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.