| Known as the"Asian Water Tower",the Qinghai-Tibet Plateau（QTP）is the birthplace of important rivers such as the Yangtze River,the Yellow River,the Lancang River and the Nu River.As a typical ecosystem in the QTP,alpine grassland plays an important role in the development of animal husbandry,climate regulation and water conservation.However,due to the characteristics of bad climate conditions,short plant growth cycle and barren soil,alpine grassland is very sensitive to environmental change.At present,the local industrial and agricultural activities in QTP and the heavy metals emitted by human activities in Southeast Asia and South Asia have caused a certain degree of heavy metal pollution to the ecosystem of QTP through atmospheric deposition.In addition,the increasing exchanges between the QTP and the mainland make the frequent transportation activities which have gradually become a new potential source of heavy metals in alpine grassland,attracting the attention of many scholars.Because of the extremely complex climate,terrain and vegetation cover,alpine grassland may have extremely complex response characteristics to the input of exogenous heavy metals.The current research still lacks a comprehensive understanding of the distribution pattern,accumulation process,pollution degree,influencing factors and main sources of heavy metals in the grassland along the transportation line of the QTP.Therefore,this study analyzes the distribution pattern,accumulation characteristics and pollution degree of heavy metals in soil and plants under the influence of different environmental factors,and reveals the potential sources of corresponding heavy metals.The results are as follows:The contents of heavy metals in the topsoil（0-5cm depth）of alpine grassland are Hg(25.95±13.72 ng g-1);Cr(69.76±35.73μg g-1)、Co(8.17±1.60μg g-1)、Ni(24.13±6.11μg g-1)、Cu(16.38±2.74μg g-1)、Zn(67.68±25.34μg g-1)、As(16.69±5.27μg g-1)、Cd(0.17±0.12μg g-1)and Pb(19.92±6.24μg g-1),respectively.In addition,controlled by various environmental factors,Hg and Zn are mainly accumulated in the surface soil,Cr and Ni are accumulated in the deep soil（>25cm）,and there is no significant difference in the contents of CO,Pb,as,Cd and Cu between the surface and deep soil.Unlike other studies,the content of all roadside metals does not show a significant negative correlation with the roadside distance,indicating a relatively limited impact of traffic activities.Soil weathering,climate（rainfall）and vegetation cover play a more important role in the accumulation process and spatial distribution of soil heavy metals in alpine grassland.Compared with alpine steppe,the higher biomass of alpine meadow promotes the litter input and return of heavy metals,enhancing the combination of organic matter and heavy metals,and the release of mineral heavy metals.Although there is a limited impact of traffic activities in the study area,about 40%of grassland soil is under moderately polluted risk（2≤PN<3）,and some sampling areas are under high pollution risk（Site of S1&S19,3≤PN<6）.Finally,the results of principal component analysis and Pearson correlation analysis show that soil Hg comes from the dry and wet deposition of atmospheric Hg.The sources of Cr,Co,Ni,As,Cu,Zn,Cd and Pb are related to the weathering of local parent materials and bedrock.The average contents of heavy metals in alpine grassland plants are Hg(24.2±13.8ng g-1)、Cr(75.6±73.7μg g-1)、Co(2.86±2.43μg g-1)、Ni(33.5±30.3μg g-1)、Cu(11.1±5.2μg g-1)、Zn(40.2±17.0μg g-1)、As(3.78±4.40μg g-1)、Cd(0.13±0.13μg g-1)and Pb(4.59±4.38μg g-1),respectively.In addition,the content of heavy metals except Zn in roots is significantly 1.58-3.69 times higher than that in aboveground parts,indicating that most of the heavy metals in the study area accumulate in plant roots.Vegetation cover type and climate（rainfall）are important factors to control the accumulation and distribution of heavy metals in alpine grassland.The results of bioaccumulation coefficient and translocation factor show that Ni,Cd,Hg and Cr can accumulate well in plant roots,and Zn can accumulate well both in aboveground parts and roots of plants.The discrepancy of physiological characteristics of plants lead to different accumulation characteristics of various heavy metals in plants.In addition,the accumulation of heavy metal elements Cr,Co,Ni,Cu,as,Pb and Hg probably has antagonistic effect with nutrient element P in plant.Zn and Cd are may not affected by other elements and accumulate synergistically in plants. |
