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Developing revised emission factors for nitrous oxide emissions from agricultural pasture treated with nitrification inhibitors

Background

The agriculture sector produces around 50% of the total greenhouse gas emissions from New Zealand. One-third of New Zealand’s agricultural emissions are nitrous oxide (N₂O) from soils. In 2004, N₂O emissions were 24% greater than those in 1990 (Ministry for the Environment 2006). Kelliher and Clark (2004) estimated that the excess of current, national, annual N₂O emissions above the 1990 level could be largely attributed to a concurrent increase in the quantity of urine excreted onto agricultural soils by grazing dairy cattle.

N₂O emissions enter the atmosphere:

1. directly from soils:
   • urine and dung excreted by sheep and cattle during grazing
   • nitrogen fertiliser application and the biological fixation of nitrogen from the atmosphere
2. indirectly from soils as a result of ammonia volatilisation and nitrate leaching.

In soils, nitrogen conversion rate into N₂O can be reduced by the application of chemical compounds called nitrification inhibitors. One such compound is dicyandiamide (DCD). Research, conducted recently in New Zealand, showed that strategic application of a nitrification inhibitor to pasture, following simulated urine excretion by grazing dairy cattle, significantly reduced direct and indirect N₂O emissions over extensive periods¹.

Currently, New Zealand’s greenhouse gas inventory does not capture and report decreased N₂O emissions as a result of nitrification inhibitor application. However, it is feasible to make the required changes. Most significantly, for areas treated by nitrification inhibitors, activity inputs would be needed seasonally at the farm scale. These include the treated area, timing and location, and the stocking and production rates because they determine excretion rate. These activity inputs and the N₂O emissions factors (EFs) will need to be scientifically defensible and robust because international, expert teams regularly review inventory methods. Further, these changes will automatically propagate into reporting under the United Nations Framework Convention on Climate Change (UNFCCC). New Zealand’s N₂O emissions inventory is determined using methods provided by the Intergovernmental Panel on Climate Change (IPCC) and adopted by the Conference of Parties to the UNFCCC.

Three New Zealand-specific emission factors are used in the N₂O emissions inventory:
EF₁ for fertiliser, EF_3(PRP) for excreta (urine and dung) from grazing animals and Frac_LEACH for leaching.

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¹ See, in particular: Clough, T.J., Di, H.J., Cameron, K.C., Sherlock, R.R., Metherell, A., and Clark, H. (2006). Effect of using eco-n^tm nitrification inhibitor technology in dairy farming on New Zealand’s agricultural greenhouse gas (GHG) inventory, and methodology for its incorporation at a national level. Report prepared for the Ministry of Agriculture and Forestry by Centre for Soil and Environmental Quality, Lincoln University.

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Sustainable Land Management and Climate Change
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