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Soil nitrous oxide (N 2 O) emissions are highly variable in space and time, making it difficult to estimate ecosystem level fluxes of this potent greenhouse gas. While topographic depressions are often evoked as permanent N 2 O hot spots and rain events are well-known triggers of N 2 O hot moments, soil N 2 O emissions are still poorly predicted. Thus, the objective of this study was to determine how to best use topography and rain events as variables to predict soil N 2 O emissions at the field scale. We measured soil N 2 O emissions 11 times over the course of one growing season from 65 locations within an agricultural field exhibiting microtopography. We found that the topographic indices best predicting soil N 2 O emissions varied by date, with soil properties as consistently poor predictors. Large rain events (>30 mm) led to an N 2 O hot moment only in the early summer and not in the cool spring or later in the summer when crops were at peak growth and likely had high evapotranspiration rates. In a laboratory experiment, we demonstrated that low heterotrophic respiration rates at cold temperatures slowly depleted soil dissolved O 2 , thus suppressing denitrification over the 2-3 day timescale typical of field ponding. Our findings show that topographic depressions do not consistently act as N 2 O hot spots and that rainfall does not consistently trigger N 2 O hot moments. We assert that the spatiotemporal variation in soil N 2 O emissions is not always characterized by predictable hot spots or hot moments and that controls on this variation change depending on environmental conditions. Plain Language Summary Soils are the primary source of nitrous oxide (N 2 O), a greenhouse gas that contributes to global warming. Nitrous oxide is produced under oxygen-depleted conditions that can occur when soils are saturated with water. Large rain events are therefore thought to lead to short periods of high soil N 2 O emissions. Landscape topography can shape spatial patterns in soil moisture and other soil properties that may regulate spatial variation in soil N 2 O emissions. The goals of this study were to determine which topographic factors best predict spatial variation in soil N 2 O emissions and under what conditions, and to determine when large rain events may not lead to bursts of soil N 2 O emissions. We found that throughout the growing season, all topographic indices were poor predictors of soil N 2 O emissions in an agricultural field. A large early summer rain event triggered a pulse of soil N 2 O emissions, but large rain events did not trigger similar pulses in the cool spring months or later in the summer when the crops were at peak growth with high soil water demand. This has important implications for predicting how soil N 2 O emissions will respond to expected future changes in temperature and rainfall, feeding back on climate change.

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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−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 h and increased in proportion to ammonium addition, exceeding 600 ng N–NO m−2 s−1 in 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 minutes 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.