| Soil humus,as the main form of soil organic matter,which content and composition characteristics can reflect the forming condition and process of soil to a certain extent.The selenium-enriched soil in Haikou not only possesses the conditions and process of soil formation under selenium enrichment,but also has the climatic characteristics of subtropical regions and the farming methods of soil.Humus in this kind of soil has its own characteristics of composition,structure and behavior.At the same time,soil humus contains a large number of carboxyl,hydroxyl and other oxygen-containing functional groups and a variety of aromatic structure,aliphatic structure,has strong adsorption and complexation with heavy metal,which will play a pivotal role in the migration and transformation of heavy metals in the soil.Based on this,this paper takes selenium-enriched soil with different levels in Haikou as the object,and extracted fulvic acids(FA),humic acid(HA)and humin(HM)using the method provided by The International Humic Substances Society(IHSS).Then,the quantitative and preliminary qualitative analysis was carried out to investigate its effect on heavy metals,and elucidate its content,structure and behavior characteristics.On this basis,the effects of humus on the migration and transformation of heavy metals in the soil-mustard system were investigated.The main results are as follows:(1)The content of humus in selenium-enriched soil was expressed as HM>>HA>FA,which conform to the general characteristics of soil.The content of each component decreased firstly and then increased with the decrease of Se content,the difference among each sample was mainly in the total amount of rhizosphere soil.(2)Among the content of these elements(C,N,H,S)in FA,HA and HM extracted from selenium-enriched soil with different levels,which HM shows the obvious differences.The main features are as fellow,the content of elements C,N and S in FA extracted from soil with high selenium level was much higher than the average level of selenium-enriched area;the content of elements C,N,H and S in HM extracted from soil with high selenium level was obviously higher than that of low selenium level,and which decreased with the decrease of selenium content,besides,the radio of C/N and H/C show the same rules.The H/C of FA and HA in the selenium-enriched soil was slightly lower than that in the control group,and which the C/N ratio was higher than that of the control group.This indicates that the FA and HA extracted from selenium-enriched soil which have a higher unsaturation degree and relatively lower humification degree.(3)The main absorption bands in infrared spectrum of various components of humus extracted from selenium-enriched soil with different selenium levels were 3400,2920,1630,1400,1380,1080cm-1,just the peak intensity were different;.It is indicated that the species of functional groups of humus in soil with different selenium levels are have-OH(individual association)and unsaturated bond.It indicateds that they all have the possibility of combining heavy metals.(4)Humus fractions FA and HA have certain adsorption on different concentrations of heavy metals(Pb,Cd,Cu,Cr),which get the best effect at 3 mg·L-1.And with the increase of FA/HA,the effect of FA/HA extracted from soil with different selenium levels on heavy metal ware enhanced at this concentrations In addition,the adsorption of these heavy metals by FA/HA extracted from soil with high selenium level ware more obvious.The effect of FA/HA on different heavy metals is different,which Cu is most obvious,Pb and Cd secondly,Cr is relatively weak.Besides,the effect of HA on each heavy metal is better than that of FA.(5)With the increase of HM dosage,the leaching concentration of lead and cadmium in the tested soil decreased apparently first and then changed slowly,which indicates that the passivation effect has a certain saturation,and the HM which extracted from soil with high selenium level have a stronger effect.The effect of cadmium by HM in soil is a process from rapid action to slow action,rapid combination and no desorption,and its effect on Pb is slightly higher than that of Cd.Meanwhile,HM extracted from soil with high selenium lever also shows good effects in the passivation time experiment.HM extracted from soil samples with different selenium level have different effects on the speciation of lead,cadmium in tested soil,which showed a marked decline in the proportion of exchangeable states,significantly increase of organic binding states,residual states also increased in varying degrees,and the other forms were not obvious.In the short time of passivation culture,the HM extracted from the soil with high selenium content showed better morphological transformation ability,while with the extend of incubation time,the ability was relatively small(6)The correlation analysis showed that the selenium level was significantly positive correlated with and FA and HA to lead and cadmium and the adsorption rate of Se and Cd adsorption rate of the adsorption rate of FA and HA to lead and cadmium,as well as the reduction rate of Pb leaching concentration in soil by HM.Which was also positively correlated with HA’s adsorption effect on lead and cadmium.It is indicated that the humus extracted from soil with high selenium content has a stronger effect on lead and cadmium,and has more obvious adsorption and passivation effect.The content of lead and cadmium in the rhizosphere and non rhizosphere soil in selenium-enriched area were both positively correlated with the selenium.It reflects the risk of heavy metal pollution in selenium-enriched soil.The content of selenium in soil was negatively correlated with the concentration coefficient(SF)and transfer coefficient(RF)of lead and cadmium in mustard,indicating that selenium in soil has certain inhibitory effect on the accumulation of lead and cadmium in mustard.Effect of each components of humus extracted from selenium-enriched soil pay more obvious influence on the enrichment coefficient of lead and cadmium,and were negatively correlated with SF and RF.It indicate that FA,HA and HM has a certain inhibitory effect on the enrichment of lead and cadmium in mustard root and stems.At the same time,the humus in selenium-enriched area can reduce the activity of lead and cadmium in the soil,in which the effects of FA and HA are great. |
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