Application progress and risk analysis of biochar in soil amelioration

Biochar has shown significant potential to improve the quality of cultivated land and crop yields. This study aims to explore the research and application of biochar returning to the field for soil improvement and crop growth. A systematic review was conducted on the preparation and equipment of the biochar, the key influencing factors on the physicochemical properties of the biochar, as well as the effects of biochar on soil (physicochemical properties, nutrients, enzyme activities and microbial communities, inorganic heavy metals and organic pollutants, carbon sink reduction) and crops. The key influencing factors and mechanisms were summarized to evaluate the risks and countermeasures of biochar application. Some suggestions were proposed for future research. The results showed that the rotary-furnace reactor was successfully applied to the industrial-scale production of biochar, due to its good adaptability and convenient operation. Furthermore, the soil bulk density decreased after the application of biochar into the soil, whereas there was an increase in the water retention capacity, cation exchange capacity, the content of macronutrients and micronutrients in the soil, as well as the utilization rate of nutrients by plants. The microbial community structure was dominated by the biochar. And then the activities of soil enzymes were enhanced, such as urease and peroxidase. Heavy metals were also adsorbed and fixed by biochar after ion exchange and complexation. The transformation of heavy metals was promoted from the exchangeable to the residual state. Organic pollutants were adsorbed and degraded by biochar after hydrophobic interaction, electrostatic interaction, and free radical interaction. The toxicity and bioaccumulation of pollutants were reduced in the soil. The returning biochar to the field can be expected to mitigate climate change through soil carbon cycling. Taking Chinese umbrella grass biochar as an example, the annual output reached 1.96-3.21 Mt, and the long-term annual carbon sequestration was 0.75-0.79 Mt, and the long-term annual reduction of CO₂ equivalent was 2.76-2.88 Mt. Generally, the behavior of returning biochar to the field had a positive effect on the crop yield. However, the extensive application of biochar in the soil can pose environmental and health risks. The pollutants that are contained in biochar itself, and the pollutants were used to adsorb from the environment (such as heavy metals and polycyclic aromatic hydrocarbons), indicating the potential to be released into the soil under the influence of the environment and time. Eventually, the nano-biochar was obtained during biochar aging after disintegration, fragmentation, and pulverization. Currently, there is a significant lack of research on the safety, stability, and toxicity of nano-biochar, particularly on the health risks of biochar to humans. Therefore, it is very necessary to conduct a systematic assessment of the long-term and large-scale biochar application in fields. Some optimal measures were also proposed to evaluate the economic, social, and ecological benefits, in order to reduce or even avoid the application risks of biochar. The research can also provide a strong reference to promote the healthy development of the biochar industry in high-quality agriculture.