View all publications

Chapter 6: Agriculture

6.1 Sector overview

The agriculture sector emissions totalled 37,445.3 Gg CO2 equivalent (Gg CO2-e) and represented 48.5 per cent of all greenhouse gas emissions in 2005. Emissions in this sector are now 4,948.2 Gg CO2-e (15.2 per cent) higher than the 1990 level of 32,497.1 Gg CO2-e (Figure 6.1.1). The increase is primarily attributable to a 2,113.3 Gg CO2-e (9.7 per cent) increase in CH4 emissions from “enteric fermentation” and a 2,673.3 Gg CO2-e (26.6 per cent) increase in N2O emissions from the “agricultural soils” category.

Figure 6.1.1 Agricultural sector emissions from 1990 to 2005

 

Year

Gg CO2 equivalent

1990

32,497.09

1991

32,314.01

1992

32,244.50

1993

32,516.66

1994

33,129.68

1995

33,706.06

1996

34,170.14

1997

34,296.12

1998

34,521.39

1999

34,923.13

2000

35,677.54

2001

36,265.60

2002

36,658.33

2003

36,919.50

2004

37,259.47

2005

37,445.29

Emissions of CH4 from “enteric fermentation” dominate, producing 63.9 per cent of CO2-e emissions in the sector (Figure 6.1.2) and 31.0 per cent of New Zealand’s total emissions. N2O emissions from “agricultural soils” are the other major component of the sector comprising 33.9 per cent of agricultural CO2-e emissions.

Agriculture is a major component of the New Zealand economy and agricultural products comprise over 50 per cent of total merchandise exports. This is because of the favourable temperate climate, the abundance of agricultural land and the unusual farming practices used in New Zealand. These practices include the extensive use of year-round grazing systems and a reliance on nitrogen fixation by legumes rather than nitrogen fertiliser.

Figure 6.1.2 Emissions from the agricultural sector in 2005 (all figures Gg CO2-equivalent)

 

Category

Gg CO2 equivalent

Percent of total

Field Burning of Agricultural Residues

14.2

0.0

Enteric Fermentation

23,919.8

63.9

Manure Management

803.4

2.1

Agricultural Soils

12,707.0

33.9

Prescribed Burning of Savannas

1.0

0.0

Since 1984, there have been changes in the proportions of the main livestock species farmed. There has been a trend for increased dairy and deer production because of high world demand and favourable prices. This has been counterbalanced by land coming out of sheep production and decreasing sheep numbers. Beef numbers have remained relatively static. There have also been productivity increases across all major animal species and classes. The land area used for horticulture has not changed significantly since 1990 although the types of produce grown have changed. There is now less grain grown, but more vegetables, fruit, and grapes (for wine production) than in 1990. There has also been an expansion of the land used for plantation forestry.

New Zealand uses a June year for all animal statistics as this reflects the natural biological cycle for animals in the southern hemisphere. The models developed to estimate emissions therefore work on a monthly timeframe beginning in July of one year and ending in June of the next year. To obtain emissions for single calendar years (January to December) emissions from the last six months of a July to June year are combined with the first six months’ emissions of the next July to June year. All emissions in the inventory are reported on a rolling three-year average of the emissions calculated for single January–December years. To ensure consistency, a single livestock population characterisation and feed intake estimate is used when estimating CH4 emissions from “enteric fermentation”, CH4 and N2O emissions from “manure management” and N2O emissions from “animal wastes deposited directly onto pasture”. Information on livestock population census and survey procedures is included in Annex 3.1.

6.2 Enteric fermentation (CRF 4A)

6.2.1 Description

Methane is produced as a by-product of digestion in ruminants, eg, cattle, and some non-ruminant animals such as swine and horses. Ruminants are the largest source of CH4 as they are able to digest cellulose. The amount of CH4 released depends on the type, age and weight of the animal, the quality and quantity of feed and the energy expenditure of the animal.

Methane emissions from “enteric fermentation” have been identified as the largest key category for New Zealand in the level assessment. In accordance with Good Practice Guidance (IPCC, 2000), the methodology for estimating CH4 emissions from “enteric fermentation” in domestic livestock was revised to a Tier 2 modelling approach for the 2001 inventory. All subsequent inventories have used this Tier 2 approach.

In 2005, emissions from enteric fermentation comprised 23,919.8 Gg CO2-e. This represents 31.0 per cent of New Zealand’s total CO2-e emissions and is the largest single category of emissions in the New Zealand inventory. The category is dominated by emissions from cattle (both dairy and non-dairy). In 2005, cattle contributed 58.0 per cent of emissions from “enteric fermentation” and sheep contributed 38.5 per cent of emissions. The current level of emissions from “enteric fermentation” is 9.7 per cent above the 1990 level. Since 1990 there have been changes in the source of emissions within the “enteric fermentation” category. The largest increase has been in emissions from dairy cattle which have increased 70.5 per cent since 1990. This increase has been partially offset by decreases in emissions from sheep (–18.2 per cent) and minor livestock populations such as goats, horses and swine.

