5.6. Antibiotic-resistant genesAntibiotics are widely used in animal production, and approximately 30–90% of the antibiotics used are excreted in feces and urine, thereby making animal manure a significant source of environmental antibiotics, which can potentially lead to the emergence of antibiotic-resistant genes (ARGs) (Heuer et al., 2011). Manure harboring antibiotic-resistant microorganisms can enter the soil after land application, which can have serious impacts on human health. Cui et al. (2016) examined the effect of biochar addition on antibiotic-resistant gene removal and found that the removal was most efficient in chicken manure containing mushroom biochar, followed by chicken manure and chicken manure containing rice straw biochar. The lower removal rate observed for rice straw biochar was explained in terms of the abundance of pathogenic bacteria. The rate at which pathogenic bacteria were removed was similar to that for ARGs, which indicates that pathogenic bacteria might have influenced the fate of ARGs.6. Recommendations for future researchFeedstock and pyrolysis conditions affect the physicochemical properties of biochar (e.g., Brunauer–Emmett–Teller surface area and pore volume) and thus the performance of biochar during composting. Currently, there is no standard method recommended for the production of biochar used as amendments during the composting of livestock and poultry manure, and accordingly, the development of a standard method would benefit all researchers working in this area. Once a standard method is made available, recommendations can be made to farmers, who may want to use biochar to improve their composting systems. In addition to developing a standard production method, adopting a recognized classification scheme would be beneficial, as the term ‘‘biochar” spans a large collection of potentially chemically distinct forms of compounds. The availability of feedstocks, energy requirements and environmental risks of field-scale applications should be also investigated. An emerging issue relating to the application of compost is the release of antibiotic-resistant genes into the environment. A larger number of studies that examine the effects of biochar use on the elimination of antibiotic-resistant genes, particularly during manure composting, would be beneficial. Furthermore, to date, no studies have reported on the use of biochar in animal mortality composting. Accordingly, investigating the effects of biochar amendment on temperature development and the elimination of pathogens during the composting of animal mortalities will potentially contribute to limiting or preventing the spread of animal disease outbreaks.
5.6. Antibiotic-resistant genes<br>Antibiotics are widely used in animal production, and approximately 30–90% of the antibiotics used are excreted in feces and urine, thereby making animal manure a significant source of environmental antibiotics, which can potentially lead to the emergence of antibiotic-resistant genes (ARGs) (Heuer et al., 2011). Manure harboring antibiotic-resistant microorganisms can enter the soil after land application, which can have serious impacts on human health. Cui et al. (2016) examined the effect of biochar addition on antibiotic-resistant gene removal and found that the removal was most efficient in chicken manure containing mushroom biochar, followed by chicken manure and chicken manure containing rice straw biochar. The lower removal rate observed for rice straw biochar was explained in terms of the abundance of pathogenic bacteria. The rate at which pathogenic bacteria were removed was similar to that for ARGs, which indicates that pathogenic bacteria might have influenced the fate of ARGs.<br>6. Recommendations for future research<br>Feedstock and pyrolysis conditions affect the physicochemical properties of biochar (e.g., Brunauer–Emmett–Teller surface area and pore volume) and thus the performance of biochar during composting. Currently, there is no standard method recommended for the production of biochar used as amendments during the composting of livestock and poultry manure, and accordingly, the development of a standard method would benefit all researchers working in this area. Once a standard method is made available, recommendations can be made to farmers, who may want to use biochar to improve their composting systems. In addition to developing a standard production method, adopting a recognized classification scheme would be beneficial, as the term ‘‘biochar” spans a large collection of potentially chemically distinct forms of compounds. The availability of feedstocks, energy requirements and environmental risks of field-scale applications should be also investigated. An emerging issue relating to the application of compost is the release of antibiotic-resistant genes into the environment. A larger number of studies that examine the effects of biochar use on the elimination of antibiotic-resistant genes, particularly during manure composting, would be beneficial. Furthermore, to date, no studies have reported on the use of biochar in animal mortality composting. Accordingly, investigating the effects of biochar amendment on temperature development and the elimination of pathogens during the composting of animal mortalities will potentially contribute to limiting or preventing the spread of animal disease outbreaks.
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