Relationships of respiratory quotient to microbial biomass and hydrocarbon contaminant degradation during soil bioremediation

This work focused on monitoring respiratory quotient, RQ (defined as a ratio of CO₂ production to O₂ uptake rates), microbial growth and residual hydrocarbon concentration during bioremediation experiments performed on laboratory soil microcosms. The aim of the study was to determine if the time course biodegradation profile of the contaminant can be related to the RQ evolution and to investigate the effect of the water content on RQ measurements. A natural soil was artificially contaminated with hexadecane and adjusted with inorganic nutrients to stimulate biodegradation. Microbial growth, CO₂ production, O₂ uptake and residual hexadecane were periodically monitored at different soil water contents ranging from 0.15 to 0.35 g water g⁻¹ of dry soil. Results showed that microbial activity and contaminant degradation were strongly dependent on soil water content. Maximal growth and hexadecane depletion were obtained at a water content of 0.20 g water g⁻¹ of dry soil, which corresponded to 46.6% of the water holding capacity. Hexadecane degradation was considerably reduced with increasing soil water content. RQ values fluctuated as a function of the hexadecane biodegradation phases. The lowest RQs corresponded to the highest hexadecane depletion and microbial growth. The water content variation did not significantly affect the shape of the RQ evolution curves as a function of time. It only modified the magnitude of RQ values. This study indicates that additional biological and chemical analyses are needed to support RQ data when monitoring contaminant degradation to have an accurate understanding of all the biotic processes, which may occur simultaneously.