Interactive effects of heavy metals and soil conditioners on rhizodegradation of long-chain petroleum hydrocarbons in soils

A microcosm experiment was performed to investigate the interactive effects of heavy metals, soil conditioners (biochar and compost), and plant on petroleum hydrocarbon (PHC)-degrading bacteria and its resultant rhizodegradation of long-chain PHCs in soils. Two levels of heavy metal contamination (50 mg/kg and 100 mg/kg for each of Cu, Ni and Zn) and three soil amendment treatments (biochar treatment, compost treatment, and biochar-compost co-treatment) were set. Comparison was made between the vegetated soils and the unvegetated counterparts and between the soils with different levels of heavy metal contamination under vegetation conditions. Plant growth consistently increased the abundance of bacterial PHC-degraders in the soils co-contaminated by PHCs and heavy metals. Elevated levels of heavy metal contamination adversely affected the growth of the plant and bacterial PHC-degraders. It also caused alteration in the structure of the PHC-degrading bacterial communities. Soil amendments enhanced plant and microbial growth. However, the enhanced effect varied among the different treatments with the performance in the following decreasing order: combined biochar and compost > compost > biochar. Plant growth and soil amendments all resulted in increased soil-borne dissolved organic compounds, which in turn caused an increase in the abundance of PHC-degrading bacterial genera and thus the rate of rhizodegradation of the long-chain PHCs in the soils co-contaminated by heavy metals. The research findings have implications for developing treatment strategies to cost-effectively remediate co-contaminated soils with PHCs and heavy metals.