Study On The Ecological Process Of Mercury In Typical Plant Communities Of Urban Forest In Shanghai

Since the Industrial Revolution,urbanization has accelerated mercury（Hg）emissions.As a major source and residence of anthropogenic Hg emission,urban forests may become a sink of Hg,providing favorable conditions for scavenging Hg in the atmosphere.In this study,three typical plant communities of urban forests in Shanghai were taken as the study objects.By monitoring the annual Hg deposition fluxes through precipitation and litterfall in the central urban,suburban and rural areas of Shanghai,where the sources of Hg pollution are significantly different,the total Hg deposition to the urban forests in Shanghai was estimated,and the relative contribution of the two types of deposition to the total Hg deposition was also obtained.In addition,this study also explored the enrichment degree of Hg in urban forest soil and discussed its migration and transformation process combined with the analysis of speciation,which focused on the vertical distribution characteristics of soil mercury.The main findings are as follows.（1）The annual volume-weighted average total mercury（VWM THg）concentration in precipitation in Shanghai was 3.04 ng·L^-1,and the annual wet deposition flux of THg was 2.79μg·m^-2·yr^-1,both of which were generally lower than those in other regions in China and abroad.VWM THg concentrations in precipitation and THg wet deposition fluxes showed obvious seasonal variation:a)with higher concentrations in winter and lower concentrations in summer,b)and with higher fluxes in spring and summer,highest fluxes in summer and lower fluxes in autumn and winter.There were no significant differences in THg concentration in different monitoring areas,while the wet deposition flux showed a significant decrease from urban to suburban（ANOVA test,p=0.04）.There was a significant positive correlation between wet deposition flux of THg and rain amount（r=0.81,p<0.01）,and no significant correlation with THg concentration,indicating that THg flux was primarily controlled by rainfall.THg concentration in precipitation showed a significant positive correlation（p<0.01）with dissolved Ca,Fe,Mg,Mn,Al,K,and S,and also with atmospheric PM_2.5,PM₁₀,and SO₂（p<0.05）.（2）The mean values of THg concentration in litterfall and THg annual deposition flux through litterfall in urban forests of Shanghai were 49.39±18.00 ng·g^-1and 32.92±12.47μg·m^-2·yr^-1,respectively.Average THg concentrations in litterfall at different communities showed the following trend:evergreen deciduous mixed community≈Cinnamomum camphora community>Metasequoia glyptostroboides community（ANOVA test,p<0.05）,which may be due to the longer leaf growth cycle of evergreen tree species.Annual fluxes of THg through litterfall at different communities showed the following trend:evergreen deciduous mixed community>Cinnamomum camphora community≈Metasequoia glyptostroboides community（ANOVA test,p<0.05）,which was consistent with the trend of annual litterfall deposition flux.A significant correlation was observed between biomass flux and litterfall deposition flux of THg（r=0.7357,p<0.01）,indicating that the flux of THg through litterfall in the urban forests of Shanghai was mainly driven by the biomass flux of the litterfall.For the evergreen deciduous mixed community and Cinnamomum camphora community,average THg concentrations in litterfall in different areas showed the following trend:central urban≈suburban>rural（ANOVA test,p<0.05）.However,there was no obvious urban-rural spatial distribution trend of the mean THg concentration in litterfall of the Metasequoia glyptostroboides community.There was no significant difference in annual fluxes of Hg through litterfall among the three areas.（3）Ratios of annual average litterfall deposition flux to annual wet deposition flux of THg in urban forests of Shanghai was 11.80.This ratio was generally consistent with the previous observations in remote forests in China（1.36-13.19）,but much higher than those observed in North America and Europe（0.42-2.57）.Among them,the contribution of litterfall deposition to total Hg deposition reached85.51%,while the contribution of wet deposition was only 7.25%,which indicated that Hg deposition through litterfall played a predominant role in the total Hg deposition budget in urban forests in Shanghai.（4）The THg and methylmercury（Me Hg）concentration of the topsoil（0-2 cm）were lower than those of other cities.Hg speciation analysis showed that the Hg in the topsoil in the three areas was mainly in the form of Hg S,followed by Hgabs1,with an average proportion of 43.56%and 36.80%,respectively.The absolute content analysis showed that the Hg concentration of different forms in the topsoil showed a downward trend from urban to rural,and the concentration of Hgads1 and Hgabs1showed a significant downward trend with the urbanization gradient（p<0.05）.Hgads1concentration in the central urban area(30.42 ng·g^-1)is about 10 times that of the rural area(3.59 ng·g^-1),indicating that the anthropogenic activity-intensive areas not only have higher levels of THg pollution but also have higher bioavailability,which will lead to higher ecological risk.For different community types in the central urban area,the topsoil THg concentration showed great differences:Metasequoia glyptostroboides community>Cinnamomum camphora community≈evergreen deciduous mixed community（ANOVA test,p<0.05）.Hg speciation analysis showed that the proportion of Hg in different forms had no significant difference in the topsoil of the Cinnamomum camphora community and mixed community,while the proportion of Hgads1 in the Metasequoia glyptostroboides community increased significantly（p<0.05）.The overall proportion of Hgads1（with high bioavailability）and Hgabs1（with potential bioavailability）in the topsoil of the Metasequoia glyptostroboides community reached 58.21%,together with the high THg concentration,indicating that the Metasequoia glyptostroboides community in the central urban area will lead to higher ecological risks.（5）There was a significant positive correlation between the THg concentration of the surface layer（0-2 cm）and the deep layer（10-15 cm）in the soil profiles of the three areas（p<0.01）.The higher the THg concentration in the 0-2 cm soil layer,the higher the THg concentration in other layers（2-5,5-10,10-15 cm）,which indicated that mercury may migrate in the vertical direction.The concentrations of THg,Me Hg,and TOC in the soil profiles of the three areas all showed a decreasing trend from the surface layer to the deep layer.The decreasing trend of THg concentration was not obvious,and the concentrations of Me Hg and TOC showed a significant decrease（p<0.05）.For soil profiles in different areas,there was a significant correlation between THg and TOC（r=0.3824,p<0.01）,and there was a significant correlation between Me Hg and TOC and THg(r _TOC=0.7434,r _THg=0.3824,p<0.01).Hg speciation analysis showed that the relative concentration changes of different forms of Hg in the vertical direction of the three regions were consistent:a)the proportions of Hgads1and Hgabs1 showed a decreasing trend with increasing depth;b)the proportion of Hg S did not change significantly in the vertical direction;3)the proportions of Hgads2 and Hgres showed an upward trend with increasing depth.In the vertical direction,mercury may transform into the most stable form.