Stability And Mechanisms Of Synergistic Effects Of Plant Growth-promoting Bacteria And Biochar On Lead And Cadmium Accumulation Of Vegetables

Vegetables are an important component of human food and the quality of vegetables is very important for human health.Vegetables growing on metal-contaminated soils may accumulate metals and pose health problems for humans or animals.Calcium superphosphate and biochar have been used for decreasing heavy metal transfer to food chain through plant accumulation by adsorption,complexation,and co-precipitation.The plant growth-promoting and metal-tolerant bacteria increase plant growth and reduce metal uptake or translocation to above-ground tissues of plants by reducing the availability of soil heavy metals.PGPB-assisted soil immobilizing agents is a practical strategy for the remediation of heavy metal-contaminated soils and the safe production of vegetables.We analyzed the impacts of heavy metal-immobilizing Bacillus megaterium H3,the immobilizers(calcium superphosphate and biochar)and their combination on the growth and Cd and Pb accumulation of two vegetables(Brassica rapa L.var.Shanghaiqing and Brassica campestris L.var.Aijiaohuang)grown in the heavy metal-contaminated soils and then analyzed the available Cd and Pb and organic matter contents in the soils.The results showed that different treatments could increase the yield of the two vegetables(27.3-196.4%and 26.2-167.1%for shanghaiqing and aijiaohuang,respectively);while reducing the Cd(18.3-51.5%and 12.8-39.7%for shanghaiqing and aijiaohuang,respectively)and Pb(17.6-54.3%and 18.0-55.0%for shanghaiqing and aijiaohuang,respectively)concentrations of edible tissues of vegetables,compared to the controls.Different treatments significantly reduced DTPA-extractable Cd and Pb contents of the rhizosphere soils of the two vegetables compared with the controls.Notably,the combined application of strain H3 and biochar was the best in reducing Cd and Pb uptake of the vegetables.To get a better combination of reducing heavy metals in vegetables,the effects of strains H3,CL-1 and biochar on the water-soluble Cd and Pb concentrations and pH of the culture medium were studied.The results showed that the treatment of biochar+H3+CL-1 has the best Cd and Pb adsorption,under the conditions of low(Cd 1 mg L-1,Pb 5 mg L-1)and high(Cd 3 mg L-1+Pb 10 mg L-1)concentrations of Cd and Pb.At low concentrations of Cd and Pb,the treatment of biochar+H3+CL-1 could reduce water-soluble Cd(81.9%)and Pb(59.0%)concentrations.At high concentrations of Cd and Pb,the treatment of biochar+H3+CL-1 could reduce water-soluble Cd(62.4%)and Pb(57.6%)concentrations.These strains combined with biochar could increase solution pH from 7.10 to 9.17.Three pot experiments were conducted to study the effects of the treatment of biochar+H3+CL-1 on the growth and heavy metal uptake of four vegetables(Brassica rapa L.var.Gaogengbai,Chinese cabbage,round white radish and Yanghua radish).The results showed that the treatment of biochar+H3+CL-1 significantly increased the yield of vegetables(First:13.1-39.2%,second:13.0-31.8%,third:11.0-34.0%)and reduced the accumulation of Cd(First:13.4-61.1%,second:19.0-64.2%,third:24.5-61.9%)and Pb(First:22.6-44.0%,second:26.8-51.4%,third:27.1-55.2%)compared to the controls.After the second and third planting,the treatment of biochar+H3+CL-1 could reduce the contents of Cd and Pb of Chinese cabbage and meet the national food safety standards in Cd and Pb(Cd 0.5,Pb 97.2 mg kg-1)-polluted soil.The result also showed that strains H3-gfp and CL-1-rfp could stably colonize the roots of the vegetables.The treatment of biochar+H3+CL-1 significantly increased the pH(0.17-0.45)and organic matter content(12.6-38.7%)in the rhizosphere soils compared to the controls.The acid exchangeable Cd and Pb contents in the rhizosphere soils were significantly decreased by 14.3-55.6%and 16.2-47.3%,respectively in the presence of biochar+H3+CL-1 compared to the controls.The easily reducible Cd and Pb contents in the rhizosphere soils were significantly increased by 50.0-157%and 5.2-19.2%in the presence of biochar+H3+CL-1 compared to the controls.Treatment of biochar+H3+CL-1 significantly increased the content of amorphous Fe oxide(11.6-23.5%)of the rhizosphere soils of Yanghua radish and Chinese cabbage compared to the controls.Further studies found that the treatment of biochar+H3+CL-1 significantly increased the relative abundance of the gene copies of Fe oxidizing Leptothrix spp.(24.3-175.2%)and Gallionella spp.(12.5-118.6%)species,which may affect the morphological distribution of Cd and Pb in the soil and reduce the contents of available Cd and Pb and their uptake of the vegetables.