Accumulation Characteristics And Processes Of Heavy Metals In Soil-vegetation System Along The Important Roads In The Tibet Plateau

Transportation has a significant effect on the accumulation of heavy metals(HMs)in soil-plant system along the roads,which induces potential ecological environmental risks.Understanding the accumulation process and the accumulation characteristics of HMs in soil-plant system can provide some key information for guiding the traffic construction.The Tibet Plateau(TP),known as the"World Roof"or the"Third Pole"with unique geographical environment and climate characteristics,is an important ecological barrier for China or even Asia.The TP is located in an alpine region,where there are few industrial activities and low population density,and the environmental quality was less affected by industrial activities for a long time.Historically,the main source of HMs in the surface ecosystem depended on long-distance transport and deposition under the action of atmospheric circulation.However,in recent years,with the continuous implementation of China's western development strategy,road construction in the TP has a great development,leading to frequent passenger and freight traffic in the surrounding areas.Transportation is believed to be an important source of HMs in the surface soil of the TP,which has gradually drawed more attentions.The phenomenon of elevated HM accumulation in soil and vegetation along urban roads has been widely found.However,the TP has a wide area and large elevation gradient,which results in a significant difference in local terrain,climate,vegetation and other environmental factors,and the traffic roads usually passs through the complex and changeable three-dimensional habitats.Until now,a systematic understanding of the distribution characteristic,accumulation process,risk level and influencing factors of HMs in soil and plants around some important roads in the TP was deficient.In addition,the source apportionment based on the isotope tracer technology is also absent.This hinders our accurate assessment of environmental risks induced by road construction and traffic activities in the TP.Therefore,in this study,the different soil layers and plants within 100 m ranges along the TP were sampled to measue the HM concentrations or natural isotopic compositions.We then demostrate the distribution characteristic,accumulation process,risk level and influencing factors of HMs in soil and plants around two important roads in the TP.Meanwhile,we further reveal the source characteristics of HMs in soils covered with different vegetation along the roads.The main results were as follows:(1)The rapid development of transportation in the TP may increase the accumulation of HMs in soil and cause harm to wildlife and human health.Here,we comprehensively assessed the HM(e.g.,Hg,Zn,As,Cd,Pb,Cu,Ag and Ni)distribution in the two soil layers(0-5 cm and15-20 cm)located at 5 m,50 m,and 100 m away from the edge of two major roads among the different vegetation covers and climatic conditions in the TP.Compared with the background values of heavy metal concentrations in the whole TP(Table 1),the average concentrations of Hg(49 ng g^-1),Zn(100.3?g g^-1),As(21.6?g g^-1),Cd(0.19?g g^-1)and Pb(32.9?g g^-1)in the surface soils(0-5 cm)are 1.1-2.3 times higher,while the average concentrations of Ni(39.4?g g^-1),Cu(22?g g^-1)and Ag(0.12?g g^-1)are comparable.The average concentrations of Ba(341?g g^-1),Tl(0.57?g g^-1),Rb(35?g g^-1),and Sr(55?g g^-1)are significantly lower than the background values by 1.2-4.2 times.For the deep soil,the average concentrations of most heavy metals such as Hg,Zn,As,Cd,Pb,Cu,Ag and Ni are significantly higher than the background values by 1.1-1.7 times Results showed that most of HMs in soils of different distance to the major road display an insignificant difference.The Nemero Synthesis indexes which represent the risk of pollution for these regions almost range 1 to 2(low pollution risk),except 12.7(extreme pollution risk)at one site.These indicate the limited impacts from the traffic activities for the whole region,but at some specific sites with the elevated traffic pollution.The statistical analysis suggests the geogenic weathering processes,climate,terrain and vegetation play more important roles in shaping HM distribution along the roadd in the TP.Due to the forest and climatic factors induce the higher atmospheric depositions and soil organic matters,the soil covered with forest at the altitude of 3700-4100 m has the highest Hg accumulation than other vegetation-covered areas.(2)As the primary components of the terrestrial ecosystem,plant plays an important role in the biogeochemical cycle of HMs.Here,the distribution and accumulation characteristics of HMs in the leaves of herbaceous plants,broad-leaved plants,conifers and moss/lichens along the main traffic roads in the TP were comprehensively evaluated.In the four plant leaves,the average content of Hg is 32.22 ng g^-1,Cr is 25.52?g g^-1,Ni is 6.96?g g^-1,Zn is 35.51?g g^-1,As is 1.93?g g^-1,Cd is 0.10?g g^-1,Pb is 3.53?g g^-1and Cu is 11.45?g g^-1.The results show that traffic activities significantly affect the accumulation of Cu and Pb in herb leaves and broad leaves,but had no significant effect on the accumulation of HMs in needles/lichens,which is different from the law that soil HMs follow.Based on the statistical analysis(PCA),it is concluded that topography,climate and vegetation succession are the important factors controlling the spatial distribution of HMs in the plant leaves.Especially,at the higher altitude,atmospheric deposition directly enhanced the accumulation of HMs in the plant leaves;however,at the lower altitudes,the accumulation of HMs was mainly controlled by vegetation succession.In addition,the As,Ni,Pb,Zn,Cu and Cr in plant leaves mainly come from anthropogenic emissions such as soil weathering process and transportation activities,while Hg and Cd mainly come from atmospheric deposition caused by anthropogenic emissions.(3)The vegetation covers have significant effects on the fate of HMs in the soils,and the isotope tracer technique of Hg can perfectly understand this peroccess.Then,the isotope compositions of Hg in soils covered with four vegetation(forest,meadow,shrub-steppe and steppe)at different elevations are measured in this study.The results show that atmospheric Hg deposition is the main source of Hg in the TP around the roads,whereas the contribution of geological sources is limited.The vegetation cover in the TP can significantly affect the pathway of atmospheric Hg deposition.The more negative?²⁰²Hg(-0.84±0.48‰)in the forested surface soil compared with that in non-forested soil(-0.84±0.48‰)suggests that the Hg accumulation is mainly affected by litters.The significant positive correlation between?²⁰²Hg and Hg concentration in steppe soil indicates that the direct deposition of Hg(0)is the dominant pathway for atmospheric Hg entering the soil.In additon,in the non-forested area of eastern TP with higher precipitation,the wet deposition of Hg(?)has a more significant effect on soil Hg accumulation than the direct deposition of Hg(0).