[Effects of Passivators on the Community Structure of Key Functional Microorganisms for Nitrogen Transformation in...

The application of passivators represents a typical remediation strategy for soil heavy metal pollution， a process that not only affects heavy metal bioavailability but also modifies nitrogen transformation processes. However， the influence of passivators on the structure of functional microbial communities governing nitrogen transformation in cadmium （Cd）-contaminated soils remains poorly understood. In this study， typical passivators （lime and biochar） were applied in a pot experiment with rapeseed cultivated in Cd-contaminated soil. High-throughput sequencing and complementary methodologies were employed to investigate shifts in gene abundance and community characteristics of soil nitrogen-fixing bacteria and ammonia-oxidizing microorganisms， along with their driving factors. The results revealed that lime application significantly reduced the Shannon index of nitrogen-fixing bacteria， ammonia-oxidizing archaea （AOA）， and ammonia-oxidizing bacteria （AOB） by 8.62%， 9.11%， and 82.80%， respectively， while increasing their Simpson index by 22.78%， 3.56%， and 55.86%. In contrast， biochar application exclusively reduced the Shannon index of AOB by 80.13% and increased its Simpson index by 71.63%. Lime significantly enhanced AOA amoA gene abundance but reduced AOB amoA gene abundance， whereas biochar induced no significant changes in amoA genes. Lime markedly altered the β-diversity of nitrogen-fixing bacteria， AOA， and AOB， while biochar only significantly affected AOA β-diversity. These findings demonstrate that passivators can restructure soil microbial communities involved in nitrogen transformation， with lime exerting more substantial effects than biochar. Additionally， both passivators significantly modified the relative abundance of dominant microbial groups （e.g.， Proteobacteria and Crenarchaeota） within functional communities. Changes in key nitrogen-transforming microbial communities showed strong correlations with soil physicochemical properties： Nitrogen-fixing bacteria were primarily governed by pH and available Cd （ACd）； AOA by ACd and pH； and AOB by soil organic matter （SOM）， pH， and ACd. Within nitrogen-fixing bacteria， Geobacter exhibited significant positive correlations with ACd， NH₄⁺， and NO₃^- but a negative correlation with pH， while Azohydromonas displayed inverse responses. Among AOA， Crenarchaeota and Thaumarchaeota positively correlated with pH but negatively with ACd， whereas Nitrososphaera showed negative correlations with pH but positive associations with soil total nitrogen （STN）. For AOB， taxa such as β-Proteobacteria demonstrated positive correlations with ACd， NH₄⁺， and NO₃^- but negative correlations with pH. By elucidating how passivators （particularly lime） significantly restructure key microbial consortia involved in soil nitrogen transformation， this study provides a theoretical foundation for understanding the mechanisms through which passivators influence nitrogen cycling processes in Cd-contaminated soils.