Microecological thresholds regulation by biochar boosts aged hydrocarbon degradation

Remediation of aged petroleum hydrocarbon (A-PHC) contamination is hindered by the recalcitrance of pollutants and the disruption of soil microbial networks. The efficacy of biochar amendments relies on balancing two opposing functions of contaminant adsorption and microbial biostimulation. Here, we propose a microecological threshold framework to resolve this trade-off. By investigating biochar application rates (0 %-5.5 %), we identified a critical threshold at approximately 1 %, where weak stimulation optimally balanced substrate bioavailability with microbial activation. This condition sustained high microbial diversity and fostered cooperative, circular network architectures, resulting in a 50.86 %-83.60 % enhancement in A-PHC degradation compared to controls. Conversely, supra-threshold dosages, while initially stimulating activity, triggered a collapse in diversity and the formation of competitive networks, ultimately stalling degradation. Community assembly analysis revealed a shift from deterministic filtering to stochastic, interaction-driven processes under threshold-optimized conditions. Spectroscopic evidence confirmed that enhanced biodegradation was driven by active microbial transformation of dissolved organic matter, distinct from the abiotic adsorption dominance observed at higher dosages. These findings validate the microecological threshold as a critical parameter for precision bioremediation, shifting the focus from simple pollutant reduction to the reconstruction of functional ecosystem networks.