Nitrous oxide emissions are known to have large variation both in time and space (e.g. Stehfest and Bouwman, 2006).
To account for temporal variation, the IPCC recommended that emission factors should only be estimated from data collected from a period of at least a year (Penman et al., 2000).
Variation in space will substantially contribute to the large confidence intervals given for the IPCC emission factors.