| The problem of soil Cd pollution in cultivated land was becoming increasingly prominent,especially the continuous enrichment and accumulation of Cd in crops such as rice,corn,and vegetables,posing a serious threat to the ecosystem and human health.Microbial induced carbonate precipitation（MICP）was a green,friendly,low-cost and highly sustainable remediation technology for remediation of Cd contaminated cultivated soil.However,there were problems with this technology,such as the susceptibility of exogenous strains to their activities and the poor stability of the repaired soil.Therefore,this paper proposes a hypothesis that using biochar to enhance the remediation effect of microorganisms on Cd contaminated farmland soil mineralization.The effects of biochar on the growth,Cd resistance,cadmium carbonate precipitation induction of Comamonas testosteroni ZG2,which could induce cadmium carbonate precipitation,and the effects of cell cells and fermentation broth on mineralization and remediation of Cd contaminated farmland soil were studied.The main results achieved are as follows:（1）When biochar was present in the growth process of strain ZG2,it not only did not affect the activity of strain ZG2,but also improved the growth activity of strain ZG2;under Cd stress,biochar could improve the resistance of strain ZG2 to Cd,reduce the toxicity of Cd to strain ZG2,and maintain the original shape of the cell of strain ZG2 without being affected,even in a severe Cd environment where strain ZG2 cannot grow,biochar provided a reproductive and sheltering place for the bacterial cells,allowing them to adhere to the surface for growth and maintain activity;the Cd adsorbed by biochar did not affect the induced formation of cadmium carbonate precipitation,the presence of biochar in the fermentation broth formation process and the presence of biochar in the precipitation formation process did not affect the formation of cadmium carbonate precipitation induced by strain ZG2,and had the same morphology as the cadmium carbonate precipitate induced by a single strain ZG2,both of which were irregularly stacked particles;XRD analysis further confirmed that the presence of biochar did not affect the induction of cadmium carbonate crystals,and its chemical formula is Cd CO3;cadmium carbonate precipitation induced by a single strain ZG2,cadmium carbonate precipitation induced by strain ZG2 on Cd adsorbed by biochar,the presence of biochar in the process of fermentation broth formation and precipitation formation was induced by strain ZG2,and cadmium carbonate precipitations were added to the soil for acidification and freeze-thaw.Acidification increased the available Cd content by16.07%,11.03%,9.45%and7.33%,respectively.Freezing and thawing increased the available Cd content by 13.07%,10.17%,9.57%,and 7.45%,respectively,ten freeze-thaw cycles increased the available Cd content by 17.56%,14.03%,10.92%and 8.15%,respectively,the stability of cadmium carbonate precipitation induced by the presence of biochar in soil were better than that induced by the absence of biochar in soil,moreover,the stability of cadmium carbonate precipitation induced by strain ZG2 during the formation process of biochar was the best.（2）Compared with the control,the effects of biochar and bacterial cells ZG2 had different degrees of remediation effects on Cd-contaminated soil.Among them,the combined repair effect of biochar and bacterial cells ZG2 were the most significant,compared with the control,the fresh weight,dry weight,plant height,and main root length of pakchoi increased by 1.97 times,2.76times,1.67 times,and 2.36 times,respectively;the total root length,root surface area,and root volume of pakchoi were increased by 4.41,5.11,and 5.63 times,respectively,and the root diameter was increased by 39.29%;the content of Cd in the edible part of pakchoi and the content of available Cd in the rhizosphere soil were decreased by 69.96%and 65.29%,respectively;the proportion of exchangeable Cd to carbonate bound Cd and residual Cd in rhizosphere soil was increased,and the ratio of carbonate bound Cd was increased by 49.03%;it significantly increased the number of OTUs of bacteria in the rhizosphere soil,increased the richness and diversity of dominant bacterial communities,improved the microbial community structure of Cd-contaminated soil,and significantly increased the relative abundance of Comamonas.（3）Biochar had a significant promoting effect on the mineralization and remediation of Cd-contaminated soil by strain ZG2 fermentation broth.Compared with the control,the order of soil remediation effect was:adding biochar first and then adding fermentation broth for mineralization remediation>adding fermentation broth first for mineralization and then adding biochar for remediation>only adding fermentation broth for remediation>only adding biochar for remediation.The available Cd content of the soil repaired by the above four methods decreased by 37.28%,47.95%,73.92%,and 78.36%,respectively,while the carbonate bound Cd increased by 37.96%,47.98%,67.97%,and 79.73%,respectively;after soil acidification,the available Cd content increased by 9.79%,8.41%,6.85%,and 3.99%,respectively,while the carbonate bound Cd decreased by 9.51%,7.79%,5.49%,and 3.58%,respectively;after 5 freeze-thaw cycles,the available Cd content of the soil repaired by four methods increased by 9.07%,8.41%,7.28%,and5.19%,respectively,after 10 freeze-thaw cycles,the available Cd content increased by 11.15%,9.91%,8.42%,and 5.88%,respectively;The above four methods of soil remediation significantly reduced the absorption of Cd by pakchoi,among them,adding biochar to soil for remediation after a period of time and then adding strain ZG2 fermentation broth had the best mineralization remediation effect on Cd-contaminated soil,the content of available Cd in soil was reduced by 78.36%,and the combined carbonate content was increased by 79.73%;the biomass,plant height,taproot length,and chlorophyll SPAD value of pakchoi were significantly increased by 393.02%、194.33%、270.44%,and 76.36%,respectively;the content of Cd in shoot and root of pakchoi decreased by 84.60%and 72.68%,respectively;the distribution ratio of Cd in the organelle components of the leaves of pakchoi decreased by 27.44%;the content of available Cd in rhizosphere soil decreased by 76.86%;the ratio of exchangeable Cd in rhizosphere soil was reduced,and the distribution ratio of carbonate bound Cd and residual Cd in rhizosphere soil was increased,resulting in a 71.25%increase in carbonate bound Cd.The above results indicated that biochar could enhance the resistance of bacterial cells to Cd,the presence of biochar did not affect the induction of cadmium carbonate precipitation by bacterial cells,both bacterial cells and fermentation broth had good remediation effects on Cd-contaminated soil,and the latter was better than the former,and biochar could enhance the stability of cadmium carbonate precipitation induced by strain ZG2,the best repair method was to first add biochar and then add fermentation broth for repair,this made the cadmium carbonate precipitation induced by strain ZG2 more stable,and pakchoi suffered the least Cd stress and accumulates the least Cd content.This indicates that biochar had important application potential in improving the remediation of Cd contaminated farmland soil by strain ZG2. |
