The production and emission of nitrous oxide from headwater streams in the Midwestern United States

The emission of nitrous oxide (N2O) from streams draining agricultural landscapes is estimated by the Intergovernmental Panel on Climate Change (IPCC) to constitute a globally significant source of this gas to the atmosphere, although there is considerable uncertainty in the magnitude of this source. We measured N2O emission rates and potential controlling variables in 12 headwater streams draining a predominantly agricultural basin on glacial terrain in southwestern Michigan. The study sites were nearly always supersaturated with N2O and emission rates ranged from -8.9 to 266.8 mu g N2O-N m(-2) h(-1) with an overall mean of 35.2 mu g N2O-N m(-2) h(-1). Stream water NO3- concentrations best-predicted N2O emission rates. Although streams and agricultural soils in the basin had similar areal emission rates, emissions from streams were equivalent to 6% of the anthropogenic emissions from soils because of the vastly greater surface area of soils. We found that the default value of the N2O emission factor for streams and groundwater as defined by the IPCC (EF5-g) was similar to the value observed in this study lending support to the recent downward revision to EF5-g. However, the EF5-g spanned four orders of magnitude across our study sites suggesting that the IPCC's methodology of applying one emission factor to all streams may be inappropriate.