Study On The Methylation Of Mercury In Soil Of Different Wetlands In Liaohe Estuary

Mercury（Hg）methylation could occur in freshwater ecosystems with low or high salinity.However,few is available about how mercury release and methylation responding to salinity change.Seawater salinity,redox condition and temperature of soils,which changed with the tidal cycles and seasonal variation,not only likely affect the release of Hg,but the synthesis and stablity of methylmercury（MeHg）in soils also affected.Using laboratory incubator to simulate the culturing process,we studied the trend of change in Hg and MeHg content of soils in response to the changing environment temperature and the salinity of the flooding water for the mudflats wetland soils and Suaeda heteroptera wetland soils in Liaohe estuary.Also,the variation characteristics of sulfate reducing bacteria（SRB）in wetland soil with flooding salinity and environment temperature and its effects on mercury methylation were analyzed.The main conclusions were drawn as follows:（1）Under different cultivation conditions,the total mercury（THg）contents in surface（0-10cm）and subsurface（10-20cm）layer were the highest when the flooding water was deionized water（CK）.The THg contents of soils in both layers showed a decreasing trend with the rising salinity of flooding water.The THg contents of flooding water were 0.854-1.243 ug·L^-1,and showed a incremental trend with the rising salinity under alternate wetting and drying conditions.The THg contents of flooding water also increased with the rising salinity under anaerobic and aerobic conditions,and showed a significantly negative correlation with THg contents in surface soil and a negatively correlation with THg content in subsurface soil.（2）The THg contents of soils showed as flows:10℃>20℃>30℃,and aerobic environment>anaerobic conditions.It was found that the THg contents of soils showed a trend of decrease-increase-decrease with the extension of incubation time through the anaerobic-aerobic experiment.（3）The MeHg contents of soils increased with the rising salinity under 10℃,and it peaked at the salinity of 1.50%under 20 and 30℃conditions.The MeHg contents showed a increasing trend with the rising temperature over the range of 10℃-30℃.The synthesis of MeHg in soils under full inundation was more favorable than that in semi-inundation conditions.The changes of flooding salinity and environment temperature were more likely to enhance the release of Hg and the synthesis of MeHg in surface（0-10 cm）soils.（4）Under alternate wetting and drying soil conditions,the soil has high MeHg contents when salinity was low（<1.00%）under the flooding period,on the contrary,when soil was dry,the MeHg contents is higher when the salinity was higher（1.00-1.80%）.Under anaerobic-aerobic conditions,the MeHg contents in soils increased first and then decreased with rising salinity of flooding water and were higher at the salinity of 0.5%-1.0%.（5）In this study,the SRB could exist in sediments under various salinity gradient,and the SRB contents in soils changed irregularly with the rising salinity.The SRB contents of soils under 30℃condition were higher than that under 20℃condition.Under the same temperature conditions,the SRB contents of soils under anaerobic condition were higher than that under aerobic condition.There were no significant correlations between the SRB contents and the MeHg contents of soils when the culture temperature was 20℃.The MeHg contents and SRB contents showed a positive correlation during the early stage of culture and had no correlation at the late stage of culture under anaerobic and aerobic conditions at 30℃.Therefore,the rising salinity could promote the release of Hg of wetland soil,and the appropriate salinity could conducive to the synthesis of MeHg of soil.The appropriately rising temperature and anaerobic conditions not only can be more conducive to the release of Hg in soils but the activity of SRB and the production of MeHg also enhanced,the methylation ability of SRB to Hg was more obvious at 30℃.This study has a very important practical significance for protecting the special ecosystem of Liaohe Estuary Wetland.