Nitrous oxide emissions are highly variable, both in space and time, and the estimation of N2O emissions from individual livestock farms based on actual measurements is very costly and impractical. Models have therefore become an important means for improving our understanding of the complex interactions between drivers of N2O emissions, for estimating N2O emissions from livestock systems and for evaluating practices that can reduce emissions. These models range from relatively simple national inventory or annual accounting models to detailed process-based models, and use factors and constants that are derived from measurements under experimental and controlled conditions.

The main example of the inventory approach is the IPCC methodology for estimating national greenhouse gas inventories (IPCC, 2006), while the DNDC model (Li et al, 1992a, 1992b) is one of the best known biophysical models for estimating N2O emissions. These inventory and modelling approaches are discussed below.

Inventory/accounting models

The IPCC inventory methodology has been developed for estimating the greenhouse gas emissions of a nation. In its simplest form, the IPCC inventory methodology estimates N2O emissions from the size of a given source of emissions (for example amount of N fertilizer applied) multiplied by N2O emission factors associated with that source. The inventory methodology includes both direct and indirect N2O emissions. Direct emissions are those that occur from N sources within the farm system. Indirect emissions are those from nitrogen that is lost through nitrate leaching or ammonia volatilization and subsequently emitted as N2O from surface waters or following re-deposition of NH3 to land (Figure 6.1). The IPCC Guidelines for National Greenhouse Gas Inventories (IPCC, 1997, 2006) provide the methodology as well as default values for the N2O emission factors and N loss fractions, but recommend that countries use 'country-specific' factors for key sources of N2O.

A detailed description of the revised 1996 IPCC methodology for N2O is provided by Mosier et al (1998). These 1996 guidelines were again revised in 2006 (IPCC, 2006) based on more recent research findings, and new knowledge and insights. The main changes between the 1996 and 2006 N2O methodologies include updated default values for some of the emission factors, removal of one source of N2O and inclusion of various new sources. In addition, adjustments to the methodology were made to remove some inconsistencies with respect to N inputs from crop residue and N leaching estimates. Table 6.1 summarizes the 2006 changes to the IPCC inventory methodology for N2O (see also Chapter 4).

Background emissions from improved pastures

Neither the 1996 nor the 2006 IPCC guidelines include estimates of 'background' N2O emissions from improved pastures as an anthropogenic source of N2O. Although the 1996 guidelines make reference to the fact that background emissions from agricultural soils may be higher than historic natural emissions as a result of enhanced mineralization of soil organic matter, only N2O emissions from added sources (for example N fertilizer, urine and manure applications) are included in the methodology. Yet evidence exists that N2O emissions from improved pastures without additional N inputs (often referred to as 'control sites' in experimental trials to determine N2O emission factors) are significantly higher than emissions from unimproved sites and should therefore be considered as anthropogenic (Bouwman, 1996; Stehfest and Bouwman, 2006). For example, Bouwman et al (2002) provided a summary of available measurement data of N2O emissions from fertilized fields (>900 observations) and suggested that annual N2O emissions from unfertilized pastures were between 0 and 1.5kg N ha-1. This is similar to 'background' emissions of 1kg N ha-1 suggested by the equation Bouwman (1996) presented:

N2O-N (kg ha-1) = 1 + (1.25 X N input in kg ha-1)/100 (6.1)

which was the origin of the default N2O emission factor for N fertilizer in the

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