Abstract
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.
Highlights
Soil drying–wetting cycles are widespread and can stimulate large emissions of both nitrous oxide (N2O) and nitric oxide (NO) (Scholes et al 1997; Homyak et al 2016; Eberwein et al 2020) with profound implications for Earth’s climate, regional air quality, and ecosystem N retention
While the production of NO and N2O is governed by both biological and chemical processes upon wetting dry soil, the magnitude of the emissions vary as a function of aridity (Wang et al 2014; Liu et al 2017; von Sperber et al 2017), with some drylands recording among the highest NO and N2O emission pulses globally (Eberwein et al 2020)
As observed in the desert, N 2O emissions did not increase in proportion to adding N O3− (p = 0.25) or N H4+ (p = 0.10, Table 2)
Summary
Soil drying–wetting cycles are widespread and can stimulate large emissions of both nitrous oxide (N2O) and nitric oxide (NO) (Scholes et al 1997; Homyak et al 2016; Eberwein et al 2020) with profound implications for Earth’s climate, regional air quality, and ecosystem N retention. While the production of NO and N2O is governed by both biological and chemical processes upon wetting dry soil, the magnitude of the emissions vary as a function of aridity (Wang et al 2014; Liu et al 2017; von Sperber et al 2017), with some drylands recording among the highest NO and N2O emission pulses globally (Eberwein et al 2020) How these emissions vary across ecosystems experiencing drying–wetting cycles and the biogeochemical processes producing them are still not well characterized. Because both biological and abiotic processes can occur simultaneously, it has been challenging to determine
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