Biological Driving Mechanism Of Robinia Pseudoacacia-Rhizobia Symbiotic System On Cadmium-Contaminated Soil Bioremediation

The problem of the mining environment is raised,and the heavy metal accumulated in the soil seriously harms human health and destroys the ecological environment.The remediation of heavy metal contaminated soil,vegetation restoration and the improvement of ecological function of soil in mining area have become the urgent tasks to solve the problems of soil environment.The legume-rhizobium symbiosis system has great application potential in heavy metal contaminated soil remediation,vegetation restoration,nutrient cycling and mining area reconstruction.However,it is not clear that the microbial driving mechanism in heavy metal pollution process.In this study,the symbiotic nitrogen fixation system of Robinia pseudoacacia and Mesorhizobium loti HZ76 was studied.To collect the soil of different cadmium pollution levels in the mine,and to carry out the pot remediation test.The characteristics of biological and non-biological factors in the remediation process were monitored.The dynamic variation of plant biomass,rhizosphere soil nutrient and rhizosphere microbial function group was analyzed.Using a common network to identify potential keystone taxa to explore its role in soil remediation,nutrient circulation,maintenance of ecosystem stability and function.In order to reveal the microbial driving mechanism of rhizobia-legume symbiotic system in the remediation of heavy metal contaminated soil.It also provides important reference for vegetation restoration and degraded soil restoration under the background of heavy metal pollution in mining area.The main research results are as follows:（1）Rhizobium inoculation promoted the growth of R.pseudoacacia,increased the transfer and enrichment of Cd,and changed soil carbon,nitrogen and phosphorus nutrients.There are differences due to the Cd pollution level and remediation time.Compared with high Cd pollution,rhizobium-enhanced R.pseudoacacia had better remediation effect on low Cd pollution soil,especially at the early remediation stage（about 30 days after planting）.（2）The structure and diversity of the microbial community of acacia are mainly affected by the level of cadmium pollution,while the rhizobia is less affected.However,it can enrich beneficial bacteria such as Mesorhizobium（involved in nitrogen fixation and resistant to heavy metals）,Arthrobacter（involved in denitrification）,Brevundimonas（carbon turnover）and Steroidobacteraceae（lignin-degrading）in the rhizosphere.Especially for low Cd pollution soil and remediation early.（3）The structure of the rhizosphere bacteria community was significantly changed by different remediation times.The rhizosphere bacterial co-occurrence network was found that rhizosphere bacterial networks were most complex at 90,60,and 30 days after planting at no,low,and high levels of Cd,respectively.Potential keystone taxa were identified according to within-module connectivities（Zi）and among-module connectivities（Pi）can be used to participate in the rhizosphere nutrient circulation.It was found that Mesorhizobium（30 d and90 d under low and high Cd）,Microvirga（90 d under low Cd）,Rhizobacter and Burkholderia（90 d under high Cd）involved in nitrogen fixation;Niastella（60 and 90 d at high Cd）and Nitrospira（30 and 90 d at high Cd）involved in denitrification.As well as Solirubrobacter（30d at low and high Cd）and Brevundimonas（60 d at low and high Cd）involved in carbon turnover were identified as keystone taxas.（4）The inoculation of rhizobium can improve the nitrogen fixation ability of rhizosphere microorganisms and reduce the nitrification and denitrification ability.The inoculation of rhizobia makes the abundance of N fixation gene（nif H）increased and the abundances of nitrification genes（amo A and amo B）and denitrification genes（nir S,nir K,nos Z,nap A,and nar G）decreased.The study also found that inoculation with rhizobia could increase N₂O emissions.（5）Rhizobium inoculation had little effect on the diversity and assembly process of abundant and rare bacterial communities in the rhizosphere.The assembly process was dominated by random processes.However,differences in community structure and function were found between abundant and rare taxa.The diversity of rare bacterial community in rhizosphere was higher than that of abundant bacterial community.Rare bacteria may regulate rhizosphere nitrogen fixation.Abundant bacterial may regulate the process of NO₃^-assimilation.