Quantifying uncertainties in N(2)O emission due to N fertilizer application in cultivated areas

Nitrous oxide (N(2)O) is a greenhouse gas with a global warming potential approximately 298 times greater than that of CO(2). In 2006, the Intergovernmental Panel on Climate Change (IPCC) estimated N(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(2)O measurements. These models were characterized by (i) the presence or absence of the explanatory variable "applied N", (ii) the function relating N(2)O emission to applied N (exponential or linear function), (iii) fixed or random background (i.e. in the absence of N application) N(2)O emission and (iv) fixed or random applied N effect. We calculated ranges of uncertainty on N(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(-1). Our uncertainty analysis shows that the uncertainty range currently used by the IPCC-Tier 1 method could be reduced.