Effect Of Straw Amendment On Selenium Bioavailability And The Mechanisms In Soil

Selenium(Se)is an essential “double-edged sword” element that may negatively affect the human health when inadequately or excessively taken.The bioavailability of Se in soil is closely related to the fraction of selenium,which is not only affected by the species of exogenous Se but also affected by soil physicochemical properties.Straw amendment is a common agronomic approach for the utility of agricultural waste and fertility enhancement.Dissolved organic matter(DOM)released from straw decomposition and mineralization is interact with Se in soil and therefore alter Se availability.Straw amendment modifies soil organic matter content and pH value in the microenvironment of soil,and thus impacts Se fraction transformation and availability.However,studies on the effect of straw amendment on the transformation and availability of Se after straw amendment in soil are rarely reported.Especially,interaction between DOM and different exogenous Se(selenate and selenite)in different types of soil and possible mechanisms had not been reported yet.Consequently,soil incubation and pot experiment were combined and DOM-Se sequential extraction and isolation method was built for Se species and fractionation identification to investigate the effect of straw amendment on fraction transformation and availability of exogenous selenate and selenite in different types of soil.The mechanisms underlying this process were also explored using various spectroscopic technique methods including UV–Vis,attenuated total reflection-Fourier-transform infrared spectroscopy(ATR-FTIR)coupled with 2D-COS and 3D-EEM coupled with PACAFAC analysis to explore the variation of DOM characteristics(quality,quantity and composition).Afterwards,comparison was made on the DOM-Se formed from straw derived DOM and inorganic Se and inorganic Se mineralized from natural organo-Se(derived from Se-enriched straws).The purpose of this study is to offer references for adjusting crop Se contents,reduce Se resources lose and environmental risk during Se biofortification by straw amendment.The main results are as follows:1.The study evidenced that pH and organic matter are the dominant influential factors for exogenous Se aging in soils.The available Se content in all soils declined rapidly at the initial stage of aging,with most of the neutral or alkaline soils reaching equilibrium after 109 d,whereas the acidic soils reached equilibrium after only 33–56 d.Se aging rates were higher in acidic soils than in alkaline soils.2.The sequential extraction and isolation method for DOM-Se is established.Through selecting the extraction solutions for SOL-Se and EX-Se as well as the amounts for resin and eluent,combined with previous studies,the extraction and isolation of DOM-Se after different exogenous Se applied in different soils is successfully realized.In addition,the application amounts of XAD-8 resin,HCl for acidification and eluent are defied to ensure the complete isolation of hydrophilic acid bound Se(Hy-Se),fulvic acid bound Se(FA-Se),hydrophobic organic matter bound Se(HON-Se)and humic acid bound Se(HA-Se),which facilites thorough study of soil DOM-Se.3.Results indicate that DOM played dual role on selenate availability in soil.FA and HON can enhance the availability of Se in the soil solution and particle surfaces of acidic soil with high organic matter content,while Se incorporation with HA can accelerate the fixation of Se into the solid phase of soil.After straw amendment,soil solution and soil particle surface are dominated by hexavalent hydrophilic acid-bound Se(Hy-Se).The amount of fulvic acid bound Se in soil solution(SOL-FA-Se)is higher than humic acid bound Se in soil solution(SOL-HA-Se),except in krasnozems.The functional groups of straw-derived and soil organic matter mineralized DOM,including hydroxyls,carboxyl,methyl,and aromatic phenolic compounds are involved in selenate reduction,ligand adsorption,and inner/outersphere complexation,which are the main incorporation approach between DOM and Se.The dual effect of DOM on selenate aging was highly reliant on the characteristics of the DOM and soil properties.4.The effect of straw amendment on selenite aging is different from selenate.Incorporation with soil particle surface and solid phase is still the main reaction for selenite and SOL-HA-Se and EX-Hy-Se are the main reflection for DOM-Se availability.Straw amount rarely affects the aging process and the effect is obvious in acidic soils than in alkaline soils.However,the soil solid organic matter plays a significant role in selenite aging,the HA-Se and RES-Se fraction are significantly enhanced.The key functional groups of straw derived DOM involved in selenite aging are hydroxyls,carboxyl and aromatic C-O,reactions between hydroxyls and selenite is the main incorporation during aging process.5.Straw amendment significantly suppresses the selenate bioavailability(p<0.05).The suppression of selenate bioavailability by straw-derived DOM is related with DOM interaction with soil Se and controlled by the aromaticity,the functional groups and compositions of soil DOM.Compared with control treatments,the reduction in wheat Se content is greater in krasnozems than in Lou soil,particularly the grain Se contents of wheat decreased by more than 34%.Straw amendment stimulates the transformation of SOL-HON-Se and EX-FA-Se fractions,which are the main derivations of grain Se in soils(p<0.01).The DOM in all treated soils tend to transform from simple-structured molecules(Hy and FA)to aromatic large molecules(HA),accompanied with the reduction and retention of Se associated with these molecules.6.The straw derived DOM can enhance selenite bioavailability and manipulated by soil properties and Se application rates.Regardless the exogenous selenite concentrations,straw amendment significantly increases wheat grain Se contents(p<0.05)in Lou soil.However,in krasnozems,only high Se treatment increases grain Se concentration after straw amendment.In addition,2D-COS analysis finds that the high Se application rate in krasnozems promotes HA mineralization and causes grain Se increase.The HA-Se can be easily formed in krasnozems which is enriched with HA fraction,since selenite was predominantly incorporated with aromatic phenolic C-O groups.Through complex with straw derived DOM by saturated and unsaturated aliphatic carboxyl,the Hy-Se and FA-Se were produced to be uptaken by plants in Lou soil.Therefore,straw amendment significantly enhances selenite availability in Lou soil and only high Se application rate in krasnozems.7.The utility of soil Se is enhanced by Se-enriched straw amendment.The Se contents of the cultivated pak choi in treatments amend with the same amount of Se-enriched wheat straw and pak choi were 1.7 and 9.7 times in the shoots and 2.3 and 6.3 times in the roots compared with control treatment.Soil respiration rate is significantly increased after all organic material amendment in soil,which accelerates the mineralization of organic materials and thus resulted in soluble Se(SOL-Se),exchangeable Se(EX-Se),and fulvic acid-bound Se(FA-Se)increasing.EX-Se is better indicator than SOL-Se for soil Se availability in organic material amendment soils,and closely relates with FA-Se.This phemonenon further demonstrates that soil OM-Se(FA-Se)was the potential source for soil available Se.The utilization of Se in P+Se treatment is significantly higher than those in WS+Se treatment because of the different mineralization rates and the amount of FA-Se in soil.The utility of Se from Se-enriched organic materials is highly dependent on the species of the straws,which needs to be considered during real application.Generally,through different approach the straw derived DOM and exogenous selenium react with each other,and thus forming different DOM-Se fractions in diverse soils.The continuous transformation of DOM-Se determined the availability of exogenous Se.The availability of Se is affected by the quantity,composition,functional groups and other characteristics of the DOM in soils.Straw amendment significantly enhanced the bioavailability of selenite but dramatically reduced the toxicity of selenate.As a result,comprehensive consideration is required when straw amendment is applied in Se biofortification or bioremediation fields.