Dual isotope and isotopomer measurements for the understanding of N₂O 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 (N₂O) 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 KNO₃ (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 (N₂O, N₂ and CO₂) and isotope signatures (δ¹⁸O-N₂O, δ¹⁵Nbulk-N₂O, δ¹⁵Nα, δ¹⁵Nβ and ¹⁵N site preference) of emitted N₂O. The maximum N₂O flux (6.9 ± 1.8 kg N ha⁻¹ day⁻¹) occurred 3 days after amendment application, followed by the maximum N₂ flux on day 4 (6.6 ± 3.0 kg N ha⁻¹ day⁻¹). The δ¹⁵Nbulk was initially -34.4[per thousand] and increased to +4.5[per thousand] during the periods of maximum N₂ flux, demonstrating fractionation during N₂O reduction, and then decreased. The δ¹⁸O-N₂O also increased, peaking with the maximum N₂ flux and remaining stable afterwards. The site preference (SP) decreased from the initial +7.5 to -2.1[per thousand] when the N₂O flux peaked, and then simultaneously increased with the appearance of the N₂ peak to +8.6[per thousand] and remained stable thereafter, even when the O₂ supply was removed. We suggest that this results from a non-homogenous distribution of NO [graphic removed] in the soil, possibly linked to the KNO₃ amendments to the soil, causing the creation of several NO [graphic removed] pools, which affected differently the isotopic signature of N₂O and the N₂O and N₂ fluxes during the various stages of the process. The N₂O isotopologue values reflected the temporal patterns observed in N₂O and N₂ fluxes. A concurrent increase in ¹⁵N site preference and δ¹⁸O of N₂O was found to be indicative of N₂O reduction to N₂.