Soil drying and wetting cycles can produce pulses of nitric oxide (NO) and nitrous oxide (N₂O) emissions with substantial effects on both regional air quality and Earth’s climate. While pulsed production of N emissions is ubiquitous across ecosystems, the processes governing pulse magnitude and timing remain unclear. We studied the processes producing pulsed NO and N₂O emissions at two contrasting drylands, desert and chaparral, where despite the hot and dry conditions known to limit biological processes, some of the highest NO and N₂O flux rates have been measured. We measured N₂O and NO emissions every 30 minutes for 24 hours after wetting soils with isotopically-enriched nitrate and ammonium solutions to determine production pathways. Nitrate was reduced to N₂O within 15 minutes of wetting, with emissions exceeding 1000 ng N-N₂O m^-2 s^-1 and returning to background levels within four hours, but the pulse magnitude did not increase in proportion to the amount of ammonium or nitrate added. In contrast to N₂O, NO was emitted over 24 hours and increased in proportion to ammonium addition, exceeding 600 ng N-NO m^-2 s^-1 in both desert and chaparral soils. Nitrogen and oxygen isotopes suggest nitrifier denitrification and nitrate reduction produced NO. Taken together, our measurements demonstrate that rapid nitrate reduction following wetting dry soils produces large pulsed N₂O emissions, and that both denitrification and nitrifier denitrification contribute to longer lasting NO emissions. These mechanisms are substantial pathways of ecosystem N loss that also contribute to regional air quality and global climate dynamics.

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