| Mercury（Hg,mercury）is a heavy metal element that can participate in global cycles.It is not involved in the metabolic process of life,but it can be concentrated in the organism,with many characteristics such as persistence,easy migration,inter-media,bioenrichment and biological toxicity.After the natural source and human source emissions enter the atmosphere,mercury can be endlessly migrated between the atmosphere and the surfaces of the earth in the global scale.The accumulation of mercury in the global ecosystem and the accumulation of mercury in organisms and the characteristics and laws of its migration have been the hot spots and frontiers of mercury science（Hg science）.The mercury cycle of the aquatic system has been studied,and the study of mercury circulation in forests,wetlands and even frozen circles has been widely reported,and the research on mercury in grassland ecosystems is scarce.At present,there are still the following problems concerning the material process of mercury（Hg0）entering grassland ecosystem along the"air-vegetation-soil"path,that is,in the process of vegetation evolution and succession,the mercury pool and mercury flux in the ecosystem and their dynamic processes and impacting factors.Specifically,it includes the inventory and dynamic characteristics of mercury in atmospheric,vegetation-soil factors,and the migration process of mercury between factor banks.That is,the law of mercury accumulation in plants through atmospheric exposure,the fixation of mercury by vegetation,the return characteristics of mercury in the process of litter return to soil and litter decomposition,the characteristics of mercury release from soil and the effects of grassland use and environmental factors are still unclear.In particular,the source/sink of mercury in the atmosphere-vegetation-soil of grassland ecosystem and the quantitative relationship between mercury are still not well understood.In the Songnen Grassland,the Leymus chinensis community,Leymus chinensis+forbs community,Puccinellia tenuiflora and Artemisia anethifolia and alkaline-saline spots in the Leymus chinensis grassland were selected as the research objects,and a series of field positioning observation and control experiments were designed.Optimized dust collection method and improved dynamic flux chamber were used.DFC combined with high resolution mercury meter（LUMEX RA915+UMA）method,dynamic flux bag method,litterfall method and litter decomposition control experiment were used to systematically analyze the atmospheric,vegetation and soil mercury stocks and their seasonal dynamics in Leymus chinensis grassland.The characteristics of mercury migration fluxes along the"air-vegetation-soil"banks and the influences of grassland use and environmental factors were clarified.The source/sink relationship between the mercury stocks and fluxes in grassland was clarified,and the turnover process and mechanism were discussed.The main results and conclusions of this study are as follows:Firstly,the diurnal pattern of atmospheric mercury inventory in grassland was"peak-valley alternating",and the relationship of seasonal concentration was"spring>autumn>summer".The annual atmospheric mercury storage（GEM）（regional environmental reference value）was 15.5±0.8 ng·m-3（2018）and 16.1±0.9 ng·m-3（2019）,respectively.The highest and lowest concentrations of atmospheric mercury occur around 14:00 and 0:00～3:00 every day,respectively.The changes of endogenous and meteorological factors of natural sources and underlying surface characteristics were the main factors affecting the dynamic changes of mercury reservoir in grassland.The atmospheric GEM mercury pool（concentration）is time-scale dependent.Mercury concentrations vary gradually from day to day and month to month.The variation of GEM concentration showed a distribution trend of"low day and high night"within each hour.The two peaks appear in two periods before and after sunrise and sunset.The variation of atmospheric mercury concentration showed obvious environmental sensitivity.The meteorological factors that change obviously with the change of day and night may be an important reason for the variation characteristics of GEM concentration.Atmospheric mercury reservoir compensates grassland with strong mercury output by dry sedimentation.The dry deposition flux intensity of mercury in non-growing season reached 0.212μg·m-2·mon-1.The maximum value of dry deposition and Hg concentration in dry deposition occurred in spring（April）,while the maximum value of Hg dry deposition flux occurred in winter（February）.The more land cover,the stronger its ability to capture atmospheric dry deposition.The intensity of mercury flux obtained by various vegetation communities was as follows:Leymus chinensis+forbs>Leymus chinensis>Puccinellia tenuiflora and Artemisia anethifoli>alkali spot.The strong dry deposition may be attributed to the combined effects of dust source,atmospheric movement,surface physical properties,grassland cover status and meteorological factors,which lead to the