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Soils are the largest source of atmospheric nitrous oxide (N 2 O), a powerful greenhouse gas. Dry soils rarely harbor anoxic conditions to favor denitrification, the predominant N 2 O-producing process, yet, among the largest N 2 O emissions have been measured after wetting summer-dry desert soils, raising the question: Can denitrifiers endure extreme drought and produce N 2 O immediately after rainfall? Using isotopic and molecular approaches in a California desert, we found that denitrifiers produced N 2 O within 15 minutes of wetting dry soils (site preference = 12.8 ± 3.92 per mil, δ 15 N bulk = 18.6 ± 11.1 per mil). Consistent with this finding, we detected nitrate-reducing transcripts in dry soils and found that inhibiting microbial activity decreased N 2 O emissions by 59%. Our results suggest that despite extreme environmental conditions-months without precipitation, soil temperatures of ≥40°C, and gravimetric soil water content of <1%-bacterial denitrifiers can account for most of the N 2 O emitted when dry soils are wetted.

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. [ABSTRACT FROM AUTHOR]

Soils are the largest source of atmospheric nitrous oxide (N2O), a powerful greenhouse gas. Dry soils rarely harbor anoxic conditions to favor denitrification, the predominant N2O-producing process, yet, among the largest N2O emissions have been measured after wetting summer-dry desert soils, raising the question: Can denitrifiers endure extreme drought and produce N2O immediately after rainfall? Using isotopic and molecular approaches in a California desert, we found that denitrifiers produced N2O within 15 minutes of wetting dry soils (site preference = 12.8 ± 3.92 per mil, δ15Nbulk = 18.6 ± 11.1 per mil). Consistent with this finding, we detected nitrate-reducing transcripts in dry soils and found that inhibiting microbial activity decreased N2O emissions by 59%. Our results suggest that despite extreme environmental conditions--months without precipitation, soil temperatures of≥40°C, and gravimetric soilwater content of <1%--bacterial denitrifiers can account for most of the N2O emitted when dry soils are wetted. [ABSTRACT FROM AUTHOR]