| Summary: | The impact of activated carbon (AC) amendments on the biodegradation of crude oil in soil was studied in batch microcosms. AC amendment slowed down biodegradation and removal of hydrocarbon pollutants and was more evident when AC when added from the beginning compared to after 5 months. The microbial sequencing analysis revealed that the overall bacterial community shifted more due to crude oil addition compared to AC amendment at the start and after 5 months. AC amendments reduced and slightly reduced the abundance of hydrocarbon degraders belonging to Actinobacteria and classes Gammaproteobacteria and Alphaproteobacteria (Such as Rhodococcus, Marinobacter, and Parvibaculum) in crude oil batches with AC from start and AC after 5 months, respectively. The effect of biofuels on the natural attenuation of toluene was also investigated. 13C/12C-CO2 batch production showed that biofuel components were preferentially degraded in blended fuels. Ethanol had a more negative effect on toluene degradation compared to biodiesel, as it was preferentially degraded even with nutrient bio-stimulation. The microbial analysis revealed nutrient effect on the microbial communities with nitrifying bacteria Nitrospira seen to make significant gains in OTU ranking and relative abundance due to the nutrient amendment. The microbial community analysis also showed a distinction in microbial communities that degrade biodiesel, ethanol and toluene in the presence or absence of nutrients. For instance the results showed that Rhodococcus can degrade toluene in the presence of biodiesel when nutrients are surplus or scarce and can degrade toluene in the presence of ethanol only when nutrients are scarce. Indicating a lower effect of biodiesel on toluene degraders compared to ethanol. Pseudomonas was identified as a key ethanol degrader and thrives in presence of both ethanol and toluene when nutrient availability is high but has preference for ethanol as a carbon source. Nocardia is main biodiesel degrader when nutrient availability is high or low. This study has shown that the use of DNA microbial community analysis gives a broader insight into microbes involved in the physiological activities and how they are affected by certain treatments and the findings in this study would possibly aid the understanding of the impact of adsorbents on hydrocarbon pollutants and the effect of blended fuels have on the microbiology within soil. It is thereby recommended that during hydrocarbon soil remediation studies/interventions, DNA microbial community analysis should be carried out in conjunction with chemical analysis such as the one carried out in this study as this would inform the proper utilization of the remediation strategies. The work was carried out in batch studies and it is important to repeat it in column, mesocosms or field studies to see if there are any significant changes in the results.
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