| The problem of soil heavy metal pollution is more and more serious.Excessive heavy metals in the soil will not only reduce the soil quality and growth of crops,but also indirectly enter the water and atmosphere and cause environment pollution,even enter the human body through the food chain,causing irreversible harm to human health.Phytoremediation is the most widely used remediation technology for soil heavy metal pollution due to its environmental protection and low cost.However,its efficiency is limited due to the small biomass and long growth cycle of the plants.Therefore,improve plant biomass is the key to ensure the efficiency of it.Microbial-plant combined remediation technology has aroused great interest of researchers.However,as one of the three major microbial groups in soil,there are few reports on the remediation of soil heavy metal pollution by bacteria.In this study,cadmium and zinc contaminated soil was taken as the test soil in Baoji City,Shaanxi Province,Brassica juncea as the test plant.Through soil culture test and pot experiment,the effects of different concentrations(10~5,10~7,10~9cfu/m L)on soil physicochemical properties,heavy metal bioavailability,enzyme activity,plant biomass,chlorophyll content,antioxidant properties and heavy metal enrichment of Lactobacillus casei,Bacillus licheniformis and Bacillus megaterium were investigated in order to provide a theory for the remediation of Cd and Zn combined contaminated soil by Brassica juncea,and to provide a scientific reference for the combined remediation of microorganism and plant.The main research results are as follows:(1)The effects of different Lactobacillus casei,Bacillus licheniformis and Bacillus megaterium on soil physicochemical properties,availability of heavy metals and enzyme activities with time were studied through soil culture experiments.The results showed that the p H value of soil decreased first,then increased and then decreased with the change of time after adding bacteria,while the change of EC value was the opposite,and the p H value reached the minimum at the end of 60 days,which decreased by 0.11?0.28 units compared with control.The available state contents of Cd and Zn increased firstly and then decreased with time,and the available state contents of Cd and Zn increased by 16-42%and 15-28%compared with the control,respectively,indicating that bacteria can activate heavy metals and improve the bioavailability of heavy metals in the soil.The trend of soil enzyme activities varied with time,but the bacteria all increased the enzyme activities,and the promotion effect on urease was stronger than that of alkaline phosphatase and sucrase.(2)The effects of different Lactobacillus casei,Bacillus licheniformis and Bacillus megaterium on biomass,heavy metal enrichment characteristics,chlorophyll content and antioxidant characteristics of Brassica juncea were studied by pot experiment.The results showed that the biomass increased by 1.21,1.14 and 0.71g compared with the control,respectively.The bacteria promoted the accumulation of Cd and Zn in the aboveground part and underground part of Brassica juncea.The content of Cd in aboveground part of Brassica juncea was 1.42-1.94,1.62-1.77 and 1.79-1.89 times of the control,respectively.Zn content was 1.58?1.86,1.72?1.75,1.41?1.82 times that of the control,respectively.The maximum Cd content in underground part was 1.46,1.64,1.23 times that of the control,and the maximum Zn content was 1.74,1.58,1.66 times that of the control,respectively.The addition of bacteria increased the content of chlorophyll,enhanced the activities of SOD,POD,CAT and other antioxidant enzymes,reduced the toxicity of reactive oxygen species(MDA)produced under the stress of Cd and Zn,and enhanced the plant resistance.(3)The effects of different Lactobacillus casei,Bacillus licheniformis and Bacillus megaterium on physicochemical properties,availability of heavy metals and enzyme activities of potted soil were studied by pot experiment.The results showed that adding bacteria could reduce the p H value of soil.The available state contents of Cd and Zn in the soil were significantly increased,and the available state contents of Cd and Zn were increased by 16?58%,54?65%and 46?57%,respectively,and the available state contents of Zn were increased by 8?38%,51?66%and 56?62%,respectively,compared with the control.Soil enzyme activities were significantly increased,alkaline phosphatase was1.14-1.21,1.02-1.10,1.02-1.15 times of that of control,and sucrase was 1.37-1.69,1.06-1.09,1.20-1.29 times of that of control,respectively.Urease was 1.10?1.21,1.04?1.09,1.04?1.15 times of the control,respectively.In conclusion,the three kinds of bacteria all had the strengthening effects on the remediation of Cd and Zn combined contaminated soil by Brassica juncea,and the strengthening effect was shown as Lactobacillus casei>Bacillus licheniformis>Bacillus megaterium,Lactobacillus casei had the best effect when the concentration was 10~5cfu/m L,while Bacillus licheniformis and Bacillus megaterium had the best effect when the concentration was 10~9cfu/m L.Therefore,all three kinds of bacteria are potential fortifiers,which can improve the remediation efficiency of Brassica juncea in the Cd and Zn compound contaminated soil. |
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