| At present,heavy metal pollution is getting more pervasive in farmland soils in China.Cadmium(Cd)is the most common soil pollution in farmland and the most contaminated heavy metal element.With abundant organic material resources in China,it is possible to apply organic materials to reduce the damage of Cd in farmland soil.This way is not only suitable for China’s national development,but also a fit match to requirements for sustainable agricultural development.In this study,paddy soil was chosen as the research object.The silkworm excrement organic fertilizer(CS)which came from Zhejiang(Z),Jiangsu(J),Yunnan(Y)and Guangxi(G)was used as the material.In addition,chicken manure(JF)was used as the reference.Elemental analysis,infrared spectroscopy,dissolved organic matter(DOM)content,humic acid(HA)and fulvic acid(FA)content,and indole acetic acid(IAA)content were first characterized and then passed through indoor soil cultivation experiment.The experiment was conducted to study the effects of CS from different sources on soil available Cd content under different culture time(7d,14d,30d and 60d),and finally the mechanism of C.S passivation repair from different sources was discussed based on the analyses of soil pH,organic matter content,CEC,HA/FA,HA and FA optical density(E4/E6),DOM content and soil Cd mineral phase composition.It attempts to find out the mechanism of how CS passivation repair works on Cd contaminated soil.The main findings are as follows:(1)The carbon(C),nitrogen(N)and sulfur(S)element content,DOM content,HA content,FA content and IAA content of CS are higher than JF;CS and JF both contain hydroxyl(-OH),sulfhydryl(-SH)and carboxyl(-COOH).In addition,by infrared spectroscopy and H/C ratio analysis,the degree of aromaticity and hydroxyl(-OH)content of CS are higher than those of JF.(2)At the end of the culture(60d),applying 3%(W/W)of CS from different sources significantly reduced the soil available Cd content compared with the blank control(CK),and the effect of using CS was better than the application of same amount(3%)of JF.The reduction of available Cd content in soils treated with CS from different sources ranged from 23.81%to 38.09%,while JF decreased by only 9.52%.(3)Soil pH,CEC,organic matter content,HA/FA and DOM content increased under different treatments.At the end of the culture,compared with CK,the increase range of the pH value and HA/FA under CS(from different sources)treatment was between 1.12-2.15 and 1.20-2.64 units respectively.While the increase range of organic matter content,CEC and DOM content was between 9.00～17.50 g·kg-1,1.69-2.98 cmol·kg-1 and 0.15～1.46 g·kg-1.The increase range of each index of JF treatment was lower than that of CS treatment.The E4/E6 of CS treatment of soil HA and FA decreased,and the E4/E6 reduction of soil HA and FA was between 0.37～1.12 and 0.47～0.64 units respectively.Meanwhile,The E4/E6 of JF treatment reduced by 0.29 and 0.40 units.(4)The increase of soil pH,organic matter content,CEC and HA/FA content is the main mechanism for reducing the effective Cd content in soil.The pH value is significantly negatively correlated with the content of available Cd;the organic matter content and CEC are negatively correlated with the content of available Cd respectively;the HA/FA is significantly negatively correlated with the content of available Cd.(5)According to the XRD spectrum analysis,at the end of the culture,Cd is mainly fixed in the soil in the form of mineral phases such as CdS,CdCO3 and Cd(OH)NO3.Apart from the results that the content of CdCO3 treated by YCS and JF is not much different,the content of CdS,CdCO3 and Cd(OH)NO3 treated by CS is higher than that of JF.In summary,silkworm excrement organic fertilizer can significantly reduce the availability of Cd in contaminated paddy soil,and its effect is better than chicken manure.The main mechanism of reducing available Cd content is due to the increase of soil pH,CEC,organic matter content and HA/FA. |
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