Study On Organic Matter Composition, Evolutionary Process And And Its Significance Of Selenium Enrichment Of Black Soil In Cold Regions

Selenium（Se）deficiency in soil exists widely in the world,and the health of more than 100 million people in the world is threatened by selenium deficiency.The distribution of selenium in soil has significant spatial heterogeneity.The release and enrichment of selenium are affected by the organic matter in soil,and the distribution and formation of organic matter（OM）are controlled by the sedimentary environment.Therefore,identifying the relationship between Se,OM and sedimentary environment is the key to the study of selenium biogeochemical process,and also an important scientific issue in academic research.In this thesis,with organic matter as the link and core element and from the perspective of sedimentary environment,we try to reveal the mechanism of selenium enrichment in black soil,combined with fluorescence and molecular scale organic matter characterization technology.The following understandings are mainly obtained:（1）The distribution of selenium content in the topsoil of Hailun City is higher in the middle and lower in the northeast and southwest.Selenium shows obvious stratification vertically.The total content of selenium in Hailun City is higher than the average level of selenium in China.The distribution of selenium in the study area is mainly affected by soil types.Black soil is most conducive to the enrichment of selenium,dark brown soil is the least conducive to the enrichment of selenium.The land use types can also affect the distribution of selenium.The vertical distribution of selenium presents a remarkable layered feature.The selenium content in black soil is about three times that in yellow-brown soil.The statistics and correlation analysis of selenium and soil physical and chemical indicators show that the main reason for controlling the selenium content in soil is the enrichment of organic matter in soil.Sedimentary environment can also affect the distribution of selenium,and the formation process of black soil leads to a large amount of selenium enrichment.In a word,although environmental factors such as soil type and land use type can affect the distribution of organic matter and selenium,the environmental formation difference of black soil layer and yellow brown soil layer is the main reason for the enrichment of selenium and organic matter in the whole black soil region.（2）Aromatic substances,tannins and other organic components with high O/C ratio and high aromaticity are the main reasons for selenium enrichment in black soil.The main forms of selenium in black soil are OM-HA-Se and SOL-Hy-Se,and the content of inorganic selenium is relatively small;However,selenium in yellow brown soil is mainly in RES-Se form,and inorganic selenium content is relatively high.Secondly,it is proved that the composition and structure of organic matter can affect the migration and transformation of selenium in soil through a variety of organic matter characterization methods.Compared with yellow brown soil,black soil contains higher proportion of aromatic and tannin components,and less proportion of protein and lipid components.Protein and lipid components with low O/C ratio and few carboxyl groups in organic matter are not conducive to the enrichment of selenium in soil,while aromatic and tannic acids which have high aromaticity and rich carboxyl groups are conducive to the enrichment of selenium in soil.The analysis results of selenium and rare earth elements show that the selenium in the black soil does not originate from the parent material,but from the oil shale of Nenjiang Formation.After weathering,selenium and other elements are released from the oil shale,which are transported to the black soil by natural transportation,combined with organic components with high O/C ratio and high aromaticity,such as aromatics and tannins,and finally enriched in the black soil.（3）The sedimentary environment significantly affects the fluorescent components and molecular composition of organic matter,among which redox conditions are the most important factors,followed by climatic conditions（temperature,humidity,salinity）,paleoproductivity and acid-base property.During the evolution of yellow brown soil into black soil,the redox conditions gradually tend to be more reductive,the climate changes from warm and humid to cooler and drier,and the soil is more acidic.The redox condition is the most important factor to control the composition and structure of organic matter;The soil in the study area belongs to oxidation environment as a whole,but when the relative oxidation is inclined to relative reduction,the humus components（C1,C2）will increase,while the protein components（C3）will decrease.And with the relative reduction of soil,the molecular weight of soil organic matter is relatively smaller,and the content of organic matter components with high O/C ratio and aromatic degree,such as tannins,aromatics and carbohydrates,increases gradually.The climate changes from relatively dry and cold to relatively warm and wet,and the content of most organic matter components will gradually decrease,which is generally not conducive to the occurrence of organic matter.However,a small amount of tannic acid,lignin,some highly unsaturated substances and phenols and polyphenols with H/C<1.25,O/C at 0.5-0.9 will increase.Paleoproductivity and acid-base mainly affect the average molecular weight（E₄/E₆）.When the paleoproductivity increases and the environment is more acidic,most components of organic matter will increase,and E₄/E₆ will increase,representing that the average molecular weight of organic matter gradually decreases,and the organic matter of small fractions will increase.（4）The change of paleoclimatic conditions promoted the enrichment of organic matter and the evolution of yellow brown soil to black soil.In this process,organic matter and selenium were combined through adsorption and complexation,thus forming Se-rich in black soil.When the climate gradually changed from 11000-7500 to7500-5500,it was cool and dry,and the grassland meadow vegetation multiplied widely,organic matter accumulated on a large scale,the thickness of humus was deepening,and black soil was gradually formed;When the climate changed from humid and warm（7500-5500）to cool and dry（5500-now）,humus had accumulated deeply.The climate change promoted the large-scale formation of black soil,in which organic matter components with high O/C ratio,high unsaturation and high aromaticity,such as tannin and aromatics,accumulated rapidly.At the same time,the selenium in the oil shale in the surrounding Nenjiang Formation is constantly moved with the wind and water flow,and accumulated in the soil.And a part of selenium（VI）is simply physically adsorbed or complexed with the soil organic matter.Selenium（Ⅳ）react with organic matter through adsorption,complexation,reduction and other physical and chemical methods.The combination of selenium and organic matter reduces the mobility of selenium,thus promoting the enrichment of selenium in black soil.