Quantifying Mercury Distribution And Source Contribution In Surface Soil Of Qinghai-Tibetan Plateau Using Mercury Isotopes

Mercury（Hg）is a global pollutant that can be released through anthropogenic activities or natural processes,then transported through atmospheric circulation and deposited into terrestrial ecosystems.As one of the largest active reservoirs of Hg in terrestrial ecosystems,soil stores at least 90%of Hg in terrestrial ecosystems.Hg deposited in soil can undergo a series of migration and transformation processes.On the one hand,Hg²⁺in soil can be reduced to Hg⁰ and re-emission into the atmosphere,thus increasing atmospheric Hg load;On the other hand,Hg²⁺in soil can be converted into bio-cumulative and neurotoxic methylmercury（Me Hg）and pose a potential health risk through the food chain.Therefore,understanding the spatial distribution and source contribution in soil is of great significance for quantifying the global ecological risks of Hg and assessing the potential risks of Hg exposure to humans and wildlife.Known as the"third pole"in the world,the Qinghai-Tibetan Plateau（QTP）,located in the southwest of China with an average altitude of more than 4,000 m,has a relatively isolated and fragile alpine ecosystem.The QTP is extremely sensitive to environmental and climate change,and its temperature is rising 2-3 times faster than the global average.As the"water tower of Asia".The QTP is the source of several major Asian rivers that affect the lives and health of more than 1.5 billion people.However,unlike the polar regions,the QTP is located in the mid-latitudes,adjacent to the more polluted parts of the world,such as South,East and Southeast Asia.Long-range transport and atmospheric deposition of gaseous Hg result in a significant accumulation of Hg in the Qinghai-Tibetan Plateau（QTP）.However,there are significant knowledge gaps in understanding the spatial distribution and source contribution of Hg in the surface soil of the QTP and the factors influencing the Hg accumulation.In this study,we comprehensively investigated Hg concentrations and isotopic signatures in the QTP to address these knowledge gaps.From 2018 to 2020,we collected surface soil samples in different areas of the QTP,combined with Hg stable isotopes and data published in previous studies,clarified the spatial distribution of Hg in surface soil of the QTP and its influencing factors,and quantitatively analyzed the sources of Hg in surface soil,and obtained the following understandings and conclusions:（1）The average Hg concentration in the surface soil of the QTP was 31.8±34.9 ng g^-1,which was significantly higher than that in the deep soil.Results show that average Hg concentration in the surface soil ranks as:forest(53.9±36.9 ng g^-1)>meadow(30.7±14.3 ng g^-1)>steppe(24.5±16.1 ng g^-1)>shrub(21.0±11.6 ng g^-1).In addition,The Hg concentrations are significantly lower than those found in low-elevation regions for similar biomes due to the low atmospheric Hg deposition.（2）Based on the correlation analysis,it was found that the Hg concentration in 0-10 cm soil was significantly correlated with SOC,precipitation and vegetation biomass.Furthermore,based on the structural equation model,it was found that the maximum direct effect on surface soil Hg concentration（β=0.56）was exhibited.Combined with the direct effect of latitude and longitude on precipitation and vegetation biomass,this result indicates that the spatial distribution of surface soil Hg concentration in QTP can be indirectly controlled by the changes of topography and climate caused by latitude and longitude through affecting precipitation,vegetation biomass and SOC.（3）The soilδ²⁰²Hg for forest shows the most negative values（mean=-1.44±0.51‰）,followed by shrub（-1.03±0.39‰）,meadow（-0.92±0.34‰）and then steppe（-0.91±0.69‰）.TheΔ¹⁹⁹Hg values in forest soil are most negative（-0.14±0.14‰）,similar to those found in total gaseous Hg⁰ in air（-0.20±0.08‰）and litter of the QTP forests（-0.18±0.09‰）.This is strong evidence that forest soil is of atmospheric origin.Comparable values are found among the samples of meadow（0.05±0.11‰）,shrub（0.00±0.13‰）and steppe（-0.05±0.14‰）.TheΔ²⁰⁰Hg values in all soil samples are close to 0.00‰.（4）The results of Hg isotopic mixing modeling highlight the dominant role of atmospheric Hg⁰ input in controlling Hg accumulation in surface soil.The highest contribution of atmospheric Hg0 input is in forest（62±12%）,followed by shrub（51±10%）,steppe（50±13%）and then meadow（45±11%）.The contribution of atmospheric Hg2+input in forest,meadow,shrub and steppe is 10±4%,18±6%,14±6%,and 18±8%,respectively.（5）Considering that the predominant role of vegetation in controlling Hg accumulation,the spatial distribution of Hg storage in surface soil of 0-10 cm depth is developed using a cokriging spatial interpolation modeling based on the relation between the soil Hg concentration and vegetation biomass production（i.e.,NPP）.The Hg pool in 0-10 cm surface soil over the QTP is estimated with 8200±3292 Mg.