N2O isotopocule measurements reveal its production/reduction pathways from a municipal sewer network in South China

Urban sewer networks are increasingly recognized as significant sources of greenhouse gas emissions, yet the mechanisms controlling nitrous oxide (N2O) production and consumption within sewers remain poorly resolved. In this study, we conducted a field campaign in a community sewer network in Shenzhen, South China, using a stable isotope approach to investigate the N2O emission potential and its regulatory mechanisms. Measurable N2O emissions were observed across all sites, with headspace concentrations (within the inspection wells) ranging from 0.48 to 6.25 ppm and fluxes ranging from 0.01 to 0.27 mg N2O h-1. Under lower redox potentials, the septic tanks (STs) exhibited stronger N2O emissions than the wastewater pipes (WPs). In situ measurements of N2O isotopocule signatures (δ15Nbulk, δ18O, and site preference) facilitated source/sink identification, showing that denitrification was the dominant process for N2O production. This interpretation was consistent with dual nitrate isotope measurements, which suggested a clear isotopic imprint from denitrification pathways. More importantly, the natural isotopic compositions of N2O allowed us to quantify the fraction of N2O being reduced to N2 before emission (70∼93%), providing an important benchmark in assessing the variability of net N2O fluxes from the sewer network. Metagenomic data provided complementary evidence for understanding the distribution of N2O emission hotspots and production/consumption mechanisms. These results show that urban sewer compartments can function simultaneously as N2O sources and internal sinks, and that net vent emissions reflect the balance between gross N2O production and reduction within sewer systems.