| Mitiika Wetland National Nature Reserve（MWNNR）has been the first important wetland in Tibet that integrates an internationally important wetland and a national nature reserve.The main river in the region,the Maidizangbo and its tributaries,is the headwater of the Lhasa River,that is the "Mother River" of the Lhasa people.Therefore,the environment quality of the wetland is significant to the ecological security of the entire Lhasa River catchments.Based on-site sample collection,pretreatment,analysis and tested,and data processed,this study took the soil environment of the MWNNR as the main research object and for the first time to evaluate the soil fertility and elements’ background values in the area.Background information on soil elements and possible risk assessment were also carried out.The major findings are as following:（1）The non-parametric test results indicated that the contents of organic matter,CEC,TOC,soil moisture,and soil salinity were related to the soil type,however,p H,contents of available potassium,available phosphorus,ammonia nitrogen were unrelated to soil type.The results demonstrated that the comprehensive soil fertility coefficient（F）in the study area was as following: alpine meadow soils（1.72）> alpine shrub meadow soils（1.66）=frigid desert soils（1.66）>bog soils（1.56）>skeletal soils（1.43）.Except the alpine meadow soil belonged to fertility grade,the other types belonged to the general grade.Overall,Mitika wetland’s soil fertility level is comparable to that of cultivated land or artificial afforestation areas in the other regions of Tibet,despite the fact that the wetland is located in the clod and high altitude northern parts of the Tibetan Plateau.（2）The analysis results of 37 elements detected in the soil od Mitika Wetland shows that.Compared with the regional soil background values,the abundance of Cs and Cd elements in the soil of the study area is higher.The results of the background value of various soil elements showed that,except for the elements whose contents exceeded the background values in the frigid desert soils,the contens of other soil elements were equivalent to the background values.Comparied the element contents of the topsoil layer and parent material layer of the profile sample points,the results show that the soil elements in the study area shows certain mobility and accumulation at the same time.（3）The source analysis results show that there are five main sources of soil elements in the study area.Among the five main influencing factors,the first three main influencing factors are the parent material of soil formation,the influence of geological activities and the influence of geological and magmatic activities,all of which are natural factors.And these three main influencing factors can explain most of the element sources in the study area.Anthropogenic activities also have a certain impact on the distribution of element content in the soil environment in the study area,but the degree of influence is low.The element content levels obtained in this study can more truly reflect the background value levels of soil elements in the Mitika Wetland National Nature Reserve,so the current soil element content in the study area can be used as the background value.（4）Based on the evaluation results of the three risk assessment methods,the results show that Cd,Hg,Cu,Mn,and Pb all have a certain degree of pollution risk in the soil environment of the study area.Combined with the source analysis results,it can be known that the above elements except Cd elements and Hg elements are mainly affected by long-distance transport,and other elements are greatly affected by the geological background.Therefore,the contents of Cd and Hg elements in this study are not suitable as background values. |
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