Nitrous oxide (N.sub.2 O) is a greenhouse gas with a global warming potential approximately 298 times greater than that of CO.sub.2 . In 2006, the Intergovernmental Panel on Climate Change (IPCC) estimated N.sub.2 O emission due to synthetic and organic nitrogen (N) fertilization at 1% of applied N. We investigated the uncertainty on this estimated value, by fitting 13 different models to a published dataset including 985 N.sub.2 O measurements. These models were characterized by (i) the presence or absence of the explanatory variable "applied N", (ii) the function relating N.sub.2 O emission to applied N (exponential or linear function), (iii) fixed or random background (i.e. in the absence of N application) N.sub.2 O emission and (iv) fixed or random applied N effect. We calculated ranges of uncertainty on N.sub.2 O emissions from a subset of these models, and compared them with the uncertainty ranges currently used in the IPCC-Tier 1 method. The exponential models outperformed the linear models, and models including one or two random effects outperformed those including fixed effects only. The use of an exponential function rather than a linear function has an important practical consequence: the emission factor is not constant and increases as a function of applied N. Emission factors estimated using the exponential function were lower than 1% when the amount of N applied was below 160 kg N ha.sup.-1 . Our uncertainty analysis shows that the uncertainty range currently used by the IPCC-Tier 1 method could be reduced.