Reducing nitrous oxide

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About Nitrous Oxide

Nitrous oxide is a powerful greenhouse gases (300 times more powerful than CO2) produced in small quantities by biological processes in soils and oceans. Because of its minor role in the nitrogen cycle on farms, it has not been extensively studied compared to other nitrogen compounds.

The addition of nitrogen (from fertilizer, dung and urine) to soil and water increases the production of nitrous oxide. Since the industrial revolution, nitrous oxide in the atmosphere has risen by about 15%.
To better assess the options for reducing nitrous oxide, some background information is important:

1. Emissions of Nitrous Oxide (N2O) result from two different processes. There is some N2O leakage during nitrification (the aerobic process of converting ammonium to nitrate) though this is probably not the major source. During the anaerobic process of denitrification (the conversion of nitrate to N2 or nitrogen gas), N2O is an obligatory intermediate product. This is probably the major source as studies show that peaks of N2O loss in the paddock tend to coincide with anaerobic/waterlogged conditions. Therefore, many of the abatement options are associated with minimising these anaerobic conditions.
   
2. Cows eating high quality pasture usually have an excess intake of nitrogen which is excreted in the urine as urea – urine patches are the biggest source of N2O loss because the N is readily available, while losses from dung are lower because it breaks down and releases the N more slowly. Therefore, many abatement options focus on minimising the amount of N in urine.

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Options for Reducing Nitrous Oxide emissions from dairy farms

Herd and feeding strategies

Animal breeding/Higher feed conversion efficiency

While not a primary breeding objective, breeding dairy cows for increased feed conversion efficiency is likely to partition more N into milk production, reducing the amount of N in dung and urine.

Condensed tannins

Condensed tannins in the diet can have a significant effect on the fate of dietary nitrogen because the tannins attach to protein and protect it from microbial digestion in the rumen. The result is more efficient digestion of the protein in the lower intestine, and/or the tannin-protein complex being excreted in the dung rather than urea being excreted in the urine.

However condensed tannins, even at relatively low concentrations in the diet suppress voluntary food intake, reduce diet digestibility and therefore reduce milk production.

Nitrification inhibitors for urine

Nitrification inhibitors are chemicals that prevent or delay the conversion of ammonium to nitrate by microbes in soil. If applied as a spray to the pasture/soil nitrogen inhibitors can reduce nitrous oxide emissions from urine patches by 60-90% and can also boost pasture yield because more of the nitrogen is retained in the soil. This spray is available commercially in New Zealand.

Balance energy and crude protein in the diet

Dairy cattle excrete a constant amount of nitrogen in the dung per unit of forage intake, so as protein content in the diet increases, most of the extra N is excreted in the urine, making N2O emissions more likely.

Lush dairy pastures are usually high in crude protein, but limited in energy, so any ‘supplement’ that has low protein but high energy can reduce N excretion in the urine. Examples include maize silage and cereal grains.

Effluent management

For dairy farms without housing or intensive, off-pasture feeding systems, effluent is a minor source of N2O emissions. However, if applying dairy effluent to pastures, N2O emissions are minimised if the effluent is applied to dry rather than to wet/waterlogged soil.

Soil based strategies

Improve drainage

N2O emissions are generally higher from poorly drained soils, so reducing waterlogging can have 2 positive impacts:

• Pastures grow better on well drained soils
• N2O emissions will also be reduced

Improve irrigation management

Because N2O emissions are generally higher from poorly drained soils, irrigation tends to increase N2O emissions because the soils are more likely to be ‘waterlogged’. However, the impact of different irrigation systems is quite complex so no clear rules are currently available.

Reduce grazing on wet soils

Grazing on wet soils increase N2O emissions in two ways:

• Directly - if dung and urine are deposited on wet soil, the conditions for N2O emissions are more likely to be triggered 
• Indirectly – if grazing wet soils increases compaction, reduces the potential for drainage and makes the soil more likely to be waterlogged during future grazing events.

Nitrification inhibitors

Using nitrification inhibitors with nitrogen fertiliser can reduce N2O emissions. Nitrification inhibitors prevent or delay the process of nitrification (the conversion of ammonium to nitrate by microbes in the soil) and therefore prevent nitrogen loss (and N2O emissions) during waterlogged conditions – they work best when soil is cool and moist.

By preventing nitrogen loss, these inhibitors can boost pasture production. Commercial products (eg Eco-N) are available in Australia with the nitrification inhibitor included in the fertiliser.

Nitrogen fertiliser management

The type, rate and timing of nitrogen fertiliser are important factors influencing the efficiency of nitrogen use and the magnitude of losses, including the scale of N2O emissions. All else being equal, N losses increase as the rate of a nitrogen fertiliser application increases.

As the major source of N2O emissions is from the denitrification process, fertilisers that do not contain nitrate are more protected from such losses. The potential here is very limited however because ammonium nitrate is no longer available (due to its potential use in bombs) and calcium nitrate is rarely used – almost all nitrogen fertiliser used on Australian dairy farms is urea of DAP (diammonium phosphate).

The farming systems reality

Nitrous oxide emissions on dairy farms can be up to 25% of total farm emissions but three distinctly different processes contribute to this total:

1. Indirect emissions, over which the farmer has little or no influence – these include NO2 emissions associated with the ‘production’ of farm inputs such as nitrogen fertiliser or purchased grain, silage or hay. Other than for feedlots, this ‘indirect’ source of NO2 is usually the largest on dairy farms, accounting for up to 50% of total NO2 emissions. Options for dairy farmers to reduce these indirect emissions are very limited.

2. Direct emissions from dung and urine, including those NO2 emissions from deposition of dung and urine on pastures, and those associated with effluent management systems. Options to reduce NO2 emissions from these sources, beyond what would currently be included as normal best practice are relatively limited.

3. Direct emissions from the use of N fertiliser. This is the smallest contributor to dairy farm NO2 emissions, often less than 20%. Because of the cost, farmers are already focussed on minimising the losses from fertiliser N, so current best practice is delivering most of the available emission reductions. However, if the use of nitrification inhibitors proves to be effective under Australian conditions, then blanket application of nitrification inhibitors to all N fertilisers during production may be a viable option – if this strategy reduced NO2 emissions from fertiliser by an optimistic 30% annually, then total dairy farm emissions would be reduced by approximately 1.5%.

In conclusion, current farming systems that are operating at or near best practice management of cows and pastures already minimise N losses and maximise dairy production. If nitrous oxide is to be significantly reduced, new options and strategies will need to be developed and tested.

The extent to which financial incentives (via the Carbon Farming Initiative) offered to farmers to reduce greenhouse gas emissions may change the economics of any of these options remains to be determined

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References and resources

De Klein CAM, and Eckard, RJ (2008) Targeted technologies for nitrous oxide abatement from animal agriculture. Australian Journal of Experimental Agriculture 48, 14-20.

Eckard RJ, Grainger C, and de Klein CAM (2008) Options for the abatement of methane and nitrous oxide from ruminant production – a review.

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