Effects Of Moisture And Organic Matter On Selenium Speciation Transformation In Soil And Mechanisms

Selenium（Se）speciation transformation plays an important role in determining Se bioavailability in soil.Soil moisture content and its variations affect soil properties,soil structure and microbial activity,etc.,especially the transformation of soil organic matter,which in turn affects the transformation of Se speciation.Therefore,to better understand effects of soil moisture and organic matter on the transformation of Se speciation and possible mechanisms is of great significance for the evaluation of Se bioavailability in soil.By using incubation experiments and sequential extraction methods,this study explored effects of soil moisture（water-deficient,-sufficient and-saturated）conditions and alternating wetting（water-saturated condition）and drying（water-deficient condition）on the aging of exogenous selenate and selenite in different types of soils（loess soil,black soil,and krasnozem）.Soil Se fractions,Se species of available Se fractions,p H,Eh,and DOC content were determined to identify the critical influential factors and possible mechanisms for Se aging.Using XAD-8resin combined with fourier transform infrared spectroscopy（FTIR）,the effects of moisture conditions on the transformation of organic matter-bound Se fractions in soils with different organic matter contents during the aging process of exogenous Se were also studied.In addition,effects and mechanisms of different sources of DOM and moisture conditions on Se speciation transformation in natural seleniferous soils were also studied,in order to find out the influential mechanisms of moisture condition and organic matter on the transformation of Se speciation in farmland soil from two aspects of exogenous Se and endogenous Se.It aims to provide a scientific basis for the regulation of soil Se availability in Se biofortification.The main findings obtained from this study are summarized as follows:1.Soil moisture conditions significantly affected the aging process of exogenous Se in different types of soils,which also depended on exogenous Se species.Regardless of soil moisture conditions,hexavalent Se（Se（VI））was the predominant species of the soluble Se fractions（SOL-Se）in selenate treated soils.Water-saturated condition could decrease soil Eh and increase soil p H and DOC content,resulting in the reduction of Se（VI）to Se（IV）,Se（0）or Se（–II）and accelerating selenate aging in different types of soils.In contrast,soil moisture content had no significant effect on the aging process of selenite in three tested soils.In addition,loess soil with high p H value and Ca CO₃ content and low Eh value and black soil with high organic matter content promoted the reduction and immobilization of exogenous Se,and accelerated Se aging processes regardless of exogenous Se species.Thus,loess soil and black soil showed the higher Se aging rates compared with krasnozem.Soil p H,Eh and organic matter content were the main factors affecting the aging of exogenous Se in different types of soils regardless of soil moisture conditions.2.Exogenous Se species played a leading role in Se aging in different types of soils under the alternating wetting and drying（AWD）condition.The available Se fractions mainly existed as Se（VI）in the SOL-Se fraction for krasnozem,while Se transformed from Se（VI）of SOL-Se to Se（IV）of EXC-Se was found in loess soil and black soil for selenate treatment.On the contrast,Se（IV）of the EXC-Se fraction was the main available Se in selenite treated soils.The aging rate of selenate in loess soil and black soil was higher than that in the corresponding selenite treated soil,while the Se aging rate in krasnozem showed the opposite result.Regardless of exogenous Se species,the aging rate of Se in loess soil was significantly higher than that of black soil and krasnozem（P<0.05）.Redundancy analysis（RDA）showed that soil p H played a key role in soil Se aging under the condition of AWD,and soil organic matter content had an important effect on the aging of exogenous selenate in black soil.3.The aging rate of soil exogenous Se increased with the raise of organic matter content under different soil moisture conditions.Regardless of exogenous Se species,the proportion of SOL-Se or EXC-Se fractions to total Se decreased with the increase of organic matter content in soils,while soil RES-Se fractions showed the opposite trend.The distribution of DOM-Se fractions in soil organic matter-bound Se fractions depended on the exogenous Se species.In selenate treatments,the