Uncoupled nitrification-denitrification reduces nitrous oxide emissions in canals affected by ship disturbance

Nitrous oxide (N₂O) emissions in rivers are influenced by hydrodynamic conditions, which are potentially altered by ship-induced disturbances in canals. To assess this impact, this study investigated N₂O emissions in the Beijing-Hangzhou Grand Canal (BHGC) in China, the longest artificial canal in the world, and compared them with emissions from nearby rivers unaffected by ships. The microbial mechanisms underlying the observed differences were elucidated using a quantitative polymerase chain reaction technique. The results showed that N₂O emissions in the BHGC were 59.1 % lower than in connected rivers undisturbed by ships. In riverine ecosystems, aerobic nitrification supplies substrates for anaerobic denitrification, and the coupling of these processes typically facilitates efficient nitrogen cycling, often leading to elevated intermediate N₂O production. However, ship-induced sediment resuspension shifts nitrogen cycling hotspots from sediments to the overlying water, where oxygen-rich conditions favor nitrification but limit denitrification, thereby disrupting their coupling. This was evidenced by nitrate accumulation and a reduced abundance of nitrifiers in the BHGC water. The findings enhance our understanding of how ship activities influence nitrogen cycling and N₂O emissions in canals.