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• Context
• Goal
• Approach
• Outcomes
  • Nitrous Oxide
  • Ruminant Methane
  • Carbon Dioxide
  • Conclusion

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1 - Summary

Context

Current predictions are that New Zealand's CO₂ equivalent emissions will rise above 1990 levels by between 50 to 75 million tonnes of carbon dioxide equivalents by the start of the first commitment period (2008-2012) of the Kyoto Protocol. As the Kyoto Protocol targets will be legally binding upon ratification, New Zealand is potentially facing a situation where these excess emissions will incur a financial penalty. The agriculture sector is responsible for over 50% of our total emissions and the adoption of practices and technologies aimed specifically at reducing emissions from this sector will have a significant impact on total emissions.

Goal

This report reviews practices and technologies that are designed to reduce nitrous oxide, methane and carbon dioxide emissions from agriculture. It evaluates their potential to reduce emissions and notes their effect on other environmental parameters. Because New Zealand's agricultural greenhouse gas emissions are dominated by the pastoral sector, the report concentrates on this sector.

Approach

Potential management practices and technologies for reducing nitrous oxide, methane and carbon dioxide emissions from agriculture were reviewed from New Zealand and international literature. Their potential to reduce emissions was estimated using existing inventory calculation methodologies. For the practices/technologies expected to have the biggest potential for reducing emissions, the financial consequences were assessed.

Outcomes

Nitrous Oxide

Nitrous oxide emissions from pastoral agriculture arise as a result of the soil processes denitrification and nitrification. Mitigation options aim to reduce the rates of these processes and ensure that nitrogen is emitted as harmless N₂ rather than N₂O. Denitrification and nitrification are affected by many soil and climatic factors, this complicates the evaluation of potential mitigation options. As a result, experimental evidence of the impact of mitigation options on N₂O emissions is limited and the estimations presented in this report are based on many assumptions. The results should be interpreted in this context. The main findings on nitrous oxide mitigation options are:

• Management practices that reduce the amount of excreta N provide the largest reductions in N₂O emissions.
• Reducing animal numbers reduces excreta N but is not a viable financial option for livestock farmers.
• Diet manipulation and changing winter management practices have the potential to each reduce national N₂O emissions by approximately 6%. These practices may, however, have a negative impact on methane emissions and carbon storage in soils.
• Based on current N fertiliser usage, national N₂O emissions could be reduced by 1-3% by reducing the amount, timing and type of N fertilisers used. However, the impact of these mitigation options could increase with time, due to a forecasted increase in N fertiliser use.
• Altering soil conditions by liming, improving drainage and avoiding compaction have the potential to reduce national N₂O emissions by 4.5%, 2.5% and 3% respectively. Improving drainage may, however, increase carbon oxidation from soils.

Ruminant Methane

The vast majority of methane emissions from pastoral agriculture arise from the breakdown of food by micro-organisms in the rumen, so called enteric methane. Reduction technologies can broadly be grouped into those that feed animals better, so that less methane is produced per unit of product, and those that directly modify the activity of rumen microbes, so that less methane is produced in total. Common to both of these approaches is a general lack of substantive experimental evidence that the technologies currently available can actually reduce methane under New Zealand conditions. The main findings on methane mitigation options are:

• Reducing livestock numbers, and at the same time maintaining the level of performance of existing livestock, is a very effective method of reducing methane emissions but is not a viable financial option for livestock farmers.
• Feeding animals better through the use of concentrate feeds will reduce emissions but is also not financially viable in New Zealand's low cost farm systems.
• The use of grass cultivars selected for improved animal performance could decrease methane emissions per unit of animal product. Total national methane emissions would only fall below projections if the rate of decline in methane emitted per unit of product is faster than the rate of increase in animal numbers.
• Some less common forage species (e.g. lotus, sulla) that contain high concentrations of condensed tannins do appear to produce less methane when digested by ruminants but their effect on emissions is constrained because they are not easily incorporated into New Zealand's farm systems.
• The effect of improved grass cultivars and alternative forage species on total emissions is estimated to produce a maximum reduction of 5%. Their impact is constrained by the limited amount of land that is re-seeded each year. The negative impact of cultivation on carbon storage also has to be considered.
• Probiotics are feed supplements that directly affect rumen function. On the evidence available they are likely to have only a small impact on methane emissions per animal. As they need to be fed daily, their use is restricted mainly to the dairy sector. Their impact on total national methane emissions is therefore expected to be less than 1.5%.
• Ionophores, antibiotic feed supplements, could reduce emissions by up to 8% if used extensively in the beef, sheep and dairy sectors. Monensin is the ionophore showing the most promise. The caveat for New Zealand is that very little of the research on monensin comes from grazing animals and no evidence is available from long term trials. To reduce methane at a reasonable cost, ionophores also need to increase animal performance. There may be consumer resistance to the routine use of ionophores as they are a type of antibiotic. Ionophores also decrease the amount of N excreted by ruminants and should therefore reduce N₂O emissions from pastures.
• Other technologies targeted at directly influencing rumen fermentation (e.g. vaccines) are only at the early research/concept stage and are unlikely be available within the next 5 years.

Carbon Dioxide

The main findings on carbon dioxide mitigation options are:

• Carbon dioxide emissions are currently not reported from the agriculture but are included within the energy sector and land use change and forestry national inventory calculations. However, agricultural management practices (e.g. re-seeding, grazing management) have implications for soil C sequestration.
• Managing grazing land to increase carbon storage requires a larger proportion of the carbon fixed in photosynthesis to be returned to the soil. This is not economically viable as it means reduced product output relative to inputs.
• Increased productivity per unit area (intensification) can be achieved without any cost in C sequestered as long as either the inputs (fertiliser, irrigation) on that area are also increased or the increased utilization of one area is matched by reduced utilization on other areas of the farm or region.
• Biofuels have the greatest potential for using agricultural land to mitigate greenhouse gas emissions because they can sequester soil C and substitute for fossil fuels.
• Energy crops have been investigated in New Zealand in the past but new advances in cellulose conversion technology and the current emphasis on greenhouse gas emissions means that a in depth re-evaluation is required.
• Using 12% of our pastoral land to grow herbaceous feedstocks for bioethanol production would provide equivalent energy to that derived from our current total petrol usage.
• The current cost of imported petrol is 67 c/l (excluding tax and levies). The costs of bioethanol production are currently 70 c/l but will fall with improvements in feedstocks and cellulose conversion technology.

Conclusion

It is clear from this review of the options available for reducing greenhouse gas emissions that simple, single solutions do not exist and that in many cases the experimental evidence by which these options can be judged is limited. A range of options is available and although each option has, in general, only a small impact, if implemented collectively they could help to defer the forecast rise in greenhouse gas emissions from agriculture. Although this report has looked at the three gases separately, in the agricultural sector they cannot be viewed in isolation from each other, as technologies which influence the emissions of any single gas often have ramifications for the other gases. At present the tools to investigate total greenhouse gas production at the farm scale are only just being developed. A more rigorous analysis of the effects of implementing mitigation strategies on whole farm greenhouse gas emissions can only be undertaken once these tools are available.

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