Agricultural soils are the most important anthropogenic source of nitrous oxide to the atmosphere. We observed large shifts with time in the emission rate (from 170 to 16 ng N cm⁻² h⁻¹) and in δ¹⁵N of N₂O emitted (from −46‰ to +5‰ relative to atmospheric N₂) from a urea-fertilized and irrigated agricultural field in Mexico. We calculated overall instantaneous enrichment factors for the sampling period, which suggest that the microbial N₂O production shifts from nitrification (week 1) to denitrification (week 2). Isotopic signatures of N₂O emissions were not always in accord with other proxies (such as NO/N₂O emission ratio or water-filled pore space) used to estimate the relative importance of nitrification and denitrification as N₂O sources. These observations strongly suggest that the soil surface emissions integrate processes occurring at different depths in the soil and a decoupling of NO and N₂O production in this system. Further clues as to the source of N₂O come from the positional dependence of ¹⁵N in the emitted N₂O, reported here for the first time in soil emissions. Enrichment at the central N position increased relative to the terminal N position by 9.3‰ during the first 4 days after irrigation, implying that nitrification preferentially enriches the central N position compared to denitrification. The overall δ¹⁵N signature we measured for N₂O emitted from N-fertilized agricultural systems is more depleted than observed δ¹⁵N values for N₂O emitted from more N-limited forest soils. Assuming that one half of the total agricultural N₂O emissions associated with the global increase in soil nitrogen fertilizer use have an isotopic composition comparable to those of the agricultural fields reported here, we predict a decline in the isotopic signature of tropospheric N₂O during this century of as much as 3‰ for ¹⁵N. Although many uncertainties remain, we suggest that measurements of δ¹⁵N-N₂O in firn air will provide constraints on how the N₂O budget has changed during the past century.