Enrichment Characteristics Of Soil Selenium And The Influencing Factors Of Its Availability In Jinqu Basin

Selenium（Se）,as an essential trace element in human body,plays an important role in anti-aging and immunity enhancement.As a Se-deficient country,China has paid widespread attention to Se-rich soils.However,for many areas with Se-rich soils in China,its enrichment characteristics of Se in the soils are not clear and the influencing factors of soil Se bioavailability,especially the mechanism of Se bioavailability effected by microorganisms remain not well studied.These knowledge gaps have constrained our understanding of the environmental biogeochemical behavior and health effects of Se.Jinqu Basin was select as a typical area with Se-rich soil for this study,to analyze the distribution and the sources of soil Se,as well as the key biotic and abiotic factors effecting the Se bioavailability,by integrating methods of ecological geochemistry and biogeochemistry.In addition,the mechanism of soil Se availability effected by indigenous microorganisms was explored.This study provided scientific evidence and technical support for the protection and utilization of Se-rich soil resources.The main achievements in this study are as follows:1.The spatial distribution of soil Se in the Jinqu Basin was heterogeneous and influenced by multiple factors.Generally,Se contents were higher in the west but lower in the east,and was enriched in the surface layer.Three Se-rich soil areas represented by Jiangtang Town were formed,with average Se content higher than 0.42mg/kg.Soil parent material is the key factor effecting the distribution of Se in soils.p H,organic matters,soil textures and soil elements were the main physical and chemical factors shaping the distribution of Se.The Se content was higher（average 0.39 mg/kg）in red soils and weakly acidic paddy soils with higher contents of organic matters and clay,which may be affected by the interactions of Mo,Cu,N,S,As,K and other elements.2.The sources of Se in the soils of Jinqu Basin were influenced by both natural and anthropogenic factors which included geological processes,soil-forming process and external inputs.The main sources of Se were the parent materials of soils with abundant Se reserves,and the middle-Pleistocene laterite weathering material was the main parent material in typical Se-rich areas,with an average Se content of 0.34-0.38mg/kg.Under the supergene geochemical processes,different soil forming processes were the key factors resulting in the differentiation of soil Se contents.The process of desilication and aluminization might be the main factors of Se enrichment in red soils,while the leaching and decalcification of purple soil and the tillage and maturation of paddy soil might have certain effects on the migration and enrichment of Se.The formation process of reticulated laterite was the key factor for Se enrichment in soils.During the redox transformation of iron oxides,Se was gradually enriched in the matrix and increased with soil depth.The redox of iron oxides promoted the gradual enrichment of Se in the matrix,and increasing with the depth of the soil profile.Exogenous inputs such as atmospheric deposition and farmland irrigation had weak effects on soil Se with contribution of 4.3%-15.2%.3.Land use pattern was the key factor affecting soil Se bioavailability.Soil moisture content,p H,ORP,TOC,soluble iron and NH₄⁺as well as the microbial communities in Se-rich soils were the key environmental factors controlling available Se content.Different land use patterns determined the distinct soil geochemistry,and further affected the bioavailability of Se in soils.The alternation of aerobic and anaerobic cultivation in dryland soils provided favorable conditions for Se enrichment by facultative anaerobic Se-rich bacteria.Microbial populations including Pseudomonas,Shewanella,Bacillus,Dyella and Comamonas could affect soil Se availability through Se transformation,iron reduction,denitrification and organic matter degradation.4.Microbial fortification could significantly improve the bioavailability of soil Se.In this study,6 selenite-reducing bacterial strains and 4 selenate-reducing bacterial strains were isolated from the typical Se rich soils in Jinqu Basin,among which the genus Chitinophaga was the first reported to be capable of selenite reduction.These selenite-reducing isolates performed strong selenite reducing abilities which ccould reduce 19.7%～44.4%of 90 mg/L（or 20 mg/L）selenite within 84h and formed nano-Se（Se NPs）.With the addition of selenite-reducing isolates Chitinophaga sp.and Comamonas testtestoni,the Se contents in rice plants were increased by 47%and 92%,respectively.The isolated selenate reducing strains could reduce 17.9%～44.6%of initial75 mg/L selenate within 48 h,which greatly increased the concentrations of water-soluble and ion-exchangeable Se in soils and the Se contents of plants.In rice plants,130%and 33%of Se were increased with the amendments of Bacillus pseudomycoides and Brevibacillus fluminis,respectively.