Effect Of Different Manure Amendments On Selenium Bioavailability And Possible Mechanism In Soil-plant System

Selenium(Se)is an essential micronutrient for human and animal health.The Se status in a human population depends on the daily dietary Se intake,which is in part governed by the amount of Se available that enters the food chain from the soil.The available Se content in soil is affected by exogenous Se,soil physicochemical properties and agronomic measures.Among the agronomic measures,the application of organic manure such as cow and chicken manure is a traditional method to maintain or increase soil fertility.The application of organic manure affects the transformation of Se fraction in soil,which in turn affects its bioavailability.But so far,only a few studies have been done on the effects of organic manure on the bioavailability of Se in soil.In this study,a combination of incubation,pot and batch experiment was used to systematically study the effect of organic manure and exogenous Se on the transformation of Se fractionation and its availability in soil.The combination of FTIR and 3-D fluorescence techniques revealed the possible mechanism related to the effect of organic manure and rhizosphere process on the bioavailability of Se.In addition,traditional extraction methods as well as DGT techniques were also used to assess their feasibility in assessing the bioavailability of selenium in rich organic soil.At the same time,the role of DOM-Se species in governing Se's bioavailability is also determined.The main results obtained are as follows: 1.The application of organic manure to soil alters the availability of Se depending on the stage.In the early stages when organic manure is applied,it tends to increase Se's availability through increasing the amount of SOL-Se.This is governed by the amount of LMWOAs released from organic manure,which is able to compete with Se into the sorption sites.However,clearly,Application cow and chicken manure in soil could significantly reduce the bioavailability of Se,especially with exogenous selenite after long-term incubation period.Interestingly,the effect of cow and chicken manure on the decline of Se availability is,however,different,which depends on the exogenous Se species.In the case of exogenous selenite,chicken manure exhibits a superior advantage leading to reduced Se availability,whereas this advantage is expressed by cow manure in the case of selenate application.DOC released from decomposition process of manure plays a role in controlling the bioavailable Se,and the speciation distribution of Se in soil.2.Generally,the DOC production period within 120 days could be divided into three stages during the decomposition period.In the early stage(0–30 d),cow and chicken manure rapid decomposition has released a large amount of DOM,which was consumed by microorganisms as a substrate in the 2nd stage,and DOC production from manure decay and biodegradation of DOC approached a quasi-equilibrium condition.The competition of low-molecular-weight DOC with Se in the early stage could inhibit Se adsorption,and thus improve its solubility and mobility.After this period,the reduction of Se(IV)to lower valence states(0 and –II)is motivated by a large number of DOC functional groups with strong reducibility,or by microorganisms.Consequently,Se was transformed from available to less available forms,or from simple-structured molecules(HY bound Se)to aromatic large molecules(HA,HON).In the soil-plant systems,cow and chicken manure continue to exhibit different levels of impact on the bioavailability of Se depending on Se species was used.With the application of selenite,Brassica juncea(L.)Czern.et Coss absorbs more Se when the soil is amended with cow manure compared to chicken manure.In contrast,the concentration of Se in Brassica juncea(L.)Czern.et Coss tissues were higher in chicken manure than cow manure when selenate is applied.This phenomenon can be attributed to differences in properties and composition between cow and chicken manure.3.Rhizosphere processes can enhance soil available Se through competition or oxidation mechanisms to Se soluble forms,thus plants can absorb more Se from soil.However,in soil-plant systems,manure amendment might plays a more dominant role than rhizosphere processes of Brassica juncea(L.)Czern.et Coss to regulate the bioavailability of Se.Reducing soil pH by cow or chicken manure application promotes Se adsorb into soil particles and reduce Se to a lower valence state,resulting in decrease bioavailability of Se.In addition,DOM derived from cow and chicken manure contains more aromatic and hydrophobic compounds with the C-O,C=O and C=C functional groups,which potentially offered reaction sites for Se,and thereby reducing and fixing Se in less available fraction,and reduced Brassica juncea(L.)Czern.et Coss Se absorption.4.In this study,the coupling of DOM-Se species extracted by PBS and SEP with DGT provided essential information on the soil Se speciation,which may be useful in understanding the Se lability and phytoavailability in soils.The PBS and SEP procedure revealed that a low-molecular weight DOM-Se species in available fraction were mobile and DGT-labile.The DGT,PBS-labile fraction(HY-Se and FA-Se),and SEP-labile fractions(SOL-HY-,-FA-Se,EXC-HY-,-FA-Se)performed comparably well in predicting the accumulation of Se in Brassica juncea(L.)Czern.et Coss.This study demonstrates that DGT is suitable for measuring bioavailable Se in soils supplemented with organic materials such as cow and chicken manure,thus is a useful test procedure in measuring bioavailability of Se in the soil-plant system.In addition,in the case of the use of chemical extraction to predict the bioavailability of Se,isolating Se available fraction into DOM-Se species is necessary to enhance the accuracy of the model.5.This study also shows that the capacity of individual LMWOAs to mobilize soil-borne Se varied significantly,which depends not only on the acid types but also on soil properties.In general,Se is mobilized from higher in added-Se soil compared to native soils.Two major Se release mechanisms in soil involving LMWOAs: ligand-enhanced dissolution,competition with Se to sorption sites and promotes Se absorption into the soil.In acid soil with high iron and organic matter content,the mobilization of Se is governed by the acid strength of a LMWOA.In contrast,the acid strength was not necessary a key factor controlling the mobilization of Se in the presence of the LMWOAs in alkaline soil.Oxalic acid exhibits superior ability to mobilize Se from contaminated soil compared to other LMWOAs,which is related to its ability to form ring complexes with metal-(hydr-)oxide surface,leading to its competitive efficiency with Se over other LMWOAs.The low efficiency of Se mobilization in the presence of acetic and citric acid is likely to be related to its role in promoting Se absorption into soil particles.The findings obtained from this study have implications for better understanding the mobilization of Se by plant root exudates in contaminated soils.In conclusion,effects of organic manure amendment on selenium bioavailability in soil-plant system involve complicated processes.Organic manure types,and application time,soil types,selenium species are all affected the interaction between DOM derived from manure and selenium,so do for selenium bioavailability in soil.Those should be taken into consideration in the biofortification.