Effects Of Several Organic Manures On Bioavailability Of Lead And Cadmium In Gray-purple Soil

The effect of organic manures on the remediation of heavy metal pollution in soils is due to their material composition and nature characteristics,because of the different sources of organic manures,methods of composting,and degrees of decay,the composition and properties of organic manures vary greatly,the effect of heavy metal activitywere also significantly differentin soil.Previously,there have been completely opposite results about remediation of organic manures in heavy metal-contaminated soil.The major reason is that the basic structural components and basic characteristics of different organic manures have not been thoroughly investigated.For this reason,this experiment firstly defined the basic composition and properties of the five organic manures,thena complete combination experimental was designed,comparingthe effects oflead?Pb?and cadmium?Cd?availabilityand the soil pH?DOC?organic matter under the treatment of five organic manures,including nutshell manure?NM?,pig manure?PM?,sludge manure?SM?,humus manure?HM?and earthworm manure?EM?,withdifferent Pb?Cd dosages?adjusted by addition of exogenous Pb/Cd as CH₃COOH?₂Pb?CdCl₂ solution)in soils,and the relationships between soil Pb,Cd availability and chemical speciation transformation and soil pH,DOC,organic matter correlation with different types and added amounts of organic manures was also discussed.The main research results are as follows:1?The basic properties of organic manures and its effect on soil properties:The five organic manures selected for this experiment were weakly acidic,ranging from 6.76 to 7.21.The five organic manures contained high total organic carbon,ranging from 174.25⁴25.57 g/kg,and a DOC content of 0.66 to 1.51g/kg.The determination of the HA and FA components found that the HA component content of each organic manures is higher than that of FA;Organic manures are similar in the composition of oxygen-containing groups such as humic substances and carboxycarbonyl groups,but their contents are different,and the content of FA components is significantly higher than that of HA components,after being applied to soil,it can affect the complexation/adsorption effect of heavy metal ions by changing the soil pH,DOC,and organic matter content,thereby affecting the transformation process of heavy metal forms in soil,and further manifesting its effect on soil heavy metals bioavailability.This study shows that adding organic manures significantly increases the pH,DOC,and organic matter content of soils contaminated with Pb and Cd.The availability of soil Pb and Cd is inhibited by the increase of soil pH and the increase of organic matter,while the increase of DOC content is significant promote.The degree of humification and aromatization of organic manures is also one of the key factors that determine the availability of soil heavy metals Pb and Cd.The size of HA/FA in the organic manures composition can also affect the availability of soil Pb and Cd:The HA/FA ratio of SM and PM is more than 3,which has a significant inhibitory effect on the availability of soil Pb and Cd.2.Effect of organic manures on Pb availability in contaminated soils:?1?The addition of organic manures reduced the contents of available Pb in soils with mild,moderate,and severe pollution of Pb,and the decline degree increased with increase of amount of organic manures which is particularly obvious in soil with moderate and severe contaminated Pb.SM and PM treatment showed the most prominent effects among the five organic fertilizer where the available Pb contents in soils were respectively decreased by 24.22%³6.10%and 19.33%³5.48%as compared to the control group.?2?After adding organic manures,the change trends of contents of available Pb in soils with time showed increase firstly and then decrease until a peak value achieved in15 days,following with gradual decline and finally approaching the stability in 6o days.The dynamics of Pb availability variation were consistent with the dynamics of Pb chemical speciation.?3?After the treatment of organic manures,the contention of exchangeable Pb?Carb-Pb?decreased by 5.36%¹2.91%while those bounded by organic matter?OM-Pb?increased slightly,and those of residual Pb?Res-Pb?increased significantly by 18.92%⁴5.47%,for mild contaminated soil;for moderate contaminated soil,the contention of Carb-Pb slightly decreased while those of Res-Pb substantially increased by 24.30%³8.04%;for severe contaminated soil,the contention of Carb-Pb did not change significantly,and those of OM-Pb and Res-Pb increased apparently,respectively raised by 14.46%⁴1.06%and 29.02%⁵6.29%.?4?The Carb-Pb had a significant positive contribution to available Pb in soil,while OM-Pb and Res-Pb demonstrated obvious negative contributions.The organic manures reduced significantly the availability of Pb in soil mainly by decreasing the contention of Carb-Pb and increasing those of OM-Pb and Res-Pb.3.Effect of organic manures on Cd availability in contaminated soils:?1?In mild Cd contaminated soil,The application of low-level?<4 g/kg?organic manures increased the availability of soil Cd,The availability Cd contention increased by 16.67%to 50.00%compared with the control,The availability Cd contention reduced only at high dosages?up to 8 g/kg?,the decrease was 4.17%²0.83%;In moderate Cd contaminated soils,all organic manures significantly reduced the availability Cd,and with the increase in the amount of increase increases,the availability Cd contention was reduced by 9.09%to 47.27%compared with the control,the NM was the most significant;In severe Cd-contaminated soil,only SM,HM and EM could significantly reduce the soil availability Cd contention.?2?After the Application of organic manures,the availability Cd contention in soil decreased firstly and then increased over time,reaching the lowest value after 15days and stabilized after 60 days.Among the treatment groups,EM had the best inhibition effect on soil Cd activity at low dosage;NM at the medium and high dosage had the best inhibition effect.?3?After treatment with organic manures,in mild Cd contaminated soil,the Exe-Cd content increased by 6.17%to 20.28%,the FeMnOx-Cd content slightly increased,the Res-Cd content decreased by 3.19%¹4.3%;In moderate Cd-contaminated soil,the Exe-Cd content decreased by 7.12%to 28.5%,Res-Cd content increased significantly from 19.74%to 65.81%;In severe Cd-contaminated soils,the Exe-Cd content decreased by 7.25%to 35.54%,and the FeMnOx-Cd and Res-Cd contents all increased slightly.?4?Exe-Cd and FeMnOx-Cd in soil had a significant positive contribution to soil available Cd and Res-Cd showed a significant negative contribution.In mild Cd contaminated soil,organic manure mainly promoted the conversion of Cd to Exe-Cd and significantly reduced the content of Res-Cd,thus increasing the availability of Cd.In medium Cd contaminated soil,Organic manure significantly decreased the Cd availability by decreasing the content of Exe-Cd and FeMnOx-Cd and significantly increasing the content of Res-Cd.In severe Cd-contaminated soils,organic manures decreased the contention of availability Cd by decreasing the content of Exe-Cd and slightly increasing the content of Res-Cd.