| As an important landscape unit connecting atmosphere and aquatic environment,forest ecosystem is one of the most active ecosystems for mercury biogeochemical cycle.After long-distance transport,atmospheric mercury deposition into remote ecosystems,which may cause mercury accumulation and bring health risks to neighboring ecosystems.Subalpine forest watershed may become an important accumulation area of atmospheric mercury due to the influence of diversified vegetation,climate,topography,hydrology and geology,but research on mercury biogeochemical cycle in those places has not pay more attention.To understand the spatio-temporal pattern and distribution characteristics of atmospheric mercury deposition in subalpine watersheds,research means of forest hydrology were used to research long time atmospheric mercury deposition in subalpine watershed in this paper,through sampling precipitation,throughfall,stemflow,surface water(stream),and litterfall in different space.By analyzing composition characteristics and spatio-temporal variation of mercury deposition,to find out the spatio-temporal rule of atmospheric mercury deposition in subalpine watershed.So as to we can understand the role of subalpine forests in mercury biogeochemical cycle.The main results are as follows:(1)The average concentrations of THg and MeHg in subalpine forest soil were110.76±11.05 ng/g and 0.79±0.18 ng/g,respectively,while the concentrations of THg and MeHg in humus layer were 115.59±7.76 ng/g,0.52±0.06 ng/g.There was no significant spatial difference in THg concentration in soil and humus layer(p>0.05),but MeHg in soil under coniferous forest at low altitude was significantly higher than that in other plots(p=0.047).The THg concentration in the stream water was 5.00±1.14ng/L and the MeHg concentration was 0.07±0.01ng/L,there was no significant seasonal change in the mercury content in the stream water(p>0.05).The concentrations of THg and MeHg in natural precipitation were 2.91±0.54 ng/L and0.04±0.01ng/L,respectively.(2)In the atmospheric mercury input pathway of subalpine forest,the concentrations of THg and MeHg in throughfall were 14.90±1.17 ng/L and 0.10±0.01 ng/L,both of which had obvious seasonal characteristics(p?0.01).The THg concentration of the stemflow was 23.30±2.08 ng/L with obvious seasonal variation(p=0.02),and the MeHg concentration was 0.59±0.10 ng/L with no obvious seasonal variation(p=0.29).The concentrations of THg and MeHg in litterfall were 58.98±4.44 ng/g and 0.46±0.03ng/g,both of them had significant time variation characteristics(p?0.001).The deposition flux of THg is 23.80?g/m~2/yr,and the deposition flux of MeHg is 0.19?g/m~2/yr.The deposition fluxes of THg in spring,summer,autumn and winter were11.57?g/m~2/qtr?4.91?g/m~2/qtr?7.11?g/m~2/qtr and 0.32?g/m~2/qtr,the deposition fluxes of MeHg were 0.11?g/m~2/qtr?0.03?g/m~2/qtr?0.05?g/m~2/qtr and 0.002?g/m~2/qtr.The deposition fluxes of THg and MeHg have significant seasonal variation.(3)The spatial characteristics of atmospheric mercury deposition are obvious.The THg concentration(18.77±2.69 ng/L)(p=0.017)and THg deposition flux(19.12±1.35?g/m~2/yr)in coniferous forest were significantly higher than those in subalpine shrub and coniferous and broad-leaved mixed forest(p=0.011),but there was no spatial difference in MeHg concentration and deposition flux,the deposition fluxes of THg(26.16±2.15?g/m~2/yr)(p=0.001)and MeHg(0.21±0.02?g/m~2/yr)(p=0.005)of litterfall in coniferous forest were higher than those of other stand types.THg deposition fluxes of subalpine shrubs,coniferous and broad-leaved mixed forests,high-altitude coniferous forests,and low-altitude coniferous forests were 22.88?g/m~2/yr,21.97?g/m~2/yr,39.50?g/m~2/yr and 43.15?g/m~2/yr;MeHg deposition fluxes were 0.24?g/m~2/yr,0.19?g/m~2/yr,0.29?g/m~2/yr,and 0.33?g/m~2/yr,respectively.Main conclusion:There is a slight enrichment of mercury in the subalpine forest soil of Jiuzhaigou,which may reflect the characteristics of atmospheric mercury deposition in the subalpine region.The MeHg content in the humus layer and mineral soil layer under the forest is relatively higher than that in the forest in other regions.The forest vegetation types and the rich humus layer and bryophyte in the lower layer of the forest may provide a more convenient microenvironment for mercury methylation,which may be the reason for the increase of MeHg content in humus layer and soil layer.The atmospheric mercury deposition of subalpine forest has obvious time variation characteristics,and the time trend of THg concentration and MeHg concentration in dry and wet deposition is higher in warm season(summer and autumn)than in cold season(winter and spring).The deposition flux also has seasonal characteristics,whether THg deposition flux or MeHg deposition flux is the highest in spring,followed by autumn,summer and winter.The atmospheric mercury deposition in subalpine forests has the characteristics of spatial variation.The THg and MeHg concentrations of litterfall and throughfall,and the input and contribution rates of litterfall and throughfall to THg and MeHg in coniferous forest plots were always higher than those of coniferous and broad-leaved mixed and shrub plots.The spatial characteristics of THg input level and input flux were obvious,while the spatial characteristics of MeHg were weak.Mercury deposition fluxes also showed that needles were higher than mixed forests and shrubs,and decreased with the increase of altitude in the spatial scale of the study,which was attributed to the strong ability of needles to capture atmospheric mercury. |
PDF Full Text Request