Technical Note: Simultaneous measurement of sedimentary N-2 and N2O production and a modified N-15 isotope pairing technique

Dinitrogen (N-2) and/or nitrous oxide (N2O) are produced through denitrification, anaerobic ammonium oxidation (anammox) or nitrification in sediments, of which entangled processes complicate the absolute rate estimations of gaseous nitrogen production from individual pathways. The classical isotope pairing technique (IPT), the most common N-15 nitrate enrichment method to quantify denitrification, has recently been modified by different researchers to (1) discriminate between the N-2 produced by denitrification and anammox or to (2) provide a more accurate denitrification rate under considering production of both N2O and N-2. In case 1, the revised IPT focused on N-2 production being suitable for the environments of a low N2O-to-N-2 production ratio, while in case 2, anammox was neglected. This paper develops a modified method to refine previous versions of IPT. Cryogenic traps were installed to separately preconcentrate N-2 and N2O, thus allowing for subsequent measurement of the two gases generated in one sample vial. The precision is better than 2% for N-2 (m/z 28, m/z 29 and m/z 30), and 1.5% for N2O (m/z 44, m/z 45 and m/z 46). Based on the six m/z peaks of the two gases, the N-15 nitrate traceable processes including N-2 and N2O from denitrification and N-2 from anammox were estimated. Meanwhile, N2O produced by nitrification was estimated via the production rate of unlabeled (N2O)-N-44. To validate the applicability of our modified method, incubation experiments were conducted using sediment cores taken from the Danshuei Estuary in Taiwan. Rates of the aforementioned nitrogen removal processes were successfully determined. Moreover, N2O yield was as high as 66 %, which would significantly bias previous IPT approaches if N2O was not considered. Our modified method not only complements previous versions of IPT but also provides more comprehensive information to advance our understanding of nitrogen dynamics of the water-sediment interface.