Abstract:

Recent studies have reported that mercury accumulation in rice grains produced in Hg mining area may pose a potential health risk to consumers of the rice. Therefore, it could be of great importance to investigate how some key biogeochemical factors affect speciation and phytoavailability of mercury (especially methylmercury, MeHg) in contaminated paddy soils. To that end, aA pot experiment was carried out in greenhouse to investigate effects of addition of humic acid on production, phytoavailability, and bioaccumulation of methylmercury in an Hg-contaminated paddy soil. Results of the experiment show that addition of humic acid significantly reduced mercury methylation in the soil (Methylation-inhibiting  effect) by 41.7% in terms of time-weighted average concentration, as compared with CK, while it increased even more or by 277.0% of methylmercury in the soil porewater in terms of time-weighted average concentration, which may be attributed to the elevated concentration of dissolved organic matter in the porewater (Methylmercury mobilizing effect). Consequently, the amount of methylmercury accumulated in shoot of the rice plants  increased by 25.6%. Furthermore, the addition of humic acid caused significant increase in methylmercury translocation coefficient from soil to rice grains (Methylmercury translocating effect), thus leading to an evident increase (26.4%) in methylmercury accumulation in rice grains. However, the addition of humic acid also increased biomass of rice grains by 25.4%, thus diluting methylmercury concentration in grains (Methylmercury biodiluting effect). Consequently, the treatment of addition of humic acid and the treatment of CK were more or less the same in methylmercury concentration in grain.

Key words: Mercury, Methylmercury, Rice, Soil organic matter, Humic acid