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

Abstract

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 N-15 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 6180, average delta N-15 (delta N-15 bulk) and N-15 site preference (SP = difference in delta N-15 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%. delta O-18 and SP of N2O fluxes ranged from 15 to 28.4 parts per thousand and from 13.9 to 29.8 parts per thousand, respectively. These ranges overlapped with isotopic signatures of N2O from denitrification reported previously. There was a negative correlation between SP and delta O-18 which is opposite to reported trends in N2O from denitrification. Variation of average N-15 signatures of N2O (delta N-15(bulk)) did not supply process information, apparently because a strong shift in precursor signatures masked process-specific effects on delta N-15(bulk). 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 parts per thousand is typical for autotrophic nitrification in soils following NH4+-fertilization. The results suggest that the delta O-18/SP fingerprint of N2O might be used as a new indicator of the dominant source process of N2O fluxes in soils. (C) 2008 Elsevier Ltd. All rights reserved.