A systematic review of biochar use in animal waste composting

The animal production industry in the United States is currently undergoing a phase of growth; however, such growth brings certain challenges. One of the most prominent concerns in this regard is the increasing amounts of animal waste produced as a natural consequence of stock population growth. For decades, composting, including that of manure and animal mortalities, has been utilized to manage animal waste. Recently, in an effort to enhance the composting process, biochar has been proposed for use as a compost amendment, and over the last few years, an increasing number of papers on composting with biochar have been published. However, although there have been a few review papers that have summarized the literature regarding biochar use in composting, none of these has focused on animal waste composting. Accordingly, the purpose of this review is to critically analyze the role of biochar in livestock and poultry waste composting, identify gaps in our current knowledge, and propose future research directions. On the basis of the studies analyzed, biochar has the potential to improve animal waste composting processes at application rates of 5-10%. Biochar can extend the thermophilic phase of the composting process, lower the pH of compost material, prevent leachate formation, and reduce ammonia, methane, and nitrous oxide emissions. Given that the feedstock used to produce biochar and the pyrolysis conditions employed in its production affect the performance of biochar, it is important to report the physicochemical properties of the biochars used to enable comparison of the results of different studies. Moreover, there is a need for further research to gain a better understanding of the impact of biochar regarding the elimination of antibiotic-resistant genes and animal mortality composting.