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.