| Selenium（Se）is an essential microelement for human body,and lack of Se or excessive selenium will have adverse effects on human health.Due to the extremelyuneven distribution of Se in the environment,attention was mainly paid to low-Se or selenosis areas at home and abroad.The research scopesof soil Se distribution in those areas are mostly based on administrative regions,which cannot reflect the integrity of the ecosystem.Guangxi features many small watersheds and widely distributed Se-enriched soils,and many of them are of large-area continuous distribution;so,it is necessary to carry out the study on Se distribution regularities and Se enrichment mechanism in those areas,and that will be very meaningful for doing macroscopic arrangement of Se industry,making development planning of Se-riched soil resources,and studying the mechanism of geographical environment on human health and longevity.This study is based on a watershed scale in GuilinSe-enrichedlongevity area.Methods such as principal component analysis,cluster analysis,path analysis,and geostatistical analysis were used to systematically study the content,distribution characteristics and control factors of Se in rocks,soils（sediment）,waters,crops,human hair and other biological or earth elements.From the perspective of watershed,this paper clarifies the distribution and correlation of Se in different media,the causes of Se enrichment in soil,and the influencing factors of available Se contents and speciations in soil.Furthermore,it reveals the influence rules of Se distribution in geographical environment media on ecological effects such as Se content in crops,dietary Se exposure level of residents,and distribution of longevity areas.Through systematic research,the achievements and understandings were obtained as follows:（1）By comparing Se contents in different environmental media in the watershed of Baishou River,it is found that the Se contents in soil,sediment and river water in the watershed gradually decreases from the upstream to the downstream.The contents of soil Se is highly correlated with elevation（r=0.41,p<0.01）,and the development of Se-enriched soil is controlled by rock lithology and parent materials,which presents a Se contents order of clastic rock area（0.84 mg/kg）>carbonate rock area（0.66 mg/kg）>quaternary sediment distribution area（0.49 mg/kg）,which are extremely significant differences in different lithologic areas.The low Se content（0.064 mg/kg）and evendistribution of rocks in the watershed are the main reasons why the soil se content has no significant correlation with the rock Se content.The content of Se in sediment is related to soil Se contentand grain size of sediment.The content of Se in river water is directly controlled by sediment Se content and indirectly controlled by soil Se content.（2）Different from the results reported in studies on high Se areas such as Enshi in Hubei Province and Ziyang in Shaanxi Province,the wide existence of Se-enriched soil in the watershed is due to climatic factors and the process of desilication and aluminization.From the earth surface downward,both the Si-Al ratio（Sa）and the Si-Al-Fe ratio（Saf）descend with the depth.The soil shows obvious characteristics of leaching and deposition,resulting in the sedimentary layer enrichment of soils which are developed from parent materials like sandstone,mudstone and Quaternary sediment and the surface layer enrichment of soils which are developed from parent materialslike limestone.According to R-type cluster analysis,the phenomenon is related to adsorption and retention of iron and aluminum oxides and leaching of basic ions.（3）Based on the structural variation characteristics of soil Se in the watershed,Kriging method is usedfor spatial interpolation of soil Seto predict the Se distribution in the watershed.The results of crossvalidationshow that the prediction accuracy of soil Se in the watershedby multivariate co-Kriging method is significantly improved through introducing terrain indice（elevation H and Compound topographic index,CTI）as covariates,and that provides a basis for the spatial distribution research of soil Se.（4）NaHCO3 and NaH2PO4 were used to extract available soilSe from clastic rock area,carbonate rock area and Quaternary sediment distribution area.The results show that the distribution of available Se in soil is consistent with that of total Se,but the differences ofavailable Se contents are low among the areas of different lithology,andtotal soil Se and pH are important factors affecting the content of available soil Se.In terms of soil Se speciations inthe watershed,the fractions of residual state,organic binding state and acid soluble state are higher（63.65%,16.40%and 11.95%,respectively）,while the fractions of exchangeable state and water soluble stateare lower（6.67%and 1.33%,respectively）.Path coefficients indicate that the key factors affecting the contents of water soluble state Se,exchangeable state Se,organic binding state Se,acid oluble state Se,and residual state Se are the contents of OM and Na2O,pH and TiO2,pH,MnO and Al2O3,OM and TiO2.By comparing the relationship between available Se and Sespeciations which are extracted with two methods,it is found that NaH2PO4 is a more suitable extractant for extracting available Sefrom acidic soil areas,and the content of extracted available Se can be used to judge the potential of soil Se supply.（5）Through the comparative study of ecological effects of Se in different lithologic areas of the watershed,it is found that crops show different characteristics of Se bioaccumulationin different lithologic areas.The high exchangeable Se content in paddy soil in carbonate rock area results in that the Se content in rice（97.23±39.59μg/kg）produced in that areais significantly higher than that produced in Quaternary sediment distribution area（51.08±15.05μg/kg）.The low activation degree of soil Se is the cause of low Se content and small difference in vegetables in different lithologic areas.Influenced by the Se content in rice,the highest dietary Se intake of residents in carbonatite area is 51.60μg/d,which is close to the recommended value of 55μg/d at home and abroad.There is a linear relationship between the content of Se in hair and the dietary Se intake,but such a relationship is not consistent with each other in different lithologic areas.The main reason why the elderly（>90 years old）are mostly distributed in carbonate rock areas may be due to the appropriate dietary Se intake of Se and the higher contents of Ca,Mn and other essential elements in the soil.The study on distribution regularities and controlling factors of Se in different environment media and onthe distribution regularities of longevity population will provide a basis for developingSe-enriched soil resources at a watershed scale and exploring the mechanism of longevity of residents;thus,it has practical application value and scientific significance. |
