Microbial Sources and Sinks of Nitrous Oxide during Organic Waste Composting

Composting is widely used for organic waste management and is also a major source of nitrous oxide (N₂O) emission. New insight into microbial sources and sinks is essential for process regulation to reduce N₂O emission from composting. This study used genome-resolved metagenomics to decipher the genomic structures and physiological behaviors of individual bacteria for N₂O sources and sinks during composting. Results showed that several nosZ-lacking denitrifiers in feedstocks drove N₂O emission at the beginning of the composting. Such emission became negligible at the thermophilic stage, as high temperatures inhibited all denitrifiers for N₂O production except for those containing nirK. The nosZ-lacking denitrifiers were notably enriched to increase N₂O production at the cooling stage. Nevertheless, organic biodegradation limited energy availability for chemotaxis and flagellar assembly to restrain nirKS-containing denitrifiers for nitrate reduction toward N₂O sources but insignificantly interrupt norBC- and nosZ-containing bacteria (particularly nosZ-containing nondenitrifiers) for N₂O sinks by capturing N₂O and nitric oxide (NO) for energy production, thereby reducing N₂O emission at the mature stage. Furthermore, nosZII-type bacteria included all nosZ-containing nondenitrifiers and dominated N₂O sinks. Thus, targeted strategies can be developed to restrict the physiological behaviors of nirKS-containing denitrifiers and expand the taxonomic distribution of nosZ for effective N₂O mitigation in composting.