Soil Nitrous Oxide Emissions Following Rain Pulses: Microbial Gene Expression Data Suggest Nitrification Is the Primary Source.

The first rainfall following the hot, dry summer in Mediterranean annual grasslands causes large and rapid emissions of nitrous oxide. The goal of this study was to understand the microbial sources of nitrous oxide emissions over time following wet-up and to assess the temporal dynamics of the organisms and processes potentially involved. Using laboratory simulations of rainfall, we monitored effluxes of carbon dioxide and nitrous oxide; pools of dissolved organic carbon, dissolved organic nitrogen, ammonium, and nitrate; and microbial gene expression (transcripts) of nitrifying bacteria, (ammonia oxidation and nitrite oxidation), and denitrifying bacteria, (nitrite reduction and nitrous oxide reduction), from 15 minutes to 72 hours following the water pulse.

Results from a California annual grassland soil show both heterotrophic activity and nitrification turn on within 3 hours of the rainfall event. Soil respiration and changes in dissolved organic carbon and nitrogen are greatest within the first three hours following wet-up. Trends in nitrification and gene expression for nitrification follow those of nitrous oxide emissions, all peaking at nine hours after the rain pulse. Denitrifier gene expression of nitrite reductase was never detected, while that of nitrous oxide reductase was detected consistently at a very low level in keeping with the amounts reported previously in studies of inactive denitrifiers in pure culture. Together, these data suggest that nitrification is the primary source of nitrous oxide in this soil at this time of year.