Effects Of The Different Exogenous Selenium Levels On Humus Characteristics In Selenium-enriched Soil And Heavy Metals Accumulation In Brassica Juncea

In recent years,while develop ing and utilizing selenium（Se）-enriched soil resources,people have also found that Se-enriched soil has certain associated effects of heavy metals.Therefore,it was critical to prevent and control the risks of heavy metal pollution that recognizing the laws and influencing factors of Se-enriched soil contaminated by heavy metals,and it was also a hot issue that must be faced during the development and utilization of Se-enriched soil.Soil humus has a large number of active oxygen-containing functional groups,thereby its components can strongly adsorb and complex with heavy metals,which may hinder the transport process of heavy metals in soil-vegetables systems.In addition,humus can be used as a source of organic-Se in the soil.Humus-bound Se,which can be released into the soil solution through mineralization for plants uptake and utilized,was a potential source of available Se.At the same time,the different types of soils that have different farming conditions and environmental conditions have different characteristics of humus.To this end,starting with the influence of exogenous Se levels,carrying out research on the effect of the characteristics of humus composition in Se-enriched soil and heavy metal enrichment in Brassica juncea（B.juncea）.It ont only can lay a foundation for recognizing the effect of humus on Se-enriched soil on heavy metals,but also was critical to the regulation of heavy metals in Se-enriched soil.Based on this,this paper takes Se-enriched soils with different exogenous Se levels and their potted B.juncea as research objects,the components of humus in Se-enriched soils with different Se levels was separated and purified by using the method provided by The International Humic Substances Society（IHSS）,which were named humic acids（HA）,fulvic acids（FA）and humin（HM）.The effects of the different exogenous Se levels on the properties and structure of humus components in Se-enriched soil,the adsorption between humus components and heavy metal ions,the rules of heavy metal enrichment in B.juncea,and the bioavailability of heavy metals in Se-edriched soil-B.juncea system were investigated.The main results are as follows:（1）The content of each component of humus in Se-enriched soil（rhizosphere soil,bulk soil）followed this order:HA<FA<<HM.The variation range of humus components in bulk soil was larger than that in rhizosphere soil,and the total amount of soil humus at different Se levels represented significant differences.There was a significant difference in the elemental composition and atomic ratio of each component of humus extracted from soils with different exogenous Se levels.The C/N and O/C of FA were larger than that of HA regardless of the treatments,but of which the H/C was smaller than that of HA.The C/N of HM increased significantly,and the effect of the S3 treatment gro up(the addition level of exogenous Se⁴⁺was 5.0 mg·kg^-1)was the best（85.08%）;the O/C change range was the smallest,and remained basically stable;the H/C decrease was the largest in the S5 treatment group(the addition level of exogenous Se⁴⁺was 17.0 mg·kg^-1)（51.79%）.There were obvious differences in the infrared spectra of the humus components,but the positions of the peaks were generally the same,while the peak strength was different.Regardless of the treatment,the main absorption ba nds of each component of humus were 3450 cm^-1,2925 cm^-1,1630 cm^-1,1383 cm^-1,1250 cm^-1,and1035 cm^-1,indicating that they all have-OH and unsaturated bonds.Compared with HA and FA,the HM has more peak positions,which mean it has more complex group types,and there are abundant associative alcohols,hydroxyl-OH.In short,the results show that HA,FA and HM all have the ability to complex and adsorb with heavy metals.（2）The adsorption capacity of HA/FA for Pb²⁺and Cd²⁺has a good correlation with the concentrations of Pb²⁺and Cd²⁺,respectively（P<0.05,P<0.01）.With the increase of the equilibrium concentration of heavy metal ions,the adsorption amount of HA/FA on Pb²⁺and Cd²⁺increased sharply,then increased slowly until it finally reached saturation.Under the conditions of the same heavy metal ion equilibrium concentration,the effects of exogenous Se levels on the adsorption of HA/FA for Pb²⁺and Cd²⁺were different,indicating that the strength of Se and HA/FA mutual bonding was different and competitive adsorption existed.Compared with Cd²⁺,HA has a larger adsorption rate of Pb²⁺（31.32%～39.95%）,and has a stronger effect;while FA has a higher adsorption strength of Cd²⁺,with an adsorption rate of up to 31.15%.The adsorption isotherms of HA and FA for Pb²⁺were well described by the Langmuir adsorption model,while the fitting effect of Freundlich adsorption model was obviously better than the Langmuir adsorption model in the adsorption of FA on Cd²⁺.（3）The p H was negatively correlated（r=-0.729）with the concentration of available Cd（0.01<P<0.05）.The concentrations of available Cd in the soils subject to three different additions of HM（H1～H3）,α-Fe₂O₃（F1～F3）and HM-α-Fe₂O₃[（F-H）1～（F-H）3]treatments were 14.21%～22.96%,21.25%～37.55%and 13.45%～27.75%,and the concentrations of exchangeable Cd were 17.77%～23.34%,33.93%～45.39%and18.56%～22.07%lower respectively,than those under the control treatment when cultured for 60 days.With a comparison of passivation effect of different repair agents,it was found that the best treatment group in individual repair agents was F2（20 d）with the maximum reduction ratio（37.55%）,but the application amount was also the largest and easily led to soil alkalization.HM achieved the best passivating effect in 5 days.The best treatment group in HM-α-Fe₂O₃ was（F-H）3（60 d）,and the decrease of the concentration of available Cd was significantly positively correlated（r=0.631,0.428）with the dosage of passivation agent and passivation time（P<0.01）.It indicated that the passivation effect showed a good linear growth relationship with the dosage and time.The application rate was low and the effect was obvious.Therefore,HM was suitable for short-term repair,while HM-α-Fe₂O₃ were the best choice for long-term repair.The application of HM combined withα-Fe₂O₃ as repair agents provided a new way for both making full use of humus and controlling metal mobility.（4）Compared to the control treatment,the application of different levels of exogenous Se reduced the Pb concentration in the roots,stems and leaves of B.juncea by5.43%～48.70%,5.13%～39.86%and 6.62%～31.91%,respectively;the Cd concentrations were reduced by 4.03%～63.52%,4.69%～67.01%and 6.65%～78.89%,respectively;significantly increased the Se concentration of each tissue,which were7.22～24.21,5.91～34.11and 12.49～57.06-fold that in the control treatment,respectively.In all treatments,compared with the control treatment,the addition of different exogenous Se levels reduced the total Pb concentration in rhizosphere soil and bulk soil by1.06%～12.48%and 2.99%～13.68%,respectively,and the total Cd decreased by2.24%～39.05%and 0.82%～22.63%,the p H in soil increased by approximately 0.06～0.21.Exogenous Se also reduces the content of DTPA extracted available Pb and Cd,exchangeable Pb and Cd fractions in soil,but increased the content of carbonate-bound Pb and Cd,residual Pb and Cd.The results of correlation analys is showed that the Se level in the soil,the content of humus components and the concentration of Pb/Cd were all negatively correlated,reflecting their mutual inhibitory effects.It was also explained the reasons,for which the decrease of Cd and Pb activity and its accumulation in Se-enrich soil-B.juncea system.