| High background concentrations of N2 and high spatio-temporal heterogeneity of sediments make it difficult to quantify denitrification in aquatic ecosystems. Traditional methods such as C2H2 inhibition, 15N tracer and mass balance measure denitrification in an indirect way, which is insufficiently accurate and time-consuming. The development of membrane inlet mass spectrometry（MIMS） makes it possible to measure N2∶Ar ratio precisely. In this study, we developed an undisturbed sediments incubation system combined with MIMS to measure net N2 fluxes directly and precisely. The incubation and sampling protocol were developed in this paper. Laboratory experiments showed that the instrument yielded high signal stability. The coefficient variations（CVs） of the measured N2∶Ar ratio of two standard solutions（with water temperature of 12 ℃ and 30 ℃） during 10 h continuous measurements were 0.26% and 0.08%. The net N2 fluxes of three replicate sediment cores also showed high reproducibility of the method（CV<9.05%）. The standard deviation of N2 concentration in triplicate water samples collected from our undisturbed sediments incubation system is less than 0.1 μmol·L-1, which was far less than the increment of dissolved N2（μmol·L-1） during the incubation（4.5 μmol·L-1 within 9 h）. Denitrification rates obtained from this method showed a Michaelis-Menten relationship with NO-3-N concentrations（R2=0.999 2, P<0.000 1）. In addition, there was a significant linear relationship between results of the N2∶Ar method and the NO-3-N disappearance method（R2=0.999 8, P<0.000 1）. This study suggests good applicability of the N2∶Ar method combined with our undisturbed sediments incubation system in determining denitrification rate of wetlands. |
