Tracing N2O production pathways in aqueous ecosystems by quasi-simultaneous online analysis of 15N in reactive nitrogen species and gaseous emissions

Natural and engineered nitrogen (N) removal processes in aqueous systems represent important sources of nitrogenous gas emissions, including the potent greenhouse gas nitrous dioxide (N₂O). The relevance of microbial and abiotic formation pathways can be assessed using ¹⁵N tracing techniques. While ¹⁵N-N₂O analysis using optical analyzers is straightforward, quantification of ¹⁵N fractions in inorganic N compounds, ammonium (NH₄⁺), nitrite (NO₂^-), and nitrate (NO₃^-), is typically time-consuming and labor-intensive.

In this study, we developed an automated sample-preparation unit coupled to a membrane-inlet quadrupole mass spectrometer (3n-ASSP-MIMS) for the online quasi-simultaneous analysis of ¹⁵N fractions in NH₄⁺, NO₂^-, and NO₃^-. The technique was designed and validated for applications at moderate (100 - 200 μmol L^-1) to high (2 – 3 mmol L^-1) N, as found in sewer systems, wastewater in treatment plants, or eutrophic surface waters, and ¹⁵N spiking (f₁₅) between 1 and 33%.

The potential of 3n-ASSP-MIMS was demonstrated in a feasibility study, where the technique, in conjunction with ¹⁵N-N₂O analyses by FTIR spectroscopy, was applied to pinpoint nitrifier denitrification as the primary N₂O formation pathway during partial NH₄⁺ oxidation to NO₂^- in a lab-scale sequencing batch reactor.