The effects of different biochars on microbial quantity, microbial community shift, enzyme activity, and biodegradation of polycyclic aromatic hydrocarbons in soil

Plant-residue derived biochars from walnut shell, corn cob, corn straw, and rice straw obtained at three heat treatment temperatures (HTTs) (250 °C, 400 °C, and 600 °C) were applied in an incubation experiment to investigate how feedstocks, HTTs, and biochar properties affect the quantity of microorganisms, microbial community shift, enzyme activity, and biodegradation of polycyclic aromatic hydrocarbons (PAHs) in an aged contaminated soil. The microbial quantities (bacteria and fungi) and enzyme activities (catechol 2,3-dioxygenase and ligninolytic enzymes) generally decreased with the increase in HTTs. Microbial quantities had significantly positive correlations with the aliphatic carbon (C) (p < 0.01) but negative correlations with the aromatic C of biochars (p < 0.01). Similar findings were observed with enzyme activities, which had significantly positive correlations with microbial quantities (p < 0.05). Meanwhile, there were significantly positive correlations between C23O activity and bacterial quantity (p < 0.01) and between ligninolytic enzyme activities and the quantity of fungi (p < 0.05). These results indicate that the increase in recalcitrant aromatic C in biochars that occurs with the increase in HTT is unfavorable for the microbial growth and enzyme activity in the soil studied. Generally, application of biochars had little influence on the biodegradation rates of total PAHs. However, the effects of biochars on the biodegradation rates of major individual PAH (e.g., Nap, Phe, Pyr, and Chr) depended on both the types of biochars and the PAH properties such as benzene ring number and angular pattern of the ring linkage.