Nitrous oxide (N₂O) is a potent greenhouse gas involved in the destruction of stratospheric ozone. Past atmospheric mixing ratios of N₂O are archived in ice cores; however, the presence of in situ N₂O production in dust-rich Antarctic ice complicates their accurate reconstruction, especially during glacial periods. This production occurs in extremely cold ice and without sunlight. This study aims to understand the reaction producing N₂O in Antarctic ice by identifying the precursors and the reaction pathway. We compared the oxygen and nitrogen bulk and position-specific isotope composition of in situ N₂O in ice cores to the isotopic composition of nitrate (NO₃^-), a possible precursor of N₂O. The ¹⁵N signature of NO₃^- is fully transferred into the central N atom (N_α) of in situ N₂O, but it is not transferred into the terminal N atom (N_β), resulting in a 50 % transfer of the ¹⁵N signature of NO₃^- into the bulk ¹⁵N isotopic composition. These findings suggest that the in situ N₂O production involves two different nitrogen precursors present in ice: the central N atom (N_α) originates from NO₃^- and the terminal N atom (N_β) from a different precursor not yet identified. Oxygen isotope analysis shows that NO₃^- cannot be the only reservoir for the O atom of in situ N₂O. Temperature, pH, and absence of sunlight in Antarctic ice point to an abiotic N-nitrosation reaction. The limiting factor of the reaction is probably associated with mineral dust and might be Fe²⁺, reducing NO₃^- to NO₂^- or the precursor of the N_β atom. The site preference (SP) values of in situ N₂O are highly variable between different ice cores and depend on the bulk ¹⁵N isotopic composition of N₂O, itself depending on the ¹⁵N isotopic composition of the NO₃^- precursor. This finding is unexpected because SP is usually determined by the production pathway through symmetric reaction intermediates that mix the N atoms in α and β positions and average out their isotopic difference. In contrast, our results provide the first evidence of a hybrid N₂O production pathway involving an asymmetric intermediate that preserves the distinct ¹⁵N signatures of two different precursors – one contributing to the N_α atom and the other to the N_β atom. This finding has important implications: in this pathway, SP reflects the isotopic difference between the two precursors rather than the pathway itself, challenging how SP is commonly interpreted in environmental studies.