Transport And Transformation Of Mercury In The Subalpine Watershed At Jinzhaigou Nature Reserve

Mercury is a pollutant that can be transported over long distances in the atmosphere and deposited into surface ecosystem by dry and wet deposition.As is a landscape unit connecting atmospheric and aquatic ecosystems,forest ecosystem is one of the most active places for biogeochemical cycle of mercury.As a typical forest ecosystem,the role of subalpine forest is not clear for the global atmospheric mercury cycle.To understand the dynamics of mercury cycle in subalpine ecosystems,it is a key link to study the input,migration and transformation of mercury in subalpine forest ecosystems.In this paper,taking forest hydrological process as a centerline,using research means of forest hydrology,spatial-temporal changes of total mercury（THg）and methylmercury（MeHg）in subalpine forest were investigated with throughfall,stemflow,litterfall,forest floor leachate,soil leachate,and surface water（stream）.Through the analysis of hydrochemical characteristics and combined with real-time precipitation observation data,the migration and transformation characteristics and influencing factors of mercury in subalpine forests were studied to provide a scientific basis for regional or global mercury cycle research and simulation.Main results are as follows:（1）The average concentrations of THg and MeHg in precipitation of subalpine forest ecosystem were 2.91±0.54 ng·L^-1 and 0.04±0.01 ng·L^-1,respectively.The contents of THg(12.70±1.17 ng·L^-1)and MeHg(0.10±0.01 ng·L^-1)in throughfall were higher than those in precipitation.The levels of THg(23.30±2.08 ng·L^-1)and MeHg(0.59±0.10 ng·L^-1)in stemflow were significantly higher than those in throughfall（p<0.003）.The average concentrations of THg and MeHg in litterfall were 58.98±4.44ng·g^-1 and 0.46±0.03 ng·g^-1,respectively.There was obvious seasonal variation in atmospheric mercury deposition,both THg and MeHg contens in throughfall were significantly higher in summer than those in winter and spring（p<0.02）,while THg level in stemflow in summer was significantly higher than that in spring（p=0.02）,there was no significant seasonal difference for MeHg concentrations.（2）The THg concentrationin forest floor leachate was lower than that of throughfall(12.70±1.17 ng·L^-1),but MeHg(0.45±0.06 ng·L^-1)in this substance was higher than that of throughfall(0.10±0.01 ng·L^-1).The average concentrations of THg and MeHg in soil leachate were 14.62±2.25 ng·L^-1 and 0.48±0.04 ng·L^-1,respectively.When compared with forest floor leachate and soil leachate,the concentrations of THg(5.00±1.14 ng·L^-1)and MeHg(0.07±0.01 ng·L^-1)in stream decreased significantly.Seasonal climate change had little effect on the dynamics of mercury in soil and stream.The contents of THg and MeHg in forest floor leachate showed an obvious seasonal variation（p<0.03）,which indicated warm season（summer and autumn）>cold season（winter and spring）.However,the concentrations of THg and MeHg in soil leachate and stream had no seasonal patterns.MeHg levels in stemflow,throughfall,forest floor leachate and soil leachate were significantly positively correlated with DOC（p<0.011）.（3）The THg deposition flux in subalpine forests was 23.91μg·m^-2·yr^-1,in whichcontribution of dry deposition(21.3μg·m^-2·yr^-1)reached 89.1%,while wet deposition(0.16μg·m^-2·yr^-1)contributed only 10.9%.The THg output fluxes from forest floor leachate and soil leachate were 2.13μg·m^-2·yr^-1 and 3.08μg·m^-2·yr^-1,respectively,accounting for only 21.8%of the total input flux,while the MeHg output flux was 0.35μg·m^-2·yr^-1 and 0.24μg·m^-2·yr^-1,respectively,and the outputin two channels was three times higher than total MeHg input.（4）The MeHg/THg ratio of precipitation is 1.35±0.07%.After entering the canopy,the proportion of MeHg/THg changed in different ways,such as throughfall（1.06±0.19%）,stemflow（3.00±0.54%）and litterfall（0.84±0.04%）.Among the output components,the proportion of MeHg/THg in forest floor leachate（4.83±0.60%）and soil leachate（3.27±0.40%）was higher than that in input pathway.（5）There was no correlation between MeHg and THg content in subalpine forest system（p=0.373）.However,MeHg was positively correlated with DOC,NO₃^-,K⁺,Ca²⁺,Mg²⁺and PO₄^3-（p=0.01）,and negatively correlated with the content of SO₄^2-（p=0.075）.The correlation between MeHg/THg and these chemical factors was similar to that of MeHg.Stepwise multiple regression analysis showed that only DOC,NO₃^-,Ca²⁺and PO₄^3-had a substantial impact on the level of MeHg,among which DOC had the greatest influence,and the total explanation of the four factors for the change of MeHg concentration was 78.3%.On the other hand,only DOC,NO₃^-and Ca²⁺have the greatest contribution to MeHg/THg,which can explain 46.3%of the MeHg/THg change,and the total explanation degree of the three factors is 62.0%.Main conclusions are as follows:（1）The dry and wet deposition of atmospheric mercury in Jiuzhaigou subalpine forest ecosystem has obvious seasonal characteristics,and there is a large deposition of mercury in the growing season（warm season）.The forest ecosystem has a significant interception effect on atmospheric mercury deposition.Some characteristics of subalpine forests may create certain conditions for the methylation of mercury,resulting in a significant increase of methylmercury production.Such ecosystems may become important sources of methylmercury in the mercury cycle.（2）The contribution of dry deposition of atmospheric mercury to mercury input in subalpine forest ecosystem is much greater than that of wet deposition,and litterfall is obviously dominant in dry deposition.The subalpine forest has a significant role of mercury sink,which shows that the annual input flux of THg is larger than the output.At the same time,subalpine forest is an important MeHg source of watershed system.DOC is a key geochemical factor that affects and controls the environmental behavior of mercury.（3）In the subalpine forest ecosystem,the forest canopy and the organic layer under the forest and the soil are the places for the transformation of mercury speciation.The forest trunk of epiphytic moss and lichen is a factor that can not be ignored.DOC is the most important factor affecting mercury methylation in subalpine forests,and the relative importance of factors is DOC,NO₃^-and Ca²⁺.