| Rice paddy plantation is one of the most prevalent land uses in South and East Asia where rice is the dominant foodstuff. Rice paddy soil, as a typical ephemeral wetland, is known to be a significant environment for Hg methylation. The methylation of inorganic mercury(IHg) in rice paddy soil constitutes a key step in the cycling of Hg in this special terrestrial ecosystem, especially in the Hg mining area. Rice consumption, not fish, was the primary pathway of methylmercury(MeHg) exposure for local residents in Hg mining areas in Southwest China. However, the compartmentalization of the Hg methylation process and the biogeochemical controls on Hg methylation in paddy soil are extremely complex and remain poorly understood. Thus, an understanding of the factors that control the MeHg production will be crucial as a basis for reliable risk assessment of emission sources and will also lead to appropriate strategies for remediating contaminated soil.Two different typical Hg-contaminated mining sites were selected in this study, include: 1) an abandoned mining site(Wukeng), with high Hg concentration in soil but low concentration in atmosphere; and 2) a current-day artisanal mining site(Gouxi), with low concentration in soil but high concentration in atmosphere, and these sampling sites are located in the Wanshan Hg mining district, eastern Guizhou province, China. Five consecutive sampling campaigns were conducted during the rice growing season(1st June-10 th September, 2012). We measured the concentration and distribution of Hg species in different compartments(ambient air, atmospheric deposition, irrigation water, paddy water, and soil cores) at two contrasting Hg mining sites during a complete rice-growing season to assess the primary source and transformation of Hg species within paddy soil. Moreover, we qualified the specific methylation/demethylation rate constants using enriched Hg isotope addition technique(202Hg2+ and Me198Hg). In order to reveal any possible controlling factors on MeHg production within the rice paddy ecosystem, the concentration and distribution of DOC, S2-, SO42-, Fe2+, and Fe3+ in pore water and organic matter, p H in soil were observed as well in this study. The primary results and conclusions are as follows:(1) In our study, the MeHgunf/HgTunf ratio was up to 11 %(MeHgunf/HgTunf) for paddy water at Gouxi and the mean ratio for this water compartment(5.9±4.4 %) was significantly higher than for irrigation water(0.71±0.93 %) and precipitation(0.031±0.028 %). However, there was no significant difference between the MeHgunf/HgTunf ratios for the various water compartments at Wukeng(K-W test, p>0.05). These results imply active net Hg methylation paddy fields at Gouxi but not at Wukeng.(2) Over the rice growing season, the mean concentrations of THg in soil was 3.2±0.75 mg kg-1 and 38±4.8 mg kg-1 at Gouxi and Wukeng, respectively. Compared to Wukeng, the highest concentration of MeHg and the maximum proportion of THg that was MeHg(MeHg/THg) was observered at the soil surface layer(top 2 cm) at Gouxi, which further illustrates active net Hg methylation in the soil surface layer at Gouxi throughout the flooded period. We found that the concentration of MeHg had a significantly positive correlation with the concentrations of THg in soil at Gouxi(r=0.53, p<0.001, n=50)but not for Wukeng(r=-0.03, p=0.82, n=50), suggesting a mechanistic relationship between these two Hg species at the artisanal mining site only.(3) Over the rice growing season, the THg flux to the rice paddy soil attributable to atmospheric deposition was 17975 mg ha–1 and 3078 mg ha–1 at Gouxi and Wukeng, respectively. The THg flux to the rice paddy soil attributable to irrigation water was 428 mg ha–1 and 1382 mg ha–1 at Gouxi and Wukeng, respectively. Our finding that high concentration of Hg in ambient air at the artisanal mining site(Gouxi) resulted in corresponding elevated atmospheric deposition, and the newly deposited mercury could be expected to rapidly methylate after deposition in the appropriate environment(such as low pH and anaerobic environment).(4) Our results showed that soil MeHg concentration was significantly positively correlated with Km but significantly negatively correlated with Kd across the two sampling sites. The strong correlation of soil MeHg with Km and Kd suggested that MeHg levels in rice paddy soil were a function of both methylation and demethylation processes. Both sites clearly showed a relatively strong positive relationship between the in situ MeHg concentration and the instantaneous net Hg methylation potential(Km/Kd) in soil(Gouxi: r=0.69, p<0.001, n=30; Wukeng: r=0.52, p=0.003, n=30), indicating that the net methylation potential in the rice paddy soil reflects the measured MeHg production at any time point. As shown above, the potentials for net MeHg production, as indicated by the Km/Kd ratios, were high, particularly so for the surface soil at Gouxi as these samples had higher Me Hg concentrations than those at Wukeng. In comparison, low Km was accompanied by high Kd in paddy soil at Wukeng, which resulted in the lower net Hg methylation potential when compared with Gouxi soil profiles.(5) The highest value of sulfide in pore water at Gouxi was present in the surface soil layer(0-2 cm), indicating that maximum microbial sulfate reduction was currently stimulated in the uppermost layer of the soil. Regression analysis revealed a significant positive correlation between the instantaneous net Hg methylation potential(Km/Kd) in soil and sulfate concentrations in pore water at Gouxi(r=0.84, p < 0.001, n=30).Because Hg methylation was coupled to the respiration of sulfate-reducing bacteria(SRB) which use dissimilatory sulfate reduction(e.g., a sulfate as terminal electron acceptor) for the generation of energy. So that sulfate could enhance microbial sulfate reduction in rice paddies of Gouxi(production of sulfide) and then stimulate Me Hg production. Significantly negative correlations between Fe2+ and sulfate in pore water at Wukeng was observed during the rice growing season(r=-0.47, p<0.001, n=30). It implied that Fe2+ could restrain the sulfur activity through the formation of solid FeS, which is known to retain Hg2+ by both adsorption and co-precipitation, and it apparently could impact the availability of Hg for methylation in pore water.(6) The concentrations of bioavailable Hg(F1: soluble and exchangeable Hg fraction) in soil profiles collected from Gouxi and Wukeng were low, averaged 0.28% and 0.15% respectively. Bioavailable Hg was considered as the most hazardous form of Hg in soil due to the potential of this phase of Hg to be readily methylated. There was a significantly positive correlation between bioavailable Hg(F1) and MeHg in soil at Gouxi(r=0.81, p<0.001, n=50), suggesting that bioavailable Hg, which may serve as the substrate for the Hg methylation process, plays an important role in MeHg production in the paddy soil of Gouxi.In summary, the peak in MeHg concentrations in the surface soil layer at Gouxi resulted from the combination of the elevated net Hg methylation potential coupled with the increasing availability of Hg. These results confirmed the importance of both bacterial activity and Hg speciation in controlling Me Hg concentration in rice paddy soil. The sulfate stimulating the activity of SRB were a potentially important metabolic pathway for Hg methylation; bioavailable Hg appears to be the primary factor regulating microbial Hg methylation rates in rice paddies, whereas Hg derived from atmospheric deposition is the primary limiting factor in permanently flooded rice paddies. |
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