Nitrous oxide emission from denitrification in stream and river networks

Nitrous oxide (N₂O) is a potent greenhouse gas that contributes to climate change and stratospheric ozone destruction. Anthropogenic nitrogen (N) loading to river networks is a potentially important source of N ₂O via microbial denitrification that converts N to N₂O and dinitrogen (N₂). The fraction of denitrified N that escapes as N₂O rather than N₂ (i.e., the N₂O yield) is an important determinant of how much N₂O is produced by river networks, but little is known about the N₂O yield in flowing waters. Here, we present the results of whole-stream ¹⁵N-tracer additions conducted in 72 headwater streams draining multiple land-use types across the United States. We found that stream denitrification produces N₂O at rates that increase with stream water nitrate (NO₃^-) concentrations, but that <1% of denitrified N is converted to N₂O. Unlike some previous studies, we found no relationship between the N₂O yield and stream water NO₃^-. We suggest that increased stream NO₃^- loading stimulates denitrification and concomitant N₂O production, but does not increase the N₂O yield. In our study, most streams were sources of N₂O to the atmosphere and the highest emission rates were observed in streams draining urban basins. Using a global river network model, we estimate that microbial N transformations (e.g., denitrification and nitrification) convert at least 0.68 Tg·y ^-1 of anthropogenic N inputs to N₂O in river networks, equivalent to 10% of the global anthropogenic N₂O emission rate. This estimate of stream and river N₂O emissions is three times greater than estimated by the Intergovernmental Panel on Climate Change.