Nitrous oxide emission from stormwater biofilters in alternating dry and wet weather

The nitrous oxide (N₂O) flux and its possible production pathways from stormwater biofilters in response to moisture content (MC) due to a shift from dry to wet weather was investigated. In this study, we evaluated the changes in the composition of the bacterial community, the relative abundance of functional genes, and N₂O emission rate in laboratory-scale stormwater biofilters in response to changes of MC. The results indicated that N₂O flux correlated positively with soil MC (r = 0.722 p < 0.01). We observed a higher rates of N₂O flux when shifting from dry to wet conditions. Notably, these values decreased substantially within 8-24 h in response to the rapid decline in MC, and then gradually decreased and stabilized at 4.4-12.0 μg/m²·h. The relative abundance of ammonia-oxidizing and denitrifying bacteria, as well as the relative abundance of functional groups associated with denitrification was higher under conditions of low soil MC than those in the high MC. Furthermore, the abundance of bacterial genes norB (r = 0.716 p < 0.01) and hao (r = 0.917 p < 0.01) was associated with higher N₂O emission in high MC soils. Studies with the stable isotope (¹⁵N) revealed that ¹⁵N enrichment in N₂O was primarily via denitrification pathways and labeled ammonium ion (¹⁵NH₄⁺). Taken together, our results suggested that nitrifier denitrification is the main pathway generating N₂O emission in soils with high MC, which may be caused by the high molar ratio of NH₃ to total nitrogen in the influent.