Technical Note: Simultaneous measurement of sedimentary N2 and N2O production and new 15N isotope pairing technique

Dinitrogen (N2) and/or nitrous oxide (N2O) are produced through denitrification, anaerobic ammonium oxidation (anammox) or nitrification in sediments, of which entangled processes obfuscate the absolute rate estimation of gaseous nitrogen production from individual pathway. Recently, the classical isotope pairing technique (IPT), the most common 15N-nitrate enrichment method to quantify denitrification, has been modified by different researchers to (1) discriminate relative contribution of N2 production by denitrification from anammox or to (2) provide more accurate denitrification rate by considering both N2O and N2 productions. Both modified methods, however, have deficiencies such as overlooking N2O production in case 1 and neglecting anammox in case 2. In this paper, a new method was developed to refine previous methods. We installed cryogenic traps to pre-concentrate N2 and N2O separately, thus, allowing simultaneous measurement for two gases generated by one sample. The precision is better than 2% for N2 (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, we further revised IPT formulae to truthfully resolve the production rates of N2 and N2O contributed from 3 specific nitrogen removal processes, i.e. N2 and N2O from denitrification, N2 from anammox and N2O from nitrification. To validate the applicability of our new method, incubation experiments were conducted using sediment cores taken from the Danshuei estuary in Taiwan. We successfully determined the rates of aforementioned nitrogen removal processes. Moreover, N2O yield was as high as 66%, which no doubt would significantly bias previous IPT approaches when N2O was not considered. Our new method not only complements the previous IPT but also provides more comprehensive information to advance our understanding of nitrogen dynamics through the water-sediment interface.