Dual isotope and isotopomer measurements for the understanding of N2O production and consumption during denitrification in an arable soil

Abstract

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, N-2 and CO2) and isotope signatures delta O-18-N2O, delta N-15(bulk)-N2O, delta N-15(alpha), delta N-15(beta) and N-15 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 N-2 flux on day 4 (6.6 +/- 3.0 kg N ha(-1) day(-1)). The delta N-15(bulk) was initially -34.4% and increased to +4.5% during the periods of maximum N-2 flux, demonstrating fractionation during N2O reduction, and then decreased. The delta O-18-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% when the N2O flux peaked, and then simultaneously increased with the appearance of the N-2 peak to +8.6 parts per thousand and remained stable thereafter, even when the O-2 supply was removed. We suggest that this results from a non-homogenous distribution of NO3- in the soil, possibly linked to the KNO3 amendments to the soil, causing the creation of several NO3- pools, which affected differently the isotopic signature of N2O and the N2O and N-2 fluxes during the various stages of the process. The N2O isotopologue values reflected the temporal patterns observed in N2O and N-2 fluxes. A concurrent increase in N-15 site preference and delta O-18 of N2O was found to be indicative of N2O reduction to N-2.