Remediation Potential Of Tobacco To Cadmium Polluted Farmland And Mechanism Of Cadmium Enrichment

At present,a part of farmland soils in China are faced with serious heavy metal pollution problem,and cadmium is the heavy metal with the highest over standard rate.Therefore,it is urgent to repair the cadmium contaminated soil.The most promising technology is phytoremediation technology.However,it is found that the biomass of cadmium enriched plants is small and the environmental adaptability is poor,which can not meet the needs of cadmium contaminated soil remediation.Tobacco has a strong ability of cadmium enrichment,large biomass and strong environmental adaptability,and is expected to become an ideal repair plant.In this study,the accumulation characteristics and mechanism of cadmium in tobacco were analyzed through hydroponics and pot experiments,and the enhancement effect of the rhizosphere of plants growth promoting bacteria on cadmium accumulation in tobacco was also analyzed.The feasibility of tobacco remediation technology was verified through field experiments.The main results are as follows:(1)The effects of foliar application of cadmium on tobacco growth and cadmium accumulation were studied by hydroponics.The results showed that with the increase of cadmium concentration(60-120 mg/L),the biomass of tobacco was smaller,and the biomass of tobacco leaves,stems and roots was the smallest under high cadmium treatment,reaching 3.35 g、0.88 g and 0.51 g respectively.Compared with the control,it decreased by 50.2%,、40.9%and 20.3%respectively.From the accumulation site of cadmium,the content of cadmium in leaves was the highest,followed by that in roots and stems,indicating that tobacco leaves could absorb cadmium,but their downward transport ability was weak.In addition,cadmium was detected from the hydroponic solution,indicating that tobacco may discharge cadmium through roots.(2)Using pot experiments to study the cadmium accumulation capacity,distribution characteristics and rhizosphere microbial community structure of tobacco under different cadmium pollution levels.The results showed that tobacco had a strong ability to accumulate cadmium,and the cadmium content in plant increased with the increase of cadmium content in soil.Under high cadmium treatment(the cadmium content in the soil is 5.8 mg/kg),the cadmium content in the aboveground and underground of tobacco was 68.1 and 40.8 mg/kg,respectively,and the enrichment coefficients were 12.6 and 7.0,respectively.From the storage site of cadmium,the content of cadmium in all treated tobacco was aboveground>underground;at the same time,it was found that the pH value of rhizosphere soil was lower than that of non rhizosphere,while the content of available cadmium was higher than that of non rhizosphere.Therefore,it was speculated that tobacco roots reduced the pH value of soil by secreting organic acids,thus improving the effectiveness of cadmium in soil.High-throughput sequencing was used to further study the microbial community structure in tobacco planting soil.The results showed that the microbial diversity of tobacco planting soil increased with the increase of cadmium pollution.Compared with non rhizosphere soil,the abundance of unidentified acidobacter and unidentified gamma proteobacter in rhizosphere soil increased,while the abundance of methylenetera,Lysobacter and Sphingomonas decreased.The change of microbial community structure may be an important reason for cadmium enrichment in tobacco.(3)The remediation potential of tobacco to cadmium contaminated soil was evaluated by field experiment.The results showed that tobacco grew rapidly and its biomass(dry weight)reached 639.7 g/plant in 80 days.Tobacco has a strong ability of cadmium enrichment,and the enrichment coefficient of different parts is between 4.03-14.8.From the point of view of cadmium storage site,the cadmium content in tobacco in different periods is leaf>stem>root,the transport coefficient from root to stem is between 0.81-0.89,and the transport coefficient from stem to leaf is between 2.74-3.12;from the view of removal rate,the apparent removal rate of cadmium in tobacco is up to 17.6%in 80 days,so tobacco is considered to be an ideal soil remediation plant for cadmium pollution.(4)Using pot experiment to study the effect of plant growth promoting rhizobacteria on the cadmium accumulation ability of tobacco.The results showed that four rhizosphere growth promoting bacteria(Bacillus gelatinosus,Bacillus megaterium,youyijun,photosynthetic bacteria)all promoted the content of cadmium in the aboveground and underground of tobacco,and increased the biomass of tobacco,among which photosynthetic bacteria was the best.The application of photosynthetic bacteria increased the cadmium content in the aboveground and underground of tobacco by 36.4%and 52.4%,the total cadmium content by 191.2%,and the dry quality of the upper and lower parts of tobacco by 100.0%.Further analysis showed that photosynthetic bacteria increased the content of available cadmium in rhizosphere soil,the content of chlorophyll and net photosynthetic rate of tobacco,and the content of organic matter,available phosphorus and total nitrogen in soil.Therefore,the application of photosynthetic bacteria is expected to be an effective measure to strengthen tobacco remediation of cadmium contaminated soil.