Bacterial denitrification drives elevated N 2 O emissions in arid southern California drylands.

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.