Functional decoupling of atrazine biodegradation and microbial respiration under viral modulation.

Atrazine persistence in agricultural soils is primarily controlled by microbial degradation. However, the combined roles of contaminant dose and viral communities in shaping microbial community assembly and biodegradation potential remain critically underexplored. Here, we established a simplified sand microcosm inoculated with a soil-derived microbial consortium to investigate how atrazine dose and viral communities jointly influence atrazine dissipation, bacterial community assembly, and microbial respiration. Atrazine degradation exhibited a clear dose-dependent response. Atrazine in the low-dose treatment (20 mg/kg) was completely degraded within 33 days, whereas most of it remained in the high-dose treatment (100 mg/kg), indicating a severe suppression of biodegradation capacity under elevated toxic stress. This divergence corresponded to fundamentally distinct assembly trajectories. Low-dose atrazine rapidly enriched Pseudomonadota-dominated communities, whereas high-dose atrazine initially selected Bacillota-dominated stress-tolerant assemblages. Alpha diversity declined sharply following atrazine exposure, with microbial richness exhibiting stronger dose sensitivity than Shannon diversity. Beta nearest taxon index analysis further revealed strong early homogeneous selection, followed by progressively weaker deterministic or stochastic assembly over time. Despite these strong dose-dependent effects on community structure and biodegradation, viral addition did not significantly alter atrazine degradation kinetics or most bacterial compositional metrics. In contrast, viruses significantly suppressed cumulative microbial respiration under the low-dose regime, demonstrating that viral regulation can alter community-scale carbon metabolism without measurably affecting specialized biodegradation capacity. Collectively, these findings reveal a functional decoupling between atrazine biodegradation and microbial respiration under viral modulation, highlighting that distinct microbial ecosystem functions differ fundamentally in their sensitivity to contaminant stress and viral regulation.IMPORTANCEPredicting contaminant effects on soil microbiomes generally relies on the assumption that microbial functions respond coherently to environmental disturbance. By integrating community assembly analysis and viral manipulation experiments across atrazine dosages, this study demonstrates that contaminant concentration acts as a master variable governing microbial community trajectory. Increasing atrazine dosage shifts assembly from degrader-dominated states to stress-persistent communities. At the functional level, specialized biodegradation and community-scale metabolism exhibit fundamentally different sensitivities. Atrazine degradation remains resilient to viral perturbation, while respiration is strongly suppressed. Viral regulation of carbon flux operates independently of contaminant removal. These findings identify contaminant concentration as a key ecological threshold that decouples community assembly and functional specialization.