6.2.2 Methodological issues

New Zealand’s methodology uses detailed livestock population characterisation and livestock productivity data to calculate feed intake for the four largest categories in the New Zealand ruminant population (dairy cattle, beef cattle, sheep and deer). The amount of CH4 emitted is calculated using CH4 emissions per unit of feed intake (Figure 6.2.1). The calculation process is explained in the full description of the Tier 2 approach in Annex 3.1 and Clark et al, (2003). A Tier 1 approach is adopted for minor species such as goats, horses and swine using either IPCC (horses and swine) or New Zealand derived default values (goats). These minor species comprised 0.3 per cent of total enteric methane emissions in 2005. Adopting a Tier 1 as opposed to a Tier 2 approach for these species has little effect on total estimated enteric methane emissions.

There has been a gradual increase in the implied emission factors for dairy cattle and beef cattle from 1990 to 2005. This is to be expected because the methodology uses animal performance data that reflects the increased levels of productivity achieved by New Zealand farmers since 1990. Increases in animal weight and animal performance (milk yield and liveweight gain) require increased feed intake by the animal to meet higher energy demands. Increased feed intake results in increased CH4 emissions per animal. The increases in productivity are shown in the agricultural worksheets in Annex 8 and in the detailed description in Annex 3.1.

Figure 6.2.1 Schematic of New Zealand’s enteric fermentation calculation methodology

6.2.3 Uncertainties and time-series consistency

Livestock numbers

Many of the calculations in this sector require livestock numbers. Both census methods and survey methods are used. Surveys occur each year between each census. Detailed information from Statistics New Zealand on the census and survey methods is included in Annex 3.1.2.

Methane emissions from enteric fermentation

In the 2001 inventory, the CH4 emissions data from domestic livestock in 1990 and 2001 were subjected to Monte Carlo analysis using the software package @RISK to determine the uncertainty of the annual estimate (Clark et al, 2003; table 6.2.1). For the 2005 inventory, the uncertainty in the annual estimate was calculated using the 95 per cent confidence interval determined from the Monte Carlo simulation as a percentage of the mean value, ie, in 2001, the uncertainty in annual emissions was ± 53 per cent.

The overall inventory uncertainty analysis shown in Annex 7 (A.7.1) demonstrates that the uncertainty in annual emissions from enteric fermentation is 12.1 per cent of New Zealand’s total emissions and removals for 2005, and is the largest single component affecting the national total. In the trend from 1990 to 2005 however, the uncertainty from enteric fermentation is only 2.4 per cent of the trend in emissions and removals. The uncertainty between years is assumed to be correlated, therefore the uncertainty in the trend is mostly in the emission factors rather than the activity data. Hence the uncertainty in the trend is much lower than uncertainty for an annual estimate.

Table 6.2.1 Uncertainty in the annual estimate of enteric fermentation emissions for 1990, 2001 and 2005 estimated using Monte Carlo simulation (1990, 2001) and the 95 per cent confidence interval (2005)

Year Enteric CH4 emissions (Gg/annum) 95% CI min 95% CI max

1990

1,015.5

478.1

1,552.9

2001

1,099.4

517.6

1,681.2

2005

1,139.0

536.2

1,741.8

Note: The methane emissions used in the Monte Carlo analysis exclude those from swine and horses.

Uncertainty in the annual estimate is dominated by variance in the measurements used to determine the “CH4 per unit of intake” factor. For the measurements made of this factor, the standard deviation divided by the mean is equal to 0.26. This uncertainty is thought to be mostly natural variation from one animal to the next. Uncertainties in the estimation of energy requirements, herbage quality and population data are thought to be much smaller (0.005–0.05), so these variables play a much smaller role.

6.2.4 Source-specific QA/QC and verification

Methane emission rates measured for 20 dairy cows scaled up to a herd have been corroborated using micrometeorological techniques. Laubach and Kelliher (2004) used the integrated horizontal flux technique and the flux gradient technique to measure CH4 flux above a dairy herd. Both techniques are comparable, within estimated errors, to scaled-up animal emissions. The emissions from the cows measured by integrated horizontal flux and averaged over three campaigns are 329 (± 153) g CH4/day/cow compared to 365 (± 61) g CH4/day/cow for the scaled-up measurements reported by Waghorn et al, (2002;2003). Methane emissions from lactating dairy cows have also been measured using the New Zealand SF6 tracer method and open-circuit respiration chamber techniques (Grainger et al, 2007). Total CH4 emissions were similar, 322 and 331 g CH4/day, when measured using chambers or the SF6 tracer technique.

6.2.5 Source-specific recalculations

The provisional livestock population data for 2005 were updated to final population numbers, and the corresponding three-year average populations for 2004, 2005 and 2006 updated. There are minor revisions in data because of to updated data precision and correcting of minor transcription errors.

