Persistent free radicals generated from a range of biochars and their physiological effects on wheat seedlings

Biochar is a promising soil conditioner and environmental remediation material. However, the amount, type, and environmental effect and risk of persistent free radicals (PFRs) associated with biochar need to be better understood. Thus, this study characterized PFRs in a range of biochar types and their effects on the growth and oxidative stress of wheat seedlings. Among the biochars prepared by pyrolysis of different types of biomass at 500 °C, the concentrations of PFRs in cow dung and egg shell biochar were the highest and the lowest, respectively. They both increased with artificial weathering treatment but decreased with aging. The dominant types of biochar PFRs were transformed from carbon-centered to oxygen and carbon/oxygen-centered free radicals with weathering. The amount and type of biochar PFRs in mixtures of biochar and soil varied with soil type and biochar dose. After 30 d incubation in different soil-biochar mixtures, measures of wheat plant germination and growth and antioxidant enzyme activity showed increases at lower biochar doses but decreases at higher doses. Catalase activity was 38.1 % greater at 20 g·kg^-1 biochar dosage and 25.2 % less at 80 g·kg^-1 dosage, on average. In contrast, leaf malondialdehyde content and staining by Evans Blue, both indicators of plant cell membrane damage, generally increased with increasing biochar dosages. Finally, soil hydrolase enzyme activity also displayed an inverted U-shaped dose response. The toxicity indicators showed an increasing trend with higher PFR concentrations in the soil-biochar combinations. While these findings provide evidence for significant potential agricultural and ecological risks associated with the application of biochar due to PFRs damage, it also points to ways that these risks could be mediated such as through biochar dosage restrictions and pre-aging. This study provides new insights into the potential toxicological mechanism and ecological risks associated with the application of biochar in agricultural and environmental settings.