Mercury Forms And Its Correlation With Properties Of Plant Rhizosphere Soil In The Water-level-fluctuation Zone Of The Three Gorges Reservoir Region

As a global pollutant,mercury（Hg）has the characteristics of strong volatility and long-distance migration and transformation.Mercury can exist in many species in the natural environment,of which methyl-mercury（MeHg）is the most toxic and most harmful form.Therefore,understanding the migration and transformation of MeHg in various environmental ecosystems is the current research hot spot.The rhizosphere environment refers to the soil micro domain environment that closely interacts with plant roots.Due to the activity of plant roots,rhizosphere soil is different from non-rhizosphere soil in physical,chemical and biological properties,thus affecting the distribution,migration and transformation of heavy metals in the soil.The Three Gorges Reservoir is China’s largest and an extra-large reservoir that has attracted worldwide attention.The special dispatching mode of Three Gorges Reservoir has led to the formation of the water-level-fluctuation zone（WLFZ）.During the dry period,a large number of plants will grow on the fluctuation zone.The growth of plants is bound to change the combined form of Hg in the soil,thus affecting the biogeochemical processes of Hg in the WLFZ.Therefore,it is of great significance to focus on the influence of typical plant growth on soil Hg migration and transformation in the WLFZ of Three Gorges Reservoir.In this thesis,we inveistigated the total mercury（THg）and MeHg content,species transformation and associated influencing factors among the rhizosphere,near rhizosphere and non rhizosphere soil of Cynodon dactylon（L.）in the 165m and 155m height ebb zones,which is the main plant of WLFZ,in Qukou（QK）of Kaizhou and Shibaozhai（SBZ）of Zhongxian of the Three Gorges Reservoir Area in Chongqing region.Our results depict that:（1）The concentration range of MeHg in QK soil is 0.25±0.03～1.65±0.46 ng·g^-1,and in SBZ soil is 0.07±0.03～1.42±0.32 ng·g^-1.The gradient of MeHg content is rhizosphere>near rhizosphere>non rhizosphere,and specifically,the MeHg content in rhizosphere soil is significantly higher than that in non rhizosphere soil（p<0.05）.Additionaly,the concentration of MeHg in the soil will first increase and then decrease during the dry period,and the variation shift of MeHg content in rhizosphere and near rhizosphere soil is larger than the range in non rhizosphere soil.（2）The concentration range of THg in QK soil is 43.67±1.69～121.66±4.85 ng·g^-1,and that in SBZ soil is 69.27±2.59～150.51±6.24 ng·g^-1.The trend of THg concentration in soil show SBZ>QK,rhizosphere>near rhizosphere>non-rhizosphere.In April,the THg concentration of near rhizosphere and rhizosphere soil is significantly higher than that in non-rhizosphere（p<0.05）.In June and August,THg concentration in the rhizosphere soil QK is significantly elevated than in non-rhizosphere soil in（p<0.05）,but such trend is not observed in SBZ（p>0.05）.This is likely attributed to the specific difference at each site during the Cynodon dactylon（L.）growth.（3）The rand gradient of the five mercury species in the QK soil and SBZ soil is Hg-e>Hg-s,Hg-o>Hg-h>Hg-w,and specifically the Hg-e is the main form.Hg-w、Hg-h、and Hg-e show rhizosphere>near rhizosphere>non-rhizosphere.Additionaly,during the dry period,both increase of concentration and propotion of Hg-w、Hg-h suggests that the rhizosphere of Cynodon dactylon（L.）can distinctly improve the concentration of bioavailable Hg,thereby promoting the methylation.（4）The concentreations of organic matter（OM）、Fe²⁺、Mn²⁺、SO₄^2-and total amount of culturable bacteria（TCB）in the QK and SBZ soil show the trend of rhizosphere>near rhizosphere>non rhizosphere.The contents of OM、Fe²⁺、Mn²⁺、SO₄^2-and TCB in rhizosphere soil is significantly higher than those in non rhizosphere soil（p<0.05）.The results indicate that the physico-chemical and biological properties in the rhizosphere soil are different from those in the non rhizosphere soil.（5）The concentration of MeHg in QK and SBZ soil exhibit extremely significant positive correlation with the concentration of OM、Mn²⁺、SO₄^2-and TCB（p<0.01）.This is attributed that OM、Mn²⁺、SO₄^2-and TCB in the soil can promote the methylation of soil.There is a significant positive correlation between Fe²⁺and MeHg content in QK soil（p<0.05）,but no significant correlation between Fe²⁺and MeHg content in SBZ soil（p>0.05）.In this study,we have verified that the physico-chemical and biological properties of typical plant rhizosphere in the WLFZ of the Three Gorges Reservoir area play an imopotrant role to shaping the Hg species and transformation.Additionally,our study provides a new insight in understanding the variation and migration and transformation of the Hg in the soil of WLFZ region.The results can provide a research basis for further studies focusing on the biogeochemical cycle and ecological risk of mercury in the Three Gorges Reservoir area.