Variation Characteristics Of Methyl Mercury In Different Types Of Paddy Soils And Their Enrichment In Rice

Mercury(Hg)is a highly toxic trace element of which can be transformed into a more toxic organic compound,i.e.,Methylmercury(MeHg)through biotic and abiotic ways in the natural environment.Previous studies have shown that the Hg methylation in paddy soil was enhanced by flooding irrigation,while the strong rice enrichment capacity for Hg,especially for MeHg in paddy field,also leads to a certain toxic effect on human body after the consumption of Hg-contaminated rice.Paddy soil has been widely recognized as the main Hg source for the rice plant,its physical and chemical properties not only affect the growth of rice plants,but also exert effects on the Hg availability to plants and the Hg methylation in paddy field.Soil pH is regarded as one of the key indicators illustrating soil physical and chemical properties,and different types of acidic soils have different effects on the growth of rice plant,soil microorganism and the activity of heavy metals.Therefore,it is of great significance to investigate the variations of Hg in paddy field with the characteristics of Hg enrichment by rice plant under different acidity.The southern China,with abundant precipitation,has been featured as the main rice producing area within China.Purple soil is widely distributed in the southern provinces of China,among which the Sichuan basin holds the largest area of such soil.Moreover,as the Sichuan basin is the main farming area in the southwestern China,it in turn becomes the key producing region of rice in China.The purple soil includes three soil genera,i.e.,acidic purple soil,neutral purple soil and calcareous purple soil of which are developed into acidic purple paddy soil,neutral purple paddy soil and calcareous purple paddy soil,respectively,through the long-term farming in paddy field.Because of the differences in physicochemical properties,Hg may have different characteristics regarding the transformation,migration and accumulation in different purple paddy soils.However,this feature is still unclear at present.For that reason,three types of typical purple paddy soil samples were collected on the representative sites.A pot experiment was conducted to analyze the transformation mechanisms of Hg during the rice growth period in different acidity purple soils,and the corresponding enrichment characteristics of Hg were also explored.The results are as follows:1.During the rice growth period,discrepancies regarding the tendencies of MeHg concentration were found within those three purple paddy soils.The MeHg concentration increased with time within the acidic purple paddy soil,while MeHg concentrations were first increased and then decreased over other types of purple paddy soils.Among those different types of soils,the MeHg concentration was found to be highest in the acidic soil,followed by the calcareous and neutral soils,respectively.2.The mean value of the MeHg/THg ratio in the acidic purple paddy soil was greater than those of the other two types of soils in the rice growth cycle,and ratios of MeHg/THg within Hg addition groups of those three purple paddy soils were all less than those of control groups.There was a significant positive correlation between MeHg and THg in purple paddy soil.Compared with soil pH value,CEC and organic matter,the correlation between CEC and 3 kinds of mercury forms in soil was the highest,showing a significant negative correlation.3.THg concentration in rice plant parts followed the trend: the content distribution of THg in rice plants is: root > grain > stalk > leaf > husk,and the content of THg in rice grain in the Hg addition group followed the trend as: acidic purple paddy soil > calcareous purple paddy soil > neutral purple paddy soil.The MeHg concentration in rice plant parts followed grain > root > stalk > leaf > husk,while the MeHg content in that acidic soil was much higher than those in neutral and calcareous purple paddy soils.The content of MeHg in the same parts of rice plants varied with THg at different growth stages.4.The ratio of Hg-w to THg in purple paddy soil was mostly lower than 0.1%.The highest concentration of Hg-w was found in acidic purple paddy soil,followed by neutral and calcareous soils,respectively.There was a significant positive correlation between MeHg and THg in these three paddy soils.The soil Hg-w was positively related with THg and MeHg,and the THg contents of rice root,stalk,grain and husk were positively correlated with soil Hg-w,the MeHg of root,stalk,leaf,ear and husk had a positive correlation with soil Hg-w as well.5.The ratios of MeHg/THg in different rice plant parts followed husk > grain > fringe > stalk > leaf > root.In the different periods of rice growth,the MeHg accumulative ability of each rice plant parts in those three purple paddy soils was different.There was a significant correlation between soil THg content and THg in roots and stalks,while MeHg in roots,stalks,leaves,fringes and grains were positively correlated with the soil MeHg content,respectively.6.Rice grain and root had very high accumulation for soil MeHg(BCF value 10~100),while BCF values of stalks and leaves were relatively low(0~10).The BCF values in those three purple paddy soils had the trend of acidic purple paddy soil> neutral purple paddy soil > calcareous purple paddy soil.In those three purple paddy soils,the MeHg/TF values of grains and most fringes were greater than 1,while others were all less than 1.