3 - Approach
3.1 - Focus of the study
For each of the greenhouse gases a separate review of New Zealand and international literature was carried out, to:
- identify currently operational practices and technologies, and long-term opportunities, for reducing nitrous oxide, methane and carbon dioxide emissions.
- note any other environmental and economic effects of these practices/technologies.
Official data for 1998 for New Zealand show that:
- in excess of 99% of agricultural methane emissions arise from grazed ruminants (MfE 2000).
- 99% of nitrous oxide emissions arise from agricultural soils (MfE 2000).
- 85% of agricultural soils are grazed by livestock (MAF 2001b).
Therefore for maximum impact, mitigation strategies will need to concentrate on the pastoral sector. This report therefore deals exclusively with practices and technologies that are applicable to pastoral agriculture but does not consider land use change as a mitigation practice. The report concentrates on technologies and practices that will influence greenhouse gases at the source of the emission or alter the size of sinks by changing management practices on an existing land use. However, there is a strong possibility that, in practice, greenhouse gas emissions will be reduced by an integrated approach that address both specific mitigation technologies and changing land use. This will have to be considered at the whole farm scale and is beyond the scope of this report. It is however an area that needs to be addressed as a matter of urgency.
3.2 - Inventory calculations
To evaluate the potential of the mitigation practices and technologies to reduce greenhouse gas emissions, inventory calculations were carried out using existing methodologies. For each mitigation option, input data and/or emission factor values were adapted as appropriate, and emission calculations were compared with those under a 'business as usual' scenario.
Nitrous Oxide
For nitrous oxide, inventory calculations were carried out based on the latest IPCC methodology (IPCC 2000), but using information on the regional distribution of sheep, dairy and beef cattle and soil drainage class, which was obtained from a recent study on the regional variations in N2O emissions (Sherlock et al 2001). For the purpose of the current report, these regional calculations were extended to include N2O emissions from all animals and from other sources such as fertiliser use, crop residue, N fixing crops and cultivated organic soils. The animal population statistics for dairy cattle, beef cattle, sheep, deer, pigs and goats were obtained from the Ministry of Agriculture & Forestry Web site. The required activity data for poultry, nitrogen fertiliser use, crop residue, N fixing crops and cultivated organic soils were obtained from the New Zealand Greenhouse Gas Inventory for 1999 (MfE 2001). The calculations were carried out on a regional basis and included information on soil drainage class distribution within each region, with corresponding N2O emission factors of 0.005 ('well drained soils'), 0.02 ('imperfectly drained soils') and 0.026 ('poorly drained soils'). This 1999 inventory was then used as a baseline against which mitigation options were evaluated by adapting either the activity data or the emission factor values.
To assess potential future impacts of the mitigation options on N2O emissions, inventory calculations were also estimated for 2010. These calculations were carried out based on the predicted N2O emissions from dairy, sheep and beef for 2010 as presented by Sherlock et al (2001) under scenario 4, i.e. projected sheep numbers of about 39 million and assuming an increase in dairy cow numbers in the southern regions of 150% and of 7% in other regions. For the other animal species no estimates of N2O emissions exists and these were therefore not included in the calculations. However, the N2O emissions from N fertiliser use were included, and it was assumed that in 2010 the total N fertiliser use would be about 275,000,000 kg N/yr, which is approximately a linear increase in fertiliser use since 1990. If N fertiliser use in 2010 is predicted based on an expected increase in fertiliser use on dairy farms of 50 kg N/ha and 4 kg N/ha on sheep and beef farms and the predicted stocking rates are 3 cow/ha and 10 SU/ha, respectively, the estimated total N fertiliser use is similar at approximately 270,000,000 kg N/yr.
Methane
Methane emission data for different classes (e.g. sheep, dairy beef) and types of animals (e.g. dairy cows, breeding ewes) were taken from Clark (2001). The inventory total was also taken from the same publication. The method used in this publication is a modification of the Ulyatt et al (1991) method and closely follows IPCC Tier 2 Good Practice guidelines.
Carbon dioxide emissions from New Zealand Agriculture are currently not reported and so no inventory data are available. Agricultural soils store large quantities of carbon but it will be voluntary whether New Zealand elects to account for carbon changes in agricultural soils during the first commitment period (2008-2012) of the Kyoto Protocol. A key determining factor on whether New Zealand will elect to account for soil carbon will be the ability to accurately establish 1990 soil carbon baselines and to accurately quantify how these are changing over time.
3.3 - Cost benefit analysis
When compiling the report it became clear that the information available on the impact and cost of the majority of mitigation options was not sufficient to allow a detailed cost benefit analysis to be carried out. For nitrous oxide and methane enough information was available for a limited assessment of the financial implications of some of the mitigation options. In these cases a simple approach was adopted. This involved estimating the cost per tonne of CO2 saved in relation to the cost of implementing the technology. The likely 'value' of the CO2 emission reductions was also estimated assuming a range of values for CO2. No cost benefit analysis was carried out for carbon dioxide mitigation options.
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