different time variation characteristics of atmospheric mercury dry deposition with the difference of grassland cover and seasonal changes.Secondly,soil mercury stocks decreased significantly with vegetation succession,soil profile and seasonal changes.The average mercury inventory in grassland soil was 7.6±0.5 mg·m-3.Among them,soil mercury storage showed a decreasing trend with vegetation degradation:Leymus chinensis>Leymus chinensis+forbs>Puccinellia tenuiflora and Artemisia anethifoli>alkali spot.Except for alkali spot（summer）,the maximum mercury storage in covered soil appeared in spring and showed a trend of decreasing gradually with seasonal changes.The mercury content in soil profile layer showed obvious vertical decreasing distribution in space.In addition,soil p H and organic matter have strong inhibition effect on mercury.The higher the soil p H value,the less the soil mercury storage.There was a positive correlation between soil organic matter content and soil mercury content distribution.At the end of growth period,mercury pools of different vegetation types showed consistent mercury sink capacity.The mean value of mercury reservoir was894.34±47.0 ng·m-2.The relationship of mercury pool among different vegetation types was:Leymus chinensis(933.60 ng·m-2)>Puccinellia tenuiflora and Artemisia anethifoli(921.80 ng·m-2)>Leymus chinensis+forbs(827.63 ng·m-2).The ratio of mercury content between vegetation stand dead body and ambient air tends to be consistent.However,the dynamic trend of mercury storage in different plant communities was obviously different during the growing season.The highest value of Leymus chinensis community was found in June,and the highest value of Leymus chinensis+forbs and Puccinellia tenuiflora and Artemisia anethifoli was found in July.The longer the stem age and leaf age,the greater the mercury storage.The opposite is true of Puccinellia tenuiflora and Artemisia anethifoli plants in September.Vegetation is the net source of soil mercury pool.There was a strong correlation between vegetation mercury storage and mercury accumulation capacity of biomass and species in different plant communities.Mercury migrates and transforms continuously among the three reservoir interfaces of atmosphere,soil and vegetation.With the change of seasons,there were source/sink reversal characteristics of mercury in vegetation-soil system and atmospheric reservoirs.The vegetation mercury reservoir was eventually attributed to the soil mercury reservoir in the form of standing blight.Thirdly,the average background concentration of gaseous mercury（GEM）was15.5±6.4 ng·m-3.Grassland is significantly higher than forest,farmland,wetland and other reported ecosystems,and lower than urban ecosystems.In addition,non-degraded grassland was higher than degraded grassland,mowing grassland was higher than untreated grassland,and July was significantly lower than other months.During the growing season,there is an obvious mercury release process from grassland to atmosphere.The presence of vegetation inhibited the release of mercury from soil,but could not change its direction.In the grassland growing season,the surface showed a net release of mercury(6.16±13.1 ng·m-2·h-1).The diurnal variation of mercury release in grassland showed an obvious peak-valley alternating trend.The mean value of mercury exchange fluxes and the ratio of positive（release）and negative（deposition）of mercury exchange fluxes showed obvious seasonal trends.The higher the degree of vegetation degradation,the higher the value of mercury exchange flux.The use of grassland（mowing,etc.）further increased the level of mercury exchange flux between surface and atmosphere.Solar radiation and air temperature play important roles in mercury exchange fluxes.Linear correlation analysis showed that solar radiation was the primary environmental factor in May,June and September,while air temperature was the primary environmental factor in July and August.Random forest analysis showed that solar radiation was the most important environmental factor for May-June and August-September,and air humidity was the most important environmental factor for July,followed by air temperature and solar radiation.Structural equation model analysis shows that solar radiation is the main environmental factor in the growing season.Fourthly,there is source/sink reversal process between grassland plants and atmosphere.The mercury exchange fluxes between Leymus chinensis and Setaria and atmosphere were net sinks of atmospheric mercury(Leymus chinensis-1.39ng·m-2·h-1,Setaria-1.46 ng·m-2·h-1)during the vegetative reproductive period.In the reproductive period,the net source was 0.34 ng·m-2·h-1of Leymus chinensis and 1.18ng·m-2·h-1of Setaria.The mercury flux was high during the day and low at night,and the peak appeared around noon.Setaria was more likely to participate in plant/atmospheric mercury exchange than Leymus chinensis.The mercury fluxes of Leymus chinensis and Setaria were basically the same when the two plants absorbed mercury