Isotope signatures of N 2 O emitted from vegetable soil: Ammonia oxidation drives N 2 O production in NH 4 + -fertilized soil of North China

Nitrous oxide (N 2 O) is a potent greenhouse gas. In North China, vegetable fields are amended with high levels of N fertilizer and irrigation water, which causes massive N 2 O flux. The aim of this study was to determine the contribution of microbial processes to N 2 O production and characterize isotopic signature effects on N 2 O source partitioning. We conducted a microcosm study that combined naturally abundant isotopologues and gas inhibitor techniques to analyze N 2 O flux and its isotopomer signatures [δ 15 N bulk , δ 18 O, and SP (intramolecular 15 N site preference)] that emitted from vegetable soil after the addition of NH 4 + fertilizers. The results show that ammonia oxidation is the predominant process under high water content (70% water-filled pore space), and nitrifier denitrification contribution increases with increasing N content. δ 15 N bulk and δ 18 O of N 2 O may not provide information about microbial processes due to great shifts in precursor signatures and atom exchange, especially for soil treated with NH 4 + fertilizer. SP and associated two end-member mixing model are useful to distinguish N 2 O source and contribution. Further work is needed to explore isotopomer signature stability to improve N 2 O microbial process identification.