hydrophobic neutral organic bound Se fraction（HON-Se）was the main DOM-Se fraction in soils with high content of organic matter,while HON-Se and humic acid bound Se（HA-Se）became the main DOM-Se fractions in soils with the low organic matter content.In selenite treatments,HON-Se was the main DOM-Se in soils regardless of organic matter content.Functional groups with O-H,alkane and aromatic C-H,aldehyde and ketone carbonyl C=O,ester C-O-C)in soil DOM were the main reaction sites for Se in different organic-matter content soils.The Se aging rates under water-saturated and-sufficient conditions were higher than that under the water-deficient condition in soils regardless of organic matter content.Water-saturated condition resulted in a decline in Eh and p H and an increase in DOC contents in high organic-matter content soils,and accelerating the aging process of exogenous Se.4.Soil organic matter content significantly affected the aging process of exogenous Se under the condition of AWD.Regardless of exogenous Se species,the main Se fraction in soils with different organic matter contents transformed from available Se fractions to RES-Se with the progresses of AWD cycles.In addition,the fraction distribution of exogenous Se in soils was affected with the organic matter content.Regardless of exogenous Se species,higher proportions of available Se fractions were observed in lower organic-matter content soils,while higher proportions of RES-Se fractions were found in higher organic-matter content soils.In terms of the distribution of DOM-Se in organic matter-bound Se fractions,HON-Se was the main Se fraction at the end of aging.The higher content of organic matter promoted the combination of Se（VI）and Se（IV）with the more aromatized and hydrophobic HON components in the organic matter,resulting in DOM-Se almost all HON-Se in soil.The binding ability of selenite with HON was stronger than that of selenate in lower organic-matter content soils,thus,the proportions of HON-Se of DOM-Se in selenite treated soils were significantly higher than that in selenate treated soils.5.Soil moisture condition and DOM had no significant influence on the activation of Se in natural seleniferous soils.Se（VI）was the main species of SOL-Se fractions in Yichun of Jiangxi（JX）,Guiping of Guangxi（GX）,and Enshi of Hubei（HB）soils under different moisture conditions,while Se（IV）plus Se（VI）became the main Se species of the SOL-Se fraction in Ziyang of Shaanxi（SX）soil with the highest Se content among the four tested seleniferous soils.The EXC-Se fractions in natural seleniferous soils mainly existed as Se（IV）,and the Se（-II）content of the EXC-Se fraction was equivalent to its Se（IV）content in JX soil.The DOM-Se of OM-Se fractions was transformed from the hydrophilic organic acid-bound Se fractions（Hy-Se）at the beginning of incubation to HON-Se at the end of incubation in Se-enriched soils.RES-Se was always the main Se fractions in natural seleniferous soils throughout the incubation period regardless of soil moisture conditions.In addition,organic matter-bound Se fractions accounted for a relatively high proportion in Hubei soil.Although the proportions of SOL-Se and EXC-Se were relatively low,the proportion of EXC-Se to total Se content increased with incubation time.However,DOM from different sources had little effect on Se activation and not significant（P<0.05）difference among four tested natural seleniferous soils.6.The AWD condition accelerated the activation of Se in natural seleniferous soils.The proportions of RES-Se to total Se in GX and SX soils were higher than that in JX and HB soils.In addition,organic matter-bound Se fractions had relatively high proportion in JX and HB soils.RES-Se fractions transformed into SOL-Se,EXC-Se and OM-Se fractions in natural seleniferous soils under the condition of AWD,and the transformation extent in JX and GX soils was greater than that in HB and SX soils.In addition,Hy-Se and HON-Se fractions were the main DOM-Se fractions,which varied dynamically during the whole incubation period under the condition of AWD.In summary,both of soil moisture and organic matter significantly affect the aging of exogenous Se in soil,and the alternating wetting and drying promoted the activation of Se in natural seleniferous soils.The transformation of exogenous Se and endogenous Se in soil was affected by Se species,soil type,soil property,especially moisture conditions and organic matter content.Therefore,these factors should be taken comprehensively into consideration in the Se biofortification practice.