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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 min for 24 h after wetting soils with isotopically-enriched nitrate and ammonium solutions to determine production pathways and their timing. Nitrate was reduced to N₂O within 15 min of wetting, with emissions exceeding 1000 ng N–N₂O m⁻² s⁻¹and 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 h and increased in proportion to ammonium addition, exceeding 600 ng N–NO m⁻² s⁻¹in desert and chaparral soils. Isotope tracers suggest that both ammonia oxidation and nitrate reduction produced NO. Taken together, our measurements demonstrate that nitrate can be reduced within minutesmore »of wetting summer-dry desert soils to produce large N₂O emission pulses and that multiple processes contribute to long-lasting NO emissions. These mechanisms represent substantial pathways of ecosystem N loss that also contribute to regional air quality and global climate dynamics.

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Topographic depressions in upland soils experience anaerobic conditions conducive for iron (Fe) reduction following heavy rainfall. These depressional areas can also accumulate reactive Fe compounds, carbon (C), and nitrate, creating potential hot spots of Fe-mediated carbon dioxide (CO2) and nitrous oxide (N2O) production. While there are multiple mechanisms by which Fe redox reactions can facilitate CO2 and N2O production, it is unclear what their cumulative effect is on CO2 and N2O emissions in depressional soils under dynamic redox. We hypothesized that Fe reduction and oxidation facilitate greater CO2 and N2O emissions in depressional compared to upslope soils in response to flooding. To test this, we amended upslope and depressional soils with Fe(II), Fe(III), or labile C and measured CO2 and N2O emissions in response to flooding. We found that depressional soils have greater Fe reduction potential, which can contribute to soil CO2 emissions during flooded conditions when C is not limiting. Additionally, Fe(II) addition stimulated N2O production, suggesting that chemodenitrification may be an important pathway of N2O production in depressions that accumulate Fe(II). As rainfall intensification results in more frequent flooding of depressional upland soils, Fe-mediated CO2 and N2O production may become increasingly important pathways of soil greenhouse gas emissions.