Response of abundant and rare microbial taxa to three iron-carbon composite amendments in metal-contaminated agricultural soil

Traditional studies of microbial succession under iron-carbon composite (Fe-C) amendment application have focused on the entire microbial community, with limited attention to the responses and ecological roles of abundant or rare taxa. Herein, a 90-day microcosm incubation was conducted to investigate the effects of three Fe-C amendments, including Fe₃O₄-modified biochar (FeC-B), ferrihydrite-natural humic acid (FeC-N), and ferrihydrite-synthetic humic-like acid (FeC-S), on distribution patterns, assembly processes, and ecological functions of both abundant and rare subcommunities. Our results showed that Fe-C amendments significantly affected the α-diversity of rare taxa, particularly under FeC-B treatment, with minimal impact on abundant taxa. Fe-C amendments also reshaped the community structures of both groups. Rare taxa, representing 63.9 % of Operational Taxonomic Unit (OTU) richness but only 1.6 % of total abundance, played a key role in community diversity and were more susceptible to Fe-C amendments. Certain rare taxa transitioned to abundant status, demonstrating their potential as a microbial seed bank. Abundant taxa were positioned more centrally within the networks, and Fe-C applications promoted cooperative interactions between abundant and rare species. Deterministic processes dominated the assembly of the rare subcommunity, while stochastic processes primarily influenced the abundant bacterial community. Fe-C amendments reduced community differentiation among rare taxa while increasing variability among abundant groups. Functional diversity of rare groups surpassed that of abundant groups, with notable enhancement in nitrogen cycling-related genes under Fe-C treatments. This study highlights the complementary roles of abundant and rare taxa in soil remediation, providing insights for optimizing remediation strategies.