Isotopologue ratios of N2O emitted from microcosms with NH4+ fertilized arable soils under conditions favoring nitrification

Soils represent the major source of the atmospheric greenhouse gas nitrous oxide (N2O) and there is a need to better constrain the total global flux and the relative contribution of the microbial source processes. The aim of our study was to determine variability and control of the isotopic fingerprint of N2O fluxes following NH4+-fertilization and dominated by nitrification. We conducted a microcosm study with three arable soils fertilized with 0–140 mg NH4+–N kg−1. Fractions of N2O derived from nitrification and denitrification were determined in parallel experiments using the 15N tracer and acetylene inhibition techniques or by comparison with unfertilized treatments. Soils were incubated for 3–10 days at low moisture (30–55% water-filled pore space) in order to establish conditions favoring nitrification. Dual isotope and isotopomer ratios of emitted N2O were determined by mass spectrometric analysis of δ18O, average δ15N (δ15Nbulk) and 15N site preference (SP = difference in δ15N between the central and peripheral N positions of the asymmetric N2O molecule). N2O originated mainly from nitrification (>80%) in all treatments and the proportion of NH4+ nitrified that was lost as N2O ranged between 0.07 and 0.45%. δ18O and SP of N2O fluxes ranged from 15 to 28.4‰ and from 13.9 to 29.8‰, respectively. These ranges overlapped with isotopic signatures of N2O from denitrification reported previously. There was a negative correlation between SP and δ18O which is opposite to reported trends in N2O from denitrification. Variation of average 15N signatures of N2O (δ15Nbulk) did not supply process information, apparently because a strong shift in precursor signatures masked process-specific effects on δ15Nbulk. Maximum SP of total N2O fluxes and of nitrification fluxes was close to reported SP of N2O from NH4+ or NH2OH conversion by autotrophic nitrifiers, suggesting that SP close to 30‰ is typical for autotrophic nitrification in soils following NH4+-fertilization. The results suggest that the δ18O/SP fingerprint of N2O might be used as a new indicator of the dominant source process of N2O fluxes in soils.