The Migration And Fate Of Mercury During Litter Decomposition In Typical Forests Of Subtropical Water Conservation Areas

Mercury,as a special heavy metal element with strong toxicity,can not only accumulate in the environment through the food chain and cause great harm to human health,but also carried out long-range migration and spread through the atmosphere and accumulate in clean areas.The accumulation of mercury in organisms in areas with the least human activity may be due to the fact that mercury from mercury contaminated areas is transported and deposited through the global atmosphere and then ingested into local organisms.Different forms of mercury have different toxicity,among which methyl mercury has been widely studied due to its high bioabsorbability,stability,accumulative and neurotoxicity.As a significant place in the global biogeochemical cycle of mercury,terrestrial ecosystems have complex biomes and are hotspots for studying the migration and transformation of mercury.Being the largest terrestrial ecosystem,forest ecosystems with rich species,complex structure and diverse functions,is considered as an important active library of mercury.Forest soil immobilizes large amounts of mercury through both biological and abiotic processes,and at the same time carries out mercury output to surrounding soil,water bodies and atmosphere through runoff or volatilization.Considered as an important part of the accumulation of mercury on the ground of forest plants,the litters formed as the main carrier of forest mercury migration,playing an important role in the migration of mercury from the atmosphere-forest vegetation-soil interface.Whether in water or on land,the migration and transformation of mercury during its decomposition process is important for studying the environmental behavior of mercury in forest ecosystems.Due to the existence of the mountain slope in southwest China,the amount of runoff after rain in the forest ecosystem of the water conservation area is large.Thus the potential ecological risk of mercury in the litter is significant.The changes of mercury concentration and organic matter in litter of typical stand during decomposition of water and land were monitored continuously,and the changes of mercury concentration in surrounding soil were monitored at the same time.The specific research results are as follow.1.During the decomposition process of the two forest litters in the water body and on the land,In general,the decomposition rate of litter on land is higher than that in water,and that in evergreen broad-leaved forest is higher than that in coniferous mixed forest.C content decreases with the decomposition process,and the N content increases first and then decreases.During the decomposition process on land,the C content of litter in terrestrial coniferous and broad-leaved mixed forests and evergreen broad-leaved forests continued to decrease,dropping to 32% and 35% of the initial concentration,respectively;the N content showed an increase first and then decreased.In the first year of summer,it reached the maximum,1.38 times and 1.28 times of the initial concentration,and fell to 94% and 90% of the initial concentration after the decomposition.During the decomposition process of litter in water,the change law of C and N is similar to that of land litter,but the decrease of litter C and the increase of N in water are lower than that of litter decomposition on land.The C/N of all litter types decreased with time,and the drop of litter in the water was more obvious than that on the land.2.During the decomposition process of the two forest litters,the total mercury content showed a similar trend,that is,during the first year of decomposing,the total mercury concentration increased first,and the total mercury concentration reached the maximum in summer.After that,the total mercury concentration gradually decreased,and in the second year of decomposition,a similar trend appeared.The total mercury concentration in the two forest soils was greater than the total mercury concentration in the corresponding litter.The total mercury concentration in the soil did not change significantly(P>0.05).During the decomposition process,the total mercury content in the litter on the land was significantly higher than that in the litter in the water(P<0.01),and the total mercury content of the litter in the water fluctuated more.3.The trends of methylmercury in the litters of the two stands were similar,both increased in the first year and reached the maximum in the summer.The difference was that there was a small increase of methylmercury on the land in the summer of the second year.The methylmercury concentration of litter in water is relatively stable in the second year.And the increase of methylmercury in litter is significantly greater than that in total mercury.The concentration of methylmercury in litter is significantly higher than that in soil(p<0.01).The methyl mercury content of litter on land varies more.4.Changes in the form of mercury in litter on land and in water are similar: two active mercury(Hg-w and Hg-h)rise from the beginning of decomposition to reach the maximum in the first summer,when the microbial activity is high and with heavy rainfall.At that time,the ecological risk of mercury in the litter is the highest.The two active mercury gradually decrease after the first year of summer;the content of inert mercury is the highest in the early and late stages of decomposition.At this time,the mercury in the litter is relatively stable,and the migration,toxicity and ecological risk of mercury are also low.5.On the land,litter microorganisms C and microbes N showed an increasing trend with decomposing,and the contents of microorganisms C and N were higher in the litter of coniferous and broad-leaved mixed forest.Higher microbial C accumulation in summer and early autumn,low values also occur in winter and early spring.There was a very good correlation between the changes in the contents of mercury and methylmercury in microorganisms C and N(P < 0.01).Different from the change of methylmercury in litter on land,the microbes C and N in the litter in the water were also less changed than the litter in the land(P>0.05).