Effects of plant rhizosphere on mercury methylation in sediments

PURPOSE: This research investigates the effects of different plant species on Hg methylation rates, examines the difference in mercury (Hg) methylation rates in the plant rhizospheres, and shows the trend in methylmercury (MeHg) content of sediment in the Second Songhua River (SSR) of China, thereby providing the basis for estimating the ecological risk of this area. MATERIALS AND METHODS: In May 2008, sediment and plant samples were collected at the Hongshi, Shihaoxian and Songhuajiang Village Segment, which were the upper, middle, and lower reaches of the SSR, respectively. The samples were treated for incubation. The plants were planted in root boxes with sediment. After the incubation ended, the sediment in each layer of the boxes was removed separately and ground to pass a 63-μm mesh sieve. Hg contents were measured prior to the experiment. MeHg contents in sediments before and after the incubation were measured. Calibration standards and blanks were analyzed with each batch of samples. The standards GSS-5(GBW-07405) and IAEA-443 were used as reference materials for Hg and MeHg measurements, respectively. RESULTS AND DISCUSSION: The plant roots promoted Hg methylation. The maximum Hg methylation rate was 2.056 μg m−3 day−1 at the plant roots where microorganisms were dense, which indicated that microorganisms could promote Hg methylation. The higher the organic matter content, the higher the Hg methylation rates. The order of roots of plant species promoting Hg methylation rates was Festuca arundinacea > Juncellus serotinus > Artemisia princeps. The order of Hg forms promoting methylation in plant roots was HgS > HgCl2 > Hg(NO3)2. CONCLUSIONS: Plant roots can promote Hg methylation. Hg methylation rates were greater: closer to plant roots for larger amounts of plant roots and for sediments with higher organic matter contents. The order of Hg forms promoting methylation was HgS > HgCl2 > Hg(NO3)2.