Study On The Influence Of Different Plant Rhizosphere DOM To Cd In Soil Passivated With Nano- Black Carbon

Soil pollution of cropland in China is dominated by heavy metal pollution,among which cadmium(Cd)is the major pollutant since Cd content of 7.0% soil samples are higher than standard.From the perspective of pollution degree,the soil pollution feature in China's farmland is slight and mild.In view of this pollution characteristic,the use of oxidized modified black carbon(MBC)in situ to immobilize heavy metal Cd in soil has become a main focus of research.Modified black carbon can inactivate Cd in soil and its passivation effect is influenced by soil pH,Eh,microorganisms,root exudates,and dissolved organic matter,et al.The effect of small molecule dissolved organic matter(DOM)on Cd can't be underestimated.So studying the effect of rhizosphere DOM on efficiency of Cd adsorbed by MBC can provide a theoretical basis for the application of MBC.In this paper,pot experiments were conducted.The effects of MBC on plant in soil remediation applications were illustrated by comparing the effects of MBC on the growth and cell ultrastructure of two plants.The immobilization effects of MBC were clarified by comparing the changes of available Cd content in the rhizosphere/nonrhizosphere of the two plants.The influence of rhizosphere DOM on Cd immobilized by MBC was analyzed by extracting the rhizosphere/non-rhizosphere DOM of two plants,characterizing the DOM content by DOC,and analyzing the DOM composition by three-dimensional fluorescence spectroscopy,and comparing the difference of DOM content between rhizosphere and non-rhizosphere of the two plants.The DOM content in MBC,the total amount of Cd adsorbed by MBC and the correlation between rhizosphere DOM and rhizosphere available Cd concentration were analyzed to investigate the mechanism of rhizospheric DOM on Cd adsorbed by MBC.The main findings of this study are as follows:(1)Application of MBC in Cd-contaminated soil can promote the growth of plants.After MBC was added to soil,the underground dry weight of ryegrass(Lolium perenne L.)increased by 60.13% and the aboveground part increased by 96.40% while the under or over ground dry weight of Beta vulgris var.cicla L.only increased by 4.73% and 28.07% separately after 50 days cultivation.It indicates that the MBC has a more pronounced promoting effect on the growth of the tolerant plant ryegrass.In addition,MBC can also protect plant cells and reduce the damage of Cd to organelles in plant root tip cells and leaf cells.(2)MBC can reduce the content of available Cd in the soil planting ryegrass(heavy metal tolerance)and Beta vulgris var.cicla L.(heavy metal hyper-accumulation).For ryegrass,MBC and the root system together contribute to the available Cd content in the soil,so that the difference of available Cd content between rhizosphere and nonrhizosphere surroundings treated with MBC is not apparent.For the Beta,the difference in Cd content between rhizosphere and non-rhizosphere was increased due to the application of MBC,indicating that MBC can highlight the effect of the root system of Beta on the rhizosphere surroundings.(3)MBC will adsorb DOM in the rhizosphere/non-rhizosphere of ryegrass and Beta at the same time as inactivation Cd,resulting in a decrease of DOM content in the soil treated with MBC.For heavy metal tolerant plant ryegrass,the content of DOM in the rhizosphere without MBC treatment decreased over time.For the hyperaccumulator Beta,the content of DOM in the rhizosphere without MBC treatment first decreased then increased toward to be stable.The reason for this difference is that the organic acids secreted by the two plant roots are different.(4)EEM spectra show that the main fluorescent substances of DOM in the rhizosphere/non-rhizosphere of Ryegrass and Beta are fulvic acids,humic acids and protein-like.In the treatment without MBC,the most intense fluorescence was fulvic acids.In the treatment with MBC,the fluorescence intensity of each component was significantly reduced,and the fluorescence peak of protein-like protein was the highest.(5)The adsorption rates of Cd in different treatments of MBC are as follows: ryegrass rhizosphere-MBC(6.67)> soil-MBC(6.17)> Beta rhizosphere-MBC(6.14).Indicate that ryegrass promoted Cd adsorption on MBC,while Beta is a competitive relationship with MBC to adsorb Cd.The decrease rates of DOM adsorption in different MBC treatments are as follows: ryegrass rhizosphere-MBC(2.65)> Beta rhizosphereMBC(2.48)> soil-MBC(2.26).It indicates that ryegrass and Beta could promote MBC preferential adsorbing Cd.(6)The effect mechanisms of rhizospheric DOM on the adsorption of Cd on MBC in soil remain following aspects:(a)DOM adsorption on MBC affects the adsorption of MB on Cd.(b)DOM forms complexes with Cd in soil,and DOM-Cd complexes can adsorb on MBC.(c)The adsorption of Cd on MBC was the result of the combined action of plants,soil,DOM,Cd,and MBC.