Nitrous Oxide: Abatement Technologies and Recommended Research
Addition of nitrogen to soil in any form (animal excreta, synthetic fertiliser, crop residues or biological fixation), results in increased nitrous oxide emissions. The bulk of the nitrogen added to New Zealand soils comes from the excreta of animals (1 282 Gg N/y) and the addition of fertiliser (213 Gg N/y) (Section 6.4). On average only 10.5% of the nitrogen in grass, silage or other feedstuff is converted by grazing animals into milk, meat, eggs or wool, and the remainder is excreted in dung and urine (Section 6.6.2.2).
Nitrous oxide is emitted directly from soil as a result of these inputs. It is also generated indirectly when nitrate lost by leaching or run-off is converted to nitrous oxide in water bodies, and when nitrogen from excreta and fertiliser is lost as ammonia to the atmosphere and subsequently deposited on land. During 2000, nitrous oxide emission from these sources amounted to 25.2 Gg N (Section 6.5), compared with the 1990 base value of 24.4 Gg N.
· Of the direct emissions, 53% came from excreta and 10% resulted from the application of fertiliser.
· Indirectly leaching and run-off of nitrogen from animal excreta or fertiliser application contributed 23%, and deposition of ammonia which had been volatilised contributed a further 11%.
Because of their larger numbers, sheep were responsible for the bulk of the direct nitrous oxide emissions from animal excreta (5.5 Gg N/y; 45.5% of the total), and dairy cattle (3.3 Gg N/y; 27.3%) generated more nitrous oxide than non-dairy cattle (3.0 Gg N/y; 24.8%) (Section 6.6.2.2).
Mitigation options need to focus on limiting the direct loss of nitrogen from animal excreta and synthetic fertilisers, and the indirect loss caused by leaching, run-off and ammonia volatilisation. Options are available that could result in considerable reductions in nitrous oxide emission from grazing animals and fertiliser application. These include:
1 For ruminants:
- Manipulating the diet of animals to influence the amount of nitrogen excreted, particularly urea nitrogen. For example, feeding dairy cattle protein that resists degradation in the rumen and high starch diets can result in less nitrogen being excreted in the urine, reduced ammonia volatilisation, and less nitrous oxide emission. In one experiment the reduction in urinary nitrogen was 24% (Section 6.6.2.2.2b).
- Breeding forage cultivars that provide an energy-to-protein ratio more in keeping with the animal's needs could improve nitrogen efficiency. Dairy cows fed grasses high in water soluble carbohydrate excreted 24% less nitrogen than those fed normal diets (Section 6.6.2.2.1d).
- Keeping cattle on feed-pads during the wet autumn/winter period, so that excreta can be collected and utilised as fertiliser later in the year. Nitrous oxide emission from dairy excreta could be reduced by 25% and nitrate leaching by 40% (Section 6.6.2.2.2c).
- Improving drainage and preventing soil compaction can reduce nitrous oxide emission by 3% each (Sections 6.6.1.1.4 and 6.6.2.2.1c)
If it is assumed that the effects are additive, and that the reductions achieved experimentally could be realised in a practical farming situation, there is the potential for nitrous oxide emission to be reduced from the sheep, dairy cattle, and beef cattle sectors by 16% (0.9 Gg N), 28% (0.9 Gg N) and 25% (0.8 Gg N) respectively (Section 6.7). It is accepted that these estimates of abatement potential will be subject to considerable variation, and that reductions of this order are unlikely to be achieved in a farm situation.
2 For cropping and forage production:
- Matching nitrogen supply with crop demand, tightening nitrogen flow cycles, and optimising tillage, irrigation and drainage could reduce nitrous oxide emissions from fertiliser use by 19% (0.6 Gg N) (Section 6.7).
- Nitrate leaching can be reduced by lowering fertiliser application rates, synchronising nitrogen supply to plant nitrogen demand, growing cover crops, and using buffer zones (Section 6.6.3.2).
If the total reduction of 3.2 Gg N was achieved, it would reduce the emissions calculated for the year 2000 to 22 Gg N (i.e. 2.43 Gg below the 1990 base value). However, even if nitrous oxide emissions were reduced below the 1990 level by implementing these options, it will only be maintained at that level if nitrogen inputs remain static. This means that fertiliser nitrogen use and animal numbers can not increase. Production could only increase by increasing the efficiency of nitrogen use by sheep and cattle, or by changing from animals with low nitrogen use efficiency (cattle 7.7%, sheep 6.2%) to those with high nitrogen use efficiency (swine 20.5%, poultry 33.8%) (Section 6.6.2.2). The options proposed for reducing nitrous oxide emission from animals could only be implemented by limiting a farmer's options for increasing production. However, making more efficient use of animal manures and slurries will have numerous indirect benefits. If the options proposed for reducing emissions from fertiliser use were implemented, they would increase rather than decrease farmers' incomes. If fertiliser nitrogen is used more efficiently, less money will be spent on fertiliser.
Research Priorities
A number of promising areas of research relating to nitrous oxide emission were revealed during the review and Workshop. These areas can be divided into three categories; mitigation, inventory and farming systems. Research needs related to the measurement and verification of the effect of abatement measures are dealt with below. As the emphasis in this project is mitigation, priority is given to this area of research, and the projects are listed in order of importance within each category.
Mitigation
Inventory
Farming Systems
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