Enhancement Of Tobacco-phytoremediation Of Cadmium Contaminated Soils By Sulfur Application And Its Mechanisms

With the development of industry,the problem of farmland soil pollution is becoming more and more serious,especially the problem of heavy metal pollution.According to survey,the total over-standard rate of cultivated land soil pollution in China is 19.4%,and cadmium pollution is most serious.Soil contaminated by cadmium will enter the human body through biological enrichment,thus endangering human health.At present,phytoremediation technology of cadmium contaminated soil is a research hotspot.Some studies have found that tobacco has a strong cadmium enrichment capacity,large biomass and extensive growth,which is expected to be used for remediation of cadmium contaminated farmland soil.In this study,tobacco varieties with high cadmium enrichment capacity were screened,and the mechanism of enhancing cadmium enrichment by application of sulfur was evaluated.The main results were as follows:(1)The cadmium bioaccumulation capabilities of seven common tobacco varieties((Yunyan 87,Zhongyan 100,Yueyan 96,Changbohuang,K326,8326,and NC628)were determined using pot experiment.The results showed that the biomass of root,stem and leaf of "Changbohuang" was the largest.The Cd concentrations in "Yunyan 87" leaves,"Changbohuang" stems,and "8326" roots were the highest compareing with corresponding values,reaching 64.67 mg/kg,27.1 mg/kg,and 23.75 mg/kg,respectively.Regarding the Cd concentration in different organs,it was highest in leaves,followed by stems and roots.The highest removal rate of Cd(3.48%)was observed in “Changbohuang”.The organic acids in rhizosphere soils were analyzed using HPLC,and the results showed that oxalic acid was secreted by all the tobacco varieties.This work indicated that tobacco,especially "Changbohuang" variety,is potential in the remediation of Cd-polluted soil.(2)The variety "Changbohuang" was found best in bioaccumulation Cd in the former work.In this work,sulfur was applied to strengthen tobacco enrichment ability,and its mechanism was clarified.The results showed that the cadmium content of tobacco leaves,roots and stems reached 66.3 mg/kg,42.5 mg/kg and 46.1 mg/kg respectively in high sulfur treatment,which was significantly higher than that without sulfur application,and did not affect the normal growth and development of tobacco.By analyzing the contents of total sulfur,non-protein sulfhydryl compounds(NPT)and total glutathione(T-GSH)in tobacco,it was found that the contents of total sulfur in tobacco increased significantly under high sulfur treatment,while the contents of NPT did not change significantly,and the contents of total glutathione(T-GSH)increased significantly.Therefore,it was found that sulfur application could affect the absorption and accumulation of cadmium in tobacco,but it was related to the amount of sulfur application.Only when the amount of sulfur application was high,it could effectively promote the absorption of cadmium in tobacco.The increase of glutathione content in tobacco after sulfur application may be one of the reasons for promoting the uptake and accumulation of cadmium in tobacco.(3)High-throughput sequencing and fluorescence quantitative PCR were used to further study the mechanism of sulfur regulating the accumulation of cadmium in tobacco rhizosphere microorganisms.The results showed that OTU1718 and OTU1216 gradually became the two OUTs with the greatest abundances with the increase of sulfur content.OTU1718 and OTU1216 belonged to Thiobacillus sp.At the generic level,the abundances of Thiobacillus reached 6.3% and 11.54% respectively in high and low sulfur treatments.Both high and low sulfur treatments significantly increased the copy numbers of dsr A and sox B genes,but not sqr gene copies.It was found that the p H of rhizosphere soil and available cadmium decreased and the content of available cadmium increased after sulfur application.Therefore,it is speculated that Thiobacillus bacteria and sox B and dsr A genes play a major role in the process of sulfur oxidation.Moderate sulfur application can reduce soil p H value and improve the bioavailability of cadmium in soil through sulfur oxidation,which may be one of the reasons for promoting the uptake of cadmium by tobacco.Sulfur application has little effect on the diversity and structure of soil bacterial community,and will not accumulate in the soil for a long time.It is a relatively effective and safe phytoremediation strengthening measure.