Dicyandiamide addition delay nitrous oxide emission and shift its production pathway from denitrification to incomplete nitrification in maturation phase of composting

Nitrous oxide (N2O) emissions contribute to nitrogen loss and exacerbate the greenhouse effect during composting. Investigating the efficacy and underlying mechanisms of dicyandiamide (DCD) in reducing N2O emissions can inform composting strategies aimed at mitigating global climate change. In this study, 5 % or 10 % DCD (based on TN content in the initial mixture) was added during the maturation phase of composting, resulting in delayed N2O production. In the control (CK) treatment without DCD, N2O production primarily stemmed from denitrification, facilitated by ammonia-oxidizing bacteria (AOB) providing nitrite substrate. The DCD-treated groups exhibited prolonged NH4+ retention times, and the significant N2O emissions were postponed by 4–9 days instead of being consistently inhibited. Notably, the Nitrosomonas genus emerged as the primary AOB contributing to N2O emissions, engaging in both nitrification and nitrifier denitrification processes, particularly through Nitrosomonas europaea carrying the nirK gene. DCD delayed nitrification via the amoA gene and persistently hindered the enrichment of hao genes. With a 10 % DCD addition, the predominant pathway for N2O production shifted from denitrification, dominated by nirK and norB genes, to incomplete nitrification, predominantly influenced by Nitrosomonas carrying the amoA gene. This study offers a novel perspective on elucidating the mechanism by which DCD influences N2O production in composting.