Additives for reducing nitrogen loss during composting: A review

NH3 and N2O emissions account for 79%-94% and 0.2%-9.9% of total nitrogen losses in composting, respectively. They not only cause serious environmental problems such as odor emission, global warming, and ozone depletion but also reduce the quality of compost products. Additives can reduce the nitrogen loss that occurs during composting by adsorbing NH3/NH4+, reducing the pH of the composting pile, forming struvite, and enhancing nitrification. However, because a wide range of additives have been investigated to reduce nitrogen loss during composting, the additives tend to be diversified and complicated. Herein, we review the recent literature on nitrogen retention of composting process done using additives and categorize the additives into physical, chemical, and microbial based on their characteristics and nitrogen conservation mechanism. Physical, chemical (except dicyandiamide), and microbial additives can reduce 38.5%, 51.3%, 33% of NH3 loss and 50.3%, 0.67%, 21.58% of N2O loss during composting, respectively. Dicyandiamide is a nitrification inhibitor that has no significant effect on NH3 emissions during composting, but it can significantly reduce N2O emissions by 63.01%. It is notable that physical (mineral) and chemical additives have high nitrogen retention properties, but they have the problems of high salt ion and unknown soil accumulation effect. Nonreusable additives are known to increase the cost of composting and affect its large-scale application. Microbial additives are advantageous in terms of cost and environmental friendliness. Regarding the future prospects, we highlight the following three research topics: compound additives, reusable porous materials, and the activity of microbial additives in various composting processes, which require further investigation to reduce nitrogen loss in composting. However, further research is needed to evaluate the economic feasibility of additives to accelerate their large-scale application in composting.