Bacterial denitrification drives elevated N <sub>2</sub> O emissions in arid southern California drylands
Alexander H. Krichels, G. Darrel Jenerette, Hannah Shulman, Stephanie Piper, Aral C. Greene, Holly Andrews, Jon Botthoff, James O. Sickman, Emma L. Aronson, Peter M. Homyak
Abstract
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.