Nitrous oxide emission from denitrification in stream and river networks

Abstract

Nitrous oxide (N 2 O) 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 2 O via microbial denitrification that converts N to N 2 O and dinitrogen (N 2 ). The fraction of denitrified N that escapes as N 2 O rather than N 2 (i.e., the N 2 O yield) is an important determinant of how much N 2 O is produced by river networks, but little is known about the N 2 O yield in flowing waters. Here, we present the results of whole-stream 15 N-tracer additions conducted in 72 headwater streams draining multiple land-use types across the United States. We found that stream denitrification produces N 2 O at rates that increase with stream water nitrate (NO 3 − ) concentrations, but that <1% of denitrified N is converted to N 2 O. Unlike some previous studies, we found no relationship between the N 2 O yield and stream water NO 3 − . We suggest that increased stream NO 3 − loading stimulates denitrification and concomitant N 2 O production, but does not increase the N 2 O yield. In our study, most streams were sources of N 2 O 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 N 2 O in river networks, equivalent to 10% of the global anthropogenic N 2 O emission rate. This estimate of stream and river N 2 O emissions is three times greater than estimated by the Intergovernmental Panel on Climate Change.