on the effects of spraying agents on plant physiological and biochemical indicators, with limited focus on their impacts on soil environment. Herein, the dynamic effects and mechanisms of organic foliar spraying materials, including earthworm liquid fertilizer (ELF), Tween 80 (T80), and citric acid (CA), on soil Hg methylation and accumulation in lettuce were investigated using pot experiment. The findings revealed that foliar spraying significantly reduced the total mercury (THg) and MeHg concentrations in mature lettuce stems and leaves, with CA demonstrating the highest efficacy, achieving reduction rates of 24-60% for THg and 64-69% for MeHg. Spraying CA and T80 also simultaneously reduced the dissolved Hg and MeHg in the soil during the lettuce maturity period. The reductions of soil Hg methylation and bioaccumulation in lettuce were related to the increased abundance of Hg-reducing bacteria, decreased tartaric acid content and Hg-methylating bacteria abundance in soils, as well as enhanced nutrient absorption by lettuce. Additionally, foliar spraying lessened Hg toxicity to the plant and facilitated Hg sequestration in cell walls and vacuoles. Thus, foliar organic spraying impacted Hg enrichment in plant through altering plant physiological and biochemical indices, soil environment and Hg methylation processes.