Biochar Amendment Promoted the Maize Growth and Changed Bacterial Community Assembly in a Phenanthrene-Contaminated Soil

Biochar amendment is considered an effective measure to improve soil fertility or alleviate the carbon-dioxide emission, but its ecological effect on the plants or microbial community in polluted soil environments remains uncertain. This study investigated the influence of biochar application on the pollutant's bioavailability and bacterial community assembly in soil contaminated by a representative polycyclic aromatic hydrocarbon (PAH)-phenanthrene, aiming to reveal the role of biochar on the toxicity mitigation and microbial community succession in PAH-contaminated soil ecosystems. A maize-pot experiment of 90 days with four treatments (no biochar or inoculation, CK; inoculation of phenanthrene-degrading bacteria, MB; biochar amendment, BC; amendment of biochar immobilized with the phenanthrene-degrading bacteria, MBC) was conducted using a phenanthrene-contaminated soil. Then the plant (biomass and activities of some PAH-degrading associated enzymes) and soil (phenanthrene content and some PAH-degrading associated enzymes) parameters were investigated, meanwhile the soil bacterial community composition was determined by Illumina Miseq sequencing and the characteristic of soil bacterial community assembly was calculated by the Null model. The biochar-amended treatments (BC and MBC) had significantly lower phenanthrene removal rate (PRR) than the no-biochar treatments (CK and MB), while the plant dry weight and height of the BC, MB, and MBC treatments were all significantly higher than those of the CK treatment, with the highest dry weight in the MBC treatment (biochar immobilized with the bacteria). Additionally, the soil SDHA (soil dehydrogenase) and leaf CYP450 (cytochrome P450) activities in the BC, MB, and MBC treatments were significantly higher and lower than those in the CK treatment, respectively. Bacterial inoculation had a negligible effect on the soil bacterial community, whereas the biochar amendment significantly changed the bacterial community structure and strengthened the bacterial co-occurrence network. Furthermore, biochar amendment enhanced the role of stochastic processes in bacterial community assembly, implying that it alleviated the toxicity or stress of PAH on soil microbes. Although biochar amendment significantly decelerated soil phenanthrene elimination, it promoted plant growth and optimized the microbial ecosystem in phenanthrene-contaminated soil.