| Mercury is one of the most important heavy metal pollutants in the world.It can be transformed into methylmercury（MeHg）with higher toxicity through biological and abiotic effects under flooding conditions,and threaten human health through food chain amplification effect.The rice field system is a hot spot of mercury methylation,and the enrichment ability of rice to soil MeHg is much higher than that of other crops,which makes rice become a more common way of mercury exposure for people.During the rice growing period,a large number of algae will propagate and grow due to the large amount of nutrients input into the rice field.During the process of algae metabolism and decay,it is easy to produce small molecular weight"algae derived organic matter"rich in sulfhydryl groups,in which mercury methylating microorganisms inhabit.The promotion effect of lake algae on mercury activation and methylation has attracted extensive attention.The large-scale growth of Lake Algae in paddy field system may produce similar environmental effects of mercury,and further enhance the environmental risk of mercury in paddy field.Therefore,exploring the key mechanism of algae intervention in mercury methylation and bioaccumulation in paddy field system,revealing the environmental effects of algae on mercury,and identifying the increased risk of bioaccumulation of Mercury will help to improve the understanding of mercury biogeochemical cycle in paddy field,and provide theoretical basis for scientific prevention and control of environmental risk of mercury in paddy field.In this paper,the mercury polluted paddy field system was taken as the research object,and the soil culture experiments with algae powder,rape straw and rice straw were constructed,as well as the rice pot experiment with algae powder,to comprehensively explore the mechanism of algae decomposition on MeHg production and rice enrichment in paddy field system.The main results are as follows:（1）In the soil culture experiment,compared with the control group,the dissolved organic carbon（DOC）and dissolved organic nitrogen（Don）in the soil pore water increased significantly after adding algae powder or straw.The DOC concentration in the"algae+soil+water"treatment group was significantly higher than that in the"rice straw+soil+water"and"rape straw+soil+water"treatment groups at the early stage of culture（15 days）（P<0.05）.UV fluorescence data showed that the concentration of DOM in pore water was increased,but the humification index of DOM was decreased.The humification index of DOM in algae treatment group was significantly lower than that in rice straw treatment group（P<0.05）,but there was little difference between algae treatment group and rape straw treatment group（P>0.05）.With the extension of incubation time,the aromaticity and humification degree of DOM in soil pore water of each treatment group increased as a whole.With the help of parallel factor analysis,two types and five kinds of fluorescent components were identified from DOM in pore water,which were humus like and protein like components.With the extension of incubation time,humus like components of soil DOM in each treatment group increased gradually,while protein like components of soil DOM in three treatment groups changed differently,in which"algae+soil+water"treatment group showed a gradual downward trend.（2）In the soil culture experiment,compared with the control group,adding straw and algae powder significantly increased the concentration of MeHg in the overlying water.With the extension of culture time,the concentration of MeHg in the"straw+soil+water"treatment group first increased and then decreased,while the concentration of MeHg in the"algae+soil+water"treatment group gradually increased,and finally significantly higher than that in the"straw+soil+water"treatment group（P<0.05）.Similar to the variation of MeHg concentration in overlying water,the addition of straw and algae powder significantly increased the MeHg concentration in pore water and the MeHg content in soil.With the extension of incubation time,the MeHg concentration in pore water and the MeHg content in soil in the"algae+soil+water"treatment group increased gradually,and finally significantly higher than that in the"soil+water"and"straw+soil+water"treatment groups（P<0.05）.Correlation analysis showed that only in the"algae+soil+water"treatment group,the pore water Don was significantly correlated with the overlying water,pore water and soil MeHg（P<0.05）,indicating that the content of MeHg increased with the increase of algal don concentration.Mercury is one of the most important heavy metal pollutants in the world.It can be transformed into methylmercury（MeHg）with higher toxicity through biological and abiotic effects under flooding conditions,and threaten human health through food chain amplification effect.Compared with dry land,paddy soil showed significantly higher mercury methylation level,and the enrichment ability of rice to soil MeHg was much higher than that of other crops.As a result,eating rice became a more common way of mercury exposure for people,which attracted extensive attention of scholars at home and abroad.During rice planting,there will be a large number of different exogenous organic matter input,such as fertilizer,etc.the large input of these nutrients will lead to the eutrophication of rice field,the reproduction and production of a large number of algae,the metabolism and decay of algae,which is easy to produce small molecular weight"algae derived organic matter"rich in sulfhydryl groups,and inhabit mercury methylating microorganisms.The promotion effect of algae on the activation and methylation of mercury in lakes has attracted extensive attention.The large-scale growth of algae in paddy field system may also produce similar environmental effects of mercury,and further enhance the environmental risk of mercury in paddy field.Therefore,exploring the key ways and mechanisms of algae involved in mercury methylation and bioaccumulation in paddy field system,revealing the environmental effects of algae on mercury,and identifying the increased risk of bioaccumulation of Mercury will help to improve the understanding of mercury biogeochemical cycle in paddy field,and provide theoretical basis for scientific prevention and control of environmental risk of mercury in paddy field.（3）In the pot experiment of rice,the addition of algae powder reduced the pH value,do concentration and redox potential of overlying water in each culture period.Compared with the control group,the addition of algae powder significantly increased the content of MeHg in rice soil,showing the rule of filling stage>mature stage>tillering stage.With the increase of algae powder,the content of MeHg in soil also increased as a whole.When the addition amount was 15g,MeHg in soil reached 9.48±2.09 ng·g-1,25.85±0.81 ng·g-1,20.36±1.18 ng·g-1at tillering stage,filling stage and maturity stage,respectively,with the increase rates of 34.32%,39.81%and30.53%.The analysis of soil microbial community showed that the input of algae powder increased the relative abundance of bacteria and archaea with the ability of mercury methylation in soil microorganisms at each stage,and the microbial abundance of potential mercury methylation was 1.3 times higher than that of the control group at the addition of 15g.Compared with the control(80.06±3.23 ng·g-1,45.93±2.56 ng·g-1),the contents of THg and MeHg in rice increased with the increase of algae powder.The contents of THg and MeHg in rice increased by 62.87%and 72.30%when the algae powder was added at 15g,which were 130.39±7.14ng·g-1and 79.14±0.85 ng·g-1,respectively.In addition,the contents of THg and MeHg were 1061.9±82.36 ng·g-1and 189.38±30.02 ng·g-1respectively,which were much higher than those of rice. |
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