from the atmosphere.With the increase of Setaria quantity and biomass ratio,the function of regional grassland as atmospheric mercury sink may be weakened.Solar radiation was the decisive factor of mercury flux in both plants during the experimental period.In addition,atmospheric mercury concentration was the main factor affecting the mercury flux during the vegetative reproductive period of the two plants,and atmospheric humidity was the main factor affecting the reproductive period of the two plants.At last,grassland vegetation is a net sink of atmospheric mercury.Vegetation showed increasing atmospheric mercury trapping capacity year by year.During the four consecutive years from 2018 to 2021,the average stock of vegetation mercury was 913.21 ng·m-2,and the inter-annual growth rates were 1.28%,3.99%and 1.66%.The mercury banks of different types of vegetation also showed the same increasing trend year by year,but the relationship of mercury occurrence among different types of vegetation in the same year was as follows:Leymus chinensis>Puccinellia tenuiflora and Artemisia anethifoli>Leymus chinensis+forbs.The grassland vegetation is the transfer reservoir of mercury between atmosphere and soil along the air-vegetation-soil pathway.The mercury flux between vegetation and soil is finally completed in the form of plant litter carrying mercury into the soil.Vegetation acts as a transfer reservoir for mercury migration between the atmosphere and soil.The decomposition trend of vegetation litter after entering the surface is"first fast and then slow".More mercury gets to the top of the soil,but it escapes into the atmosphere all the time.Soil is the net sink of mercury in grassland vegetation.The decomposition characteristics of litters of different vegetation types were similar and their compensation effects on soil mercury pools were similar.The relationship between decomposition speed and slow of grassland plants was:Puccinellia tenuiflora and Artemisia anethifoli>Leymus chinensis+forbs>Leymus chinensis.The long decomposition process of the vertical body carrying mercury into the soil surface may be the main reason for the formation of the mercury reservoir in the soil profile layer,and the main reason for the active mercury reservoir in the soil surface layer（0～5cm）.Based on the above experimental results,this study obtained an in-depth understanding and understanding of mercury stocks and their dynamics,mercury flux characteristics and action factors,and the source-sink relationship among the repositories in the atmospheric and vegetation-soil systems of grassland ecosystems.Atmospheric mercury reservoir exported mercury into grassland ecosystem along the"air-vegetation-soil"path,and formed a reservoir in various elements,with atmospheric(15.5～16.1 ng·m-3),vegetation(894.34 ng·m-2)and soil(7.6 mg·m-3)stocks.Atmospheric,vegetation-soil and soil-atmosphere mercury transport fluxes between reservoirs were distinct,which were affected by community changes,grassland use and environmental factors,and showed different seasonal characteristics.Grassland use（mowing,etc.）did not change the net release process and characteristics of surface（soil）to the atmosphere,and its inhibition effect was limited.However,vegetation still acts as a net sink of the atmosphere and becomes a net source of mercury in soil by means of standing dead body.Vegetation played the role of"temporary reservoir"and"regulator"during mercury migration in grassland.Mercury migrates along the atmospheric,vegetation-soil mercury reservoirs in a cyclic manner,and there is an inventory measurement relationship of 10 ng→103ng→107ng among atmosphere→vegetation→soil.In this paper,the occurrence level of mercury in various elements of atmosphere,vegetation and soil in grassland ecosystem（regional environmental reference value）was proposed from the perspective of pool stock,and the characteristics and action factors of mercury exchange flux（flux）were described along the"air-vegetation-soil"banks.In particular,it was found that the utilization of grassland did not change the trend and characteristics of surface mercury release.In addition,grassland plants released mercury into the atmosphere,but it was still a net sink in the atmosphere,and different plant communities had nearly the same mercury sink capacity at the dead body stage.The bio-carbon stability of litter inhibits the intense physical and chemical exchange of mercury between soil and atmosphere.These findings are helpful for re-understanding the"vegetation-soil"system of grassland and the occurrence of atmospheric factor banks and the process of mercury flux between them,explaining the mechanism and mechanism of mercury migration in grassland along atmospheric,vegetation-soil factors,and evaluating the role and function of vegetation degradation trend（reverse succeeding）in global and regional mercury circulation.It provides sufficient basis and scientific basis for grassland utilization and management. |
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