Understanding GHG emissions from nitrous oxide

A review commissioned by the Fertiliser Association of New Zealand seeks to understand how much nitrous oxide and ammonia is typically emitted from ammonium-based nitrogen fertilisers - and whether New Zealand's emission factors for these gases are appropriate.

Nitrous oxide is the second most important greenhouse gas from agriculture after methane.

"For a typical livestock farm, methane makes up about 70% of the total agricultural GHG emissions footprint," says Tony van der Weerden, who conducted the review with Bob Longhurst and Martin Kear. "The other 30% pretty much comes from nitrous oxide, except for a small amount of carbon dioxide from urea fertiliser and lime. Livestock urine is the largest source of nitrous oxide from agriculture, with fertiliser being the second most important."

Each year New Zealand reports its national greenhouse gas footprint to the United Nations Framework on Climate Change Convention. This is calculated using a method developed by the Intergovernmental Panel on Climate Change (IPCC). The IPCC encourages individual nations to develop country-specific emission factors for significant sources of greenhouse gases.

"The nitrous oxide emission factors for each product, describes the amount emitted per kilogram of nitrogen deposited in the form of urine or dung during grazing or applied to land as effluent or fertiliser," says Tony.

The latest national inventory estimates that nitrous oxide emissions from fertilisers make up 4.3% of our total agricultural emissions.

"While this isn't a huge amount, any opportunity to get a more accurate picture is going to help with our reporting and help identify mitigation options to reduce emissions," says Tony. "We already have a country specific emission factor for urea fertiliser, which makes up about 70% of total nitrogen fertiliser use. The question is, should we consider developing a country specific emission factor for non-urea fertilisers?"

The researchers used data collated from publications from around the world to calculate mean nitrous oxide emission factor values for ammonium sulphate and diammonium phosphate (DAP). When studies were restricted to those where urea was also applied, to provide for comparisons under the same conditions, nitrous oxide emission factors were similar for ammonium sulphate and urea, at around 0.4 - 0.6% of the applied nitrogen. There was not enough data to calculate an average value for DAP.

The researchers also worked out the ammonia volatilisation values for these fertilisers. Under New Zealand's farming conditions, ammonia losses from ammonium sulphate are typically less than 3% of applied nitrogen while losses from DAP were typically 50% of the losses from urea fertiliser.

In addition, they looked at carbon dioxide lime and dolomite values. It was found that the IPCC carbon dioxide emission factors for lime represent the absolute maximum losses, at 12% of calcium carbonate. The IPCC acknowledge that actual emissions may be half of this value, based on overseas research. The guidelines also state that data on the purity of the lime (that is, the neutralising value of the lime) can be used to improve the accuracy of national greenhouse gas reporting.

"I feel there is justification to explore the concept of developing country specific emission factors for these N fertilisers and to determine whether there is sufficient national data on lime purity to provide accurate information for national greenhouse gas reporting," says Tony.

"However, for developing new emission factors, the cost of running field trials across the country would be pretty high too - so it really comes down to how far we want to go in improving the accuracy of the inventory."

The Fertiliser Association is keen to promote work that enables farmers and growers to get a more accurate picture of on-farm impacts and help identify mitigation options to reduce emissions.

Read the report 'Greenhouse gas emissions from ammonium-based nitrogen fertilisers and lime'