Effects of C/N ratio on NO emissions and nitrogen functional genes during vegetable waste composting.

Nitrous oxide (NO) generation during composting not only leads to losses of nitrogen (N) but also reduces the agronomic values and environmental benefits of composting. This study aimed to investigate the effect of the C/N ratio on NO emissions and its underlying mechanisms at the genetic level during the composting of vegetable waste. The experiment was set up with three treatments, including low C/N treatment (LT, C/N = 18), middle C/N treatment (MT, C/N = 30), and high C/N treatment (HT, C/N = 50). The results showed that NO emission was mainly concentrated in the cooling and maturation periods, and the cumulative NO emissions decreased as the C/N ratio increased. Specifically, the cumulative NO emission was 57,401 mg in LT, significantly higher than 2155 mg in MT and 1353 mg in HT. Lowering the C/N ratio led to increasing TN, NH-N, and NO-N contents throughout the composting process. All detected nitrification-related gene abundances in LT continued to increase during composting, significantly surpassing those in MT during the cooling period. By contrast, in HT, there was a slight increase in the abundance of detected nitrification-related genes but a significant decrease in the abundance of narG, napA, and norB genes in the thermophilic and cooling periods. The structural equation model revealed that hao and nosZ genes were vital in NO emissions. In conclusion, increasing the C/N ratio effectively contributed to NO reduction during vegetable waste composting.