Effect of digestate application on microbial respiration and bacterial communities' diversity during bioremediation of weathered petroleum hydrocarbons contaminated soils

Highlights: • Digestate application with bulking agent or immobilized bacteria improves TPH removal. • Digestate effect on soil respiration depends on soil texture. • Sewage sludge digestate contains high concentrations of alkB genes. • Application of digestate increases and maintains alkB genes content in the soil. • Distinct microbial groups developed in amended and non-amended soils. -- Abstract: Digestate is an organic by-product of biogas production via anaerobic digestion processes and has a great potential as soil fertilizer due to concentrated nutrients. In this study, we examined digestate as a potential nutrient and microbial seeding for bioremediation of weathered (aged) petroleum hydrocarbon contaminated soils. We analysed 6 different treatments in microcosm using two industrial soils having different textures: a clay rich soil and a sandy soil. After 30 days of incubation, the highest total petroleum hydrocarbons (TPH) removal was observed in microcosms containing digestate together with bulking agent (17.8% and 12.7% higher than control in clay rich soil and sandy soil, respectively) or digestate together with immobilized bacteria (13.4% and 9% higher than control in clay rich soil and sandy soil, respectively). After digestate application microbial respiration was enhanced in sandy soil and inhibited in clay rich soil due to aggregates formation. After bulking agent addition to clay rich soil aggregates size was reduced and oxygen uptake was improved. Application of digestate to soil resulted in the development of distinct microbial groups in amended and non-amended soils. Genera containing species able to degrade TPH like Acinetobacter and Mycobacterium were abundant in digestate and in soil amended with digestate. Quantification of alkB genes, encoding alkane monoxygenase, revealed high concentration of these genes in digestate bacterial community. After application of digestate, the level of alkB genes significantly increased in soils and remained high until the end of the treatment. The study revealed great potential of digestate as a nutrient and bacteria source for soil bioremediation.