Source,Bioavailability And Controlling Factors Of Soil Selenium In Yanqi Basin,Xinjiang

Selenium（Se）is one of the essential trace elements for human beings and animals,and its deficiency or excess will affect human health.The Se ingested by human diet comes from soil through the core of food chain,and Se content in crops is greatly affected by the bioavailability of soil Se.A large area of Se-enriched soil was found in Yanqi basin of Xinjiang by multi-purpose regional geochemical survey.However,the source and the factors affecting the bioavailability of Se in the soil（saline-alkaline soil）in oasis are still lacking in the world.The main planting areas of wheat and rice in Yanqi Basin,Xinjiang were chosen as the research objects,wheat and rice samples and their corresponding surface soil samples were collected and analysed respectively.Geostatistical analysis was used to analyze the spatial distribution characteristics of total Se and available Se content in soil-crop system.Meanwhile,the main factors affecting the Se bioavailability were explored by multivariate statistical analysis method,and the source of Se in soil was preliminarily analyzed.Finally,the content of heavy metals in soil and crops was comprehensively analyzed and the safety was evaluated.The purpose is to provide a basis for the rational and safe development of Se-enriched soil in the study area.Main results are as follows:1.In Yanqi basin of Xinjiang,the total Se content in dryland soil ranged from 0.105mg/kg to 1.100 mg/kg（average of 0.330 mg/kg）,all paddy soil samples reached moderate and Se-enriched level,and the paddy soil was 0.209-0.767 mg/kg（average of 0.447 mg/kg）.while 81.06%of the dryland soil reached moderate Se-enriched level.On the whole,the soil total Se content in the region reached an appropriate level or above,and the Se-enriched areas were distributed in blocks,with the distribution area of Bohu county being the largest.The maximum total Se content in wheat grain was 1.086 mg/kg（average of 0.140 mg/kg）,reaching a moderate level and Se enrichment accounted for 84.85%.The spatial distribution of Se content in wheat grain was as follows:Bohu>Hejing>Yanqi>Heshuo.The total Se content in rice grains ranged from 0.050 mg/kg to 0.345 mg/kg（average of 0.127 mg/kg）,and all rice grain samples reached moderate or above levels.2.Available Se content in dryland soils ranged from 0.004 mg/kg to 0.198 mg/kg,with an average of 0.033 mg/kg and MF was 9.42%.The content of available Se in paddy soil ranged from 0.028 mg/kg to 0.097 mg/kg,with an average of 0.052 mg/kg and MF was11.87%.High-value regions of A-Se and MF were mainly concentrated in Yanqi County and Bohu county,and the spatial distribution was similar to that of wheat grain Se content,which showed local planar or punctiform distribution.ORG-Se was the main fraction of Se in the area.OM,A-P,Si O₂and Fe/Al oxide were the main factors affected the bioavailability of Se in dryland soil,and p H mainly had an indirect effect.While A-P,Si O₂,and Fe/Al oxide were the main factors affecting the bioavailability of Se in paddy soil.Different varieties of crops also affected the bioavailability of Se.Among the seven wheat varieties tested,wheat XM517 had the strongest ability to absorption and accumulation Se in soil.3.The redundancy analysis showed that p H,OM,A-P,A-K,B,A-N,Mn O,Si O₂,Ca O,T-Fe,dryness,altitude,TWI and NDVI were the main contributors to the enrichment of total Se in topsoil.Different physicochemical properties had different effects on Se enrichment in topsoil in different regions.Se enrichment in topsoil was increased with the increasing of soil p H and CIA.The enrichment of total Se increased gradually with the decrease of altitude and precipitation,and the increase of dryness and vegetation cover index.Jurassic carbonaceous slate had the highest Se content in the main strata in Kaidu River Basin,followed by glutenite,phyllite and red bed,which were the main sources of Se-enriched soil in the study area.Long-term agricultural activities（fertilization,irrigation,etc）were also important causes of Se enrichment.4.There was no heavy metal pollution in the topsoil of the study area.Geochemical comprehensive score of soil nutrients reached the third standard of comprehensive grade of nutrients.There was no phenomenon that the content of heavy metals in all crops exceeds the standard.The daily intake of HMs and Se by rice and wheat per capita and the target hazard coefficient were within the safe range,therefore no health risk.In conclusion,the enrichment and distribution of Se in soil of Yanqi Basin were influenced by soil physicochemical properties,geological background climatic and topographic conditions,while the contents of OM,A-P,Si O₂and Fe/Al oxide in soil were the main controlling factors of soil Se bioavailability.Yanqi basin is a good base for producing healthy selenium-enriched agricultural products because of its high bioavailability of soil Se and great land environmental quality.