Dual isotope and isotopomer measurements for the understanding of N ₂ O production and consumption during denitrification in an arable soil

The aim of our research was to obtain information on the isotopic fingerprint of nitrous oxide (N 2 O) associated with its production and consumption during denitrification. An arable soil was preincubated at high moisture content and subsequently amended with glucose (400 kg C ha −1 ) and KNO 3 (80 kg N ha −1 ) and kept at 85% water‐filled pore space. Twelve replicate samples of the soil were incubated for 13 days under a helium‐oxygen atmosphere, simultaneously measuring gas fluxes (N 2 O, N 2 and CO 2 ) and isotope signatures ( δ 18 O‐N 2 O, δ 15 N bulk ‐N 2 O, δ 15 N α , δ 15 N β and 15 N site preference) of emitted N 2 O. The maximum N 2 O flux (6.9 ± 1.8 kg N ha −1 day −1 ) occurred 3 days after amendment application, followed by the maximum N 2 flux on day 4 (6.6 ± 3.0 kg N ha −1 day −1 ). The δ 15 N bulk was initially −34.4‰ and increased to +4.5‰ during the periods of maximum N 2 flux, demonstrating fractionation during N 2 O reduction, and then decreased. The δ 18 O‐N 2 O also increased, peaking with the maximum N 2 flux and remaining stable afterwards. The site preference (SP) decreased from the initial +7.5 to −2.1‰ when the N 2 O flux peaked, and then simultaneously increased with the appearance of the N 2 peak to +8.6‰ and remained stable thereafter, even when the O 2 supply was removed. We suggest that this results from a non‐homogenous distribution of NO in the soil, possibly linked to the KNO 3 amendments to the soil, causing the creation of several NO pools, which affected differently the isotopic signature of N 2 O and the N 2 O and N 2 fluxes during the various stages of the process. The N 2 O isotopologue values reflected the temporal patterns observed in N 2 O and N 2 fluxes. A concurrent increase in 15 N site preference and δ 18 O of N 2 O was found to be indicative of N 2 O reduction to N 2 .