Effects Of Dissolved Organic Matter(DOM) On The Selenium Transformation In Soil

Selenium（Se）is an essential micronutrient for human and animal health.The addition of organic amendment is a traditional method to maintain or improve soil fertility.The decomposition of organic materials will increase the content of dissolved organic matter（DOM）and other active organic components,which affects the transformation of Se fractions and oxidation states in soil,and then affects its bioavailability.Therefore,it is of great significance to find out the influence of DOM on Se mobilization and transformation in soil and reveal its regulation on Se availability.In this study,batch experiments were conducted to explore the effect of low molecular organic acids（LMWOAs）（acetic acid,citric acid,and malic acid）on Se adsorption and desorption in selenate or selenite treated soil.The differences of acetic acid,citric acid,and oxalic acid in Se mineralization release from four seleniferous soil with different physical and chemical properties were compared.Moreover,the changes of valence states or fractions of DOM extracted from cow manure and straw during soil aging were studied,and the mechanism of DOM influencing soil Se availability was preliminarily revealed.The main results are as follows:1.The adsorption and desorption effects of organic acids on Se in soil vary with different soil types,exogenous Se species,and concentrations.The adsorption of the two kinds of Se types（selenite and selenate）by loess soil basically reached the adsorption equilibrium within 24 h,and the treatment with LMWOAs didn’t affect the equilibrium time but only reduced the equilibrium adsorption capacity.The adsorption of selenite in krasnozems soil reached saturation in 1 h,but the adsorption of selenite in krasnozems soil was similar to the loess soil.The second-order kinetic equation can better describe the adsorption kinetic characteristics of the two kinds of exogenous Se in soil.The isothermal adsorption results showed that the adsorption capacity of selenate and selenite was decreased by the addition of malic acid treatment（MA）and citric acid（CA）.Whereas the adsorption of selenite was increased by acetic acid treatment（AA）at low concentration of selenite(<3 mg·L^-1).Moreover,Langmuir model can better describe the adsorption of Se in soil.The adsorption of selenite in krasnozems soil was almost complete,which had nothing to do with the addition of LMWOAs.However,the adsorption of selenate in soil with three kinds of LMWOAs was significantly higher than that without the addition of LMWOAs.The desorption rate of Se in soil treated with LMWOAs was higher than that in control treatment,but not in krasnozems soil.In conclusion,most treatments with LMWOAs decreased the absorption into the Se and promoted the desorption of selenite and selenate in loess soil.However,the adsorption capacity of the krasnozems soil to Se was improved,and the desorption of selenite and selenate in the krasnozems soil was inhibited.2.Compared with the natural seleniferous soils,the artificial Se-enriched soil is more easily mineralized and released.The release of Se in acidic seleniferous soil is mainly related to the number of carboxyl groups of LMWOAs;The more carboxyl groups,the stronger the release ability of Se in soil.The release of Se in alkaline seleniferous soil is mainly related to the acidity of LMWOAs.Compared with other LMWOAs,oxalic acid（OA）has a batter ability to mobilize Se from alkaline seleniferous soil.The results of this study are of great significance to better understand the mobilization of Se by LMWOAs in Se-enriched soil.3.The addition of exogenous DOM promoted the retention of available Se in soil.With the extension of incubation time,the valence states of Se（IV）and Se（VI）in the available Se（SOL-Se and EXC-Se）are transformed into the Se（-II）until it is stable.Moreover,after DOM application,the decrease rate of the valence states of Se（IV）or Se（VI）content in most treatment of available Se increased or the Se（-II）content increased.For different DOM,it can be found that different content have no significant effect on the immobilization of available Se,and（cow manure-derived DOM）NDOM has stronger immobilization of Se than straw-derived DOM（JDOM）.4.OM-Se in soil changed from HY-Se and FA-Se with strong activity to HA-Se and HON-Se with the extension of incubation time.It is worth noting that NDOM treatment in selenite has stronger retention on OM-Se than the corresponding JDOM treatment,while DOM from different sources in selenate has less influence on OM-Se.The increase of HON-Se and HA-Se in soil with Se aging is greater than the decrease of HY-Se and FA-Se in loess soil,which may be due to the available Se will transform into more stable fraction.In krasnozems soil,the increase of HON-Se and HA-Se is basically the same as the decrease of HY-Se and FA-Se,and the decrease of stable HA-Se indicates that there is a trend of transformation to more stable Se fraction.To sum up,LMWOAs affects the immobilization of the soil with the application of exogenous Se and the mobilization of Se in Se-enriched soil,and animal-derived DOM（NDOM）has a stronger retention effect on Se than plant-derived DOM（JDOM）.The influence of DOM on the mobilization and transformation of exogenous Se was influenced by the soil types and the forms of exogenous Se.Therefore,comprehensive consideration should be taken in Se biofortification.