Bacterial denitrification drives elevated NO emissions in arid southern California drylands.

Soils are the largest source of atmospheric nitrous oxide (NO), a powerful greenhouse gas. Dry soils rarely harbor anoxic conditions to favor denitrification, the predominant NO-producing process, yet, among the largest NO emissions have been measured after wetting summer-dry desert soils, raising the question: Can denitrifiers endure extreme drought and produce NO immediately after rainfall? Using isotopic and molecular approaches in a California desert, we found that denitrifiers produced NO within 15 minutes of wetting dry soils (site preference = 12.8 ± 3.92 per mil, δN = 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 NO 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 NO emitted when dry soils are wetted.