Methane and Nitrous Oxide Mitigation Options
Methane and nitrous oxide emissions have the potential to be a major issue for New Zealand's primary production sector. The following article by Harry Clark and Cecile De Klein presents the results from a MAF-commissioned report Potential Management Practices and Technologies to Reduce Nitrous Oxide, CH4 and Carbon Dioxide Emissions from New Zealand Agriculture.
Introduction
Ratification of the Kyoto Protocol will commit New Zealand to reducing its average greenhouse gas emissions between 2008 and 2012 to 1990 levels. Emissions of methane (CH4) and nitrous oxide (N2O) from the agricultural sector make up about 54 percent of New Zealand's total emissions and these are forecast to rise by 2010. This presents a considerable challenge to New Zealand if it wants to avoid having to potentially obtain "carbon credits" to offset any excess emissions.
What is the Source of CH4 and Nitrous Oxide Emissions?
Almost all of the CH4 arising from agricultural sources is a by-product of the fermentation of feed, especially fibrous feeds, in the rumen. Micro-organisms in the rumen break down feed to produce volatile fatty acids, which are utilised as an energy source, and carbon dioxide (CO2) and CH4, which are expelled in the breath.
N2O emissions from agriculture are produced by soil bacteria, that transform nitrogen, mainly from animal excreta, into N2O via the soil processes denitrification and nitrification.
How can Methane Emissions be Reduced?
Reducing animal numbers is one obvious solution, although this is only guaranteed to reduce CH4 emissions if the productivity per animal remains unchanged. This has, however, major financial consequences, as farm revenues will fall in direct proportion to the reduction in product output.
Improving the quality of the diet of ruminants tends to result in higher feed intakes, which in turn tends to increase productivity and CH4 output per animal. However, if CH4 is expressed per unit of product, then using a smaller number of high-producing animals to produce a given amount of product emits less CH4 than using a larger number of lower producing animals. This is because a smaller proportion of the feed eaten is required to maintain the animal and because high feed intakes tend to reduce CH4 yield per unit of feed eaten. Concentrate diets produce less CH4 than forage diets but are too expensive for extensive use in New Zealand. Research undertaken by AgResearch and Dexcel indicates that certain forage species e.g. white clover, lotus and sulla, improve animal performance and produce less CH4 per unit of feed eaten. Experiments are currently underway to look at whether ryegrass cultivars selected for improved animal performance also result in lower CH4 yields per unit of product.
It may also be possible to reduce CH4 by directly influencing the rumen fermentation process through feed additives. A group of commonly used compounds, ionophores, have, under some circumstances, been found to reduce the amount of CH4 emitted per unit of product and to decrease the amount of CH4 produced per unit of feed eaten. There is also some evidence that they reduce the amount of nitrogen excreted and can therefore reduce N2O emissions from pastures. Unfortunately very little of the evidence comes from the type of diets commonly consumed by New Zealand ruminants and the extent to which ionophores will reduce CH4 emissions is difficult to quantify under our conditions. There may also be some consumer resistance to their use as they are a type of antibiotic. A second group, probiotics, are microbial feed additives that can increase feed conversion efficiency and perhaps reduce CH4 production per kg of feed. However no information is yet available from controlled trials carried out under New Zealand conditions.
Extensive publicity has been given to a vaccine being
developed in Australia that is claimed to reduce CH4 production per kilogram of food by 11 percent to 23 percent. This approach shows great promise as it may involve a once-in-a-lifetime injection and be applicable to all classes of ruminants. It is, however, still at the development and testing stage and is unlikely to be available, even for evaluation, for another three to four years.
How can Nitrous Oxide Emissions be Reduced?
The single largest source of N2O in New Zealand's pastoral systems is animal excreta and therefore reducing the amount of excreta nitrogen has the largest potential to reduce N2O emissions. This can be done by reducing the number of animals or by reducing the amount of nitrogen excreted per animal. As with CH4, producing a given amount of product from a smaller number of high-producing animals reduces N2O emissions. Options for reducing the amount of nitrogen excreted per animal include the replacement of nitrogen-boosted grass with low-protein feed such as maize and the breeding of grasses that have a better balance between energy and protein so that they reduce the amount of nitrogen excreted and at the same time improve animal performance.
N2O emissions can also be reduced by increasing the efficiency with which excreta and applied nitrogen are utilised. N2O emissions from dung and urine patches are highest during the wet autumn/winter period. If dairy and beef cattle are kept on feed-pads during these high-risk periods and the excreta collected and re-utilised as effluent, emissions could be reduced as emissions from excreta are higher than for effluent applied to the soil. This management practice could also reduce nitrate leaching losses. Other options include the substitution of synthetic fertilisers by effluent wherever possible, timing nitrogen applications so that they do not occur after periods of heavy rain or onto wet soils and, in the future, the incorporation of specific nitrification inhibitors into synthetic fertilisers.
Recent New Zealand research has found that N2O emissions from urine patches on poorly drained soils are much higher than from free-draining soils, suggesting that improving drainage can help to reduce N2O emissions. However, improved drainage could increase nitrate leaching. Soil compaction can also increase N2O emissions and minimising treading damage by cattle is therefore beneficial. Liming can also decrease N2O emissions from soil but this is offset by CO2 emissions from the lime itself.
Summary
There are a variety of options for reducing CH4 and nitrous oxide emissions from agriculture but it must be emphasised that it is a complex problem and we should be wary of simple solutions. No single approach is likely to be suitable for all circumstances and reductions are likely to require a basket of reduction methods. Some of these have been outlined above and others will emerge from current research and development.
An important issue is that emissions of N2O and CH4 should not be viewed in isolation from each other as some of the potential options for reducing emissions of one gas can affect emissions of the other. This reflects the complexity of farming systems and the tradeoffs in greenhouse gas mitigation options that are likely to arise in practice. The Kyoto protocol sets emission targets for the total quantity of greenhouse gases emitted by a country, not targets for specific gases, and potential mitigation options therefore need to be evaluated at a farm scale and for all greenhouse gases collectively.
|
Harry Clark Harry joined MAF Tech (now AgResearch) in 1991 after spending six years working for MAFF, UK as a specialist livestock adviser. His current research involves quantifying CH4 emissions from grazing ruminants and the development of more accurate methods for estimating CH4 emissions from ruminant livestock at the national scale. He trained at the University College of North Wales, Bangor.
|
|
Cecile de Klein Cecile de Klein is a soil scientist with AgResearch's Land & Environmental Management Group. Her current research involves quantifying nitrous oxide emissions from soils, to develop more accurate estimates of agricultural nitrous oxide emissions, and management practices for reducing nitrous oxide emissions. She was trained at the University of Utrecht in the Netherlands and has been working with AgResearch since 1995.
|
Contact for Enquiries
Amber Duncalfe
Editor - RM Update
Ministry of Agriculture and Forestry
PO Box 2526
Wellington
NEW ZEALAND
Tel: +64 4 894 0710
Fax: +64 4 894 0745
Contact this person
