Porewater iron dynamics drive microbial metabolic stratification and carbon transformation in a subtropical sphagnum multifibrosum peatland

Abstract

Peatlands are the densest terrestrial carbon stocks, and iron is a key redox-active element that strongly influences microbial metabolism and carbon preservation. However, the role of microbial metabolic stratification and its coupling with geochemical factors, such as iron in governing carbon stability remain poorly understood. Here, we employed a metagenomic approach to investigate the vertical distribution of microbial communities and their functional potential in a subtropical Sphagnum multifibrosum peatland in Southwest China. By integrating genetic data with porewater geochemistry across a depth profile (0-60 cm), we identified a coherent tripartite microbial metabolic pattern. The aerobic surface layer (0–20 cm) was dominated by Pseudomonadota harbouring genes for labile carbon degradation and nitrogen fixation. The microaerophilic middle layer (20–40 cm), enriched with Acidobacteriota, was a hotspot for denitrification (narG, nirK) and sulfate reduction (dsrA). The anaerobic deep layer (40–60 cm) was characterized by Euryarchaeota and genes for methanogenesis (mcrA) and sulfur disproportionation (TST). Porewater Fe2+ concentrations were exceptionally high and strongly correlated with this stratification. Iron-reducing bacteria (Geobacter) linked carbon mineralization to the potential for re-stabilization via iron oxide formation. Plant community composition, specifically the shift from Sphagnum- to vascular plant-dominance, was a primary determinant of carbon quality and the resulting microbial functional network. Our findings provide a genomic blueprint for carbon transformation in peatlands, in which a tightly coupled plant-iron-microbe nexus shapes metabolic stratification and carbon sequestration potential. This integrative framework generates testable predictions about the stability of carbon stocks under disturbances such as vascular plant encroachment.