ghg-inventory-1990-2013

Frac LEACH-H is the fraction of N added to, or mineralised from, agricultural soils
where leaching and runoff occur that is lost through leaching and runoff (table
5.5.3), and,
EF 5 is the emission IPCC 2006 default emission factor for N 2 O emissions from
leaching and runoff.
New Zealand uses a country-specific value for the fraction of nitrogen applied to
agricultural land that is lost through leaching and runoff 30 . Scientific research and a
literature review in New Zealand have shown lower rates of nitrogen leaching than are
suggested in the 1996 and 2006 IPCC guidelines. A New Zealand parameter for
Frac LEACH of 0.15 was used in inventories submitted before 2003. However, using a
Frac LEACH of 0.15 for different farm systems, IPCC-based estimates were found, on
average, to be 50 per cent higher than those estimated using the OVERSEER ® nutrientbudgeting
model (Wheeler et al, 2003).
The OVERSEER ® model provides average estimates of the fate of nitrogen for a range of
pastoral, arable and horticultural systems. In pastoral systems, nitrate leaching is
determined by rainfall, soil type, and the amount of nitrogen entering the farm system, for
example in nitrogen-based fertilisers and dung and urine applied as dairy farm effluent
and directly excreted by grazing animals. The latter is calculated from the difference
between nitrogen intake by grazing animals and nitrogen retained in animal products –
milk, meat, velvet etc. This is based on user inputs of stocking rate or production and an
internal database with information on the nitrogen content of pasture and animal products,
and calibrated against field measurements.
The IPCC estimates were closer for farms using high rates of nitrogen fertiliser,
indicating that the IPCC-based estimates for nitrogen leaching associated with animal
excreta were too high for New Zealand dairy and drystock (sheep and non-dairy cattle)
farming systems (Thomas et al, 2005). When the IPCC method was applied to field sites
where nitrogen leaching was measured (four large-scale, multi-year animal grazing
trials), it resulted in values that were double the measured values. This indicated that a
value of 0.07 for Frac LEACH more closely followed actual field leaching in New Zealand
(Thomas et al, 2005) and is used for all years as it best reflects New Zealand’s national
agricultural circumstances.
In 2013, N 2 O emissions from leaching and runoff made up 1.3 per cent (496.1 kt CO 2 -e)
of agricultural emissions, an increase of 27.0 per cent from the 1990 value of
390.8 kt CO 2 -e.
Incorporation of nitrous oxide mitigation technologies into the
Agriculture Inventory
Urease inhibitor (UI)
A methodology to include a greenhouse gas mitigation technology, urease inhibitor (UI),
into the Agriculture sector of the Inventory has been developed, based on research by
Saggar et al (2013). Urea is the predominant synthetic nitrogen fertiliser for grazed
pastures. Urease inhibitors restrict the action of the enzyme, urease, which is a catalyst for
the volatilisation of the nitrogen contained in urea fertiliser and urine into ammonia gas,
which can act as a secondary source of N 2 O.
Urease inhibitor mitigation is included into New Zealand’s Agriculture Inventory by
adjusting the value of the existing country-specific N 2 O parameter: Frac GASF . In
particular, Saggar et al (2013) considered the mitigating effect of a widely used UI,
30
For reporting under the 1996 IPCC guidelines this parameter was defined as Frac LEACH ; under the
2006 IPCC guidelines, it is defined as Frac LEACH – (H) ).
172 New Zealand’s Greenhouse Gas Inventory 1990–2013