Response of microbial communities in capillary zone to toluene natural attenuation under leaching/capillary rise

The capillary zone, a crucial transition between the unsaturated zone and groundwater, presents two key pollution scenarios for toluene pollutants: entry from the unsaturated zone or groundwater through leaching (case LS) or capillary rise (case CS). However, how these different pollutant entry pathways influence microbial responses and degradation behaviors remains poorly understood. In this study, laboratory column experiments were conducted to systematically compare distinct scenarios of toluene attenuation driven by capillary action, focusing on microbial process related to the attenuation patterns. We integrated toluene concentration monitoring with 16S rRNA sequencing with co-occurrence network analysis and null modeling (βNTI) to investigate the link between attenuation dynamics and microbial community assembly. Steeper substrate gradients during CS enhanced microbial activity and competition, leading to deterministic assembly (βNTI > 2; m = 0.079) and higher attenuation efficiency (up to 76 %). In contrast, weaker gradients during LS resulted in stochastic assembly (|βNTI| < 2; m = 0.61) and slower degradation (18-26 %). Pseudomonas acted as the keystone degrader, while Sphingobium and Pseudoxanthomonas exhibited competitive or cooperative roles under CS and LS, respectively. Our findings establish the mechanistic link between capillary-structured toluene attenuation dynamics and microbial community assembly, thereby providing a scientific basis to improve natural attenuation predictions and bioremediation strategies for petroleum hydrocarbon contamination.