6.2.6 Source-specific planned improvements

A national inter-institutional ruminant CH4 expert group was formed to identify the key strategic directions for research into the CH4 inventory and mitigation, and to develop a collaborative approach to improve the certainty of CH4 emissions. This expert group is funded through the Ministry of Agriculture and Forestry. The Pastoral Greenhouse Gas Research Consortium has been established to carry out research, primarily into mitigation technologies and management practices but also on-farm inventory considerations. The consortium is funded by both public and private sectors. The implementation of the Tier 2 approach for CH4 emissions from enteric fermentation and manure management is a consequence of the research conducted by the expert group.

6.3 Manure management (CRF 4B)

6.3.1 Description

Emissions from the “manure management” category comprised 803.4 Gg CO2-e (2.1 per cent) of emissions from the agriculture sector.

Livestock manure is composed principally of organic material. When the manure decomposes in the absence of oxygen, methanogenic bacteria produce CH4. The emissions of CH4 are related to the amount of manure produced and the amount that decomposes anaerobically. Methane from “manure management” has been identified as a key category for New Zealand in both the 1990 and 2005 key category level assessments (table 1.5.2).

This category also includes emissions of N2O related to manure handling before the manure is added to the soil. The amount of N2O released depends on the system of waste management and the duration of storage. With New Zealand’s extensive use of all-year-round grazing systems, this category is relatively small at 63.9 Gg CO2-e in 2005. In comparison, agricultural soil emissions of N2O totalled 12,707.0 Gg CO2-e.

6.3.2 Methodological issues

Methane

Methane emissions from ruminant animal wastes in New Zealand have been calculated using an IPCC Tier 2 approach. The methodology adopted is based on the methods recommended by Saggar et al, (2003) in a review commissioned by the Ministry of Agriculture and Forestry.

The approach is based on;

  1. an estimation of the total quantity of faecal material produced
  2. the partitioning of this faecal material between that deposited directly onto pastures and that stored in anaerobic lagoons
  3. the development of specific New Zealand emission factors for the quantity of methane produced per unit of faecal dry matter deposited directly onto pastures and that stored in anaerobic lagoons.

The quantity of faecal dry matter produced is calculated by multiplying the feed intake by the dry matter digestibility of the feed. The feed intake estimates and monthly dry matter digestibility values are the same as in the current enteric fermentation and N2O inventories.

In New Zealand, only dairy cows have a fraction (5 per cent) of the excreta stored in anaerobic lagoon waste systems. The remaining 95 per cent of excreta from dairy cattle is deposited directly on pasture. These fractions relate to the proportion of time dairy cattle spend on pasture compared to the time they spend in the milking shed. All other ruminant species (sheep, beef cattle, deer and goats) graze outdoors all year round and deposit all of their faecal material directly onto pastures. Values for the quantity of CH4 produced per unit of faecal dry matter deposited on pastures for cattle are obtained from New Zealand research by Saggar et al, (2003) and Sherlock et al (2003). The value for sheep comes from a New Zealand study by Carran et al, (2003). So the average of cattle and sheep values is used. No values for deer are available.

Methane emissions from anaerobic lagoons are estimated assuming that faecal material deposited in lagoons is diluted with 90 litres of water per kilogram of dung dry matter (Heatley, 2001). This gives a total volume of effluent stored. A New Zealand study on emissions from anaerobic lagoons by McGrath and Mason (2002) quotes an emission rate for an effluent pond of 4.6 m depth. Using this depth figure as “typical” for the industry it is then possible to arrive at a surface area of the faecal material produced by dairy cows stored in anaerobic lagoons. McGrath and Mason (2002) quote specific emissions values of 0.33–6.21 kg CH4/m2/year from anaerobic lagoons and the mean value of 3.27 CH4/m2/year of this range is assumed in the New Zealand Tier 2 calculations.

Table 6.3.1 Derivation of methane emissions from manure management

Animal species Proportion of faecal material deposited on pasture CH4 from animal waste on pastures (g CH4/kg faecal dry matter) Proportion of faecal material stored in anaerobic lagoons Water dilution rate (litres water/kg faecal dry matter) Average depth of a lagoon (metres) CH4 from anaerobic lagoon(g CH4/m2/year)

Dairy cattle

0.95

0.98

0.05

90

4.6

3.27

Beef cattle

1.0

0.98

0.0

Sheep

1.0

0.69

0.0

Deer

1.0

0.92

0.0

 

New Zealand-specific emissions factors are not available for CH4 emissions from manure management for swine, horses and poultry. These are minor livestock categories in New Zealand and emissions estimates for these species use IPCC default emission factors (refer to the agricultural worksheets in Annex 8.4).

Nitrous oxide

For the N2O calculation, six alternative regimes for treating animal manure, known as animal waste management systems (AWMS) are identified in the IPCC Guidelines (1996). New Zealand farming uses four AWMS:

  1. anaerobic lagoons
  2. pasture, range and paddock
  3. solid storage and dry-lot
  4. other systems (poultry without bedding and swine deep litter).

With the exception of dairy cattle, animals were allocated to the different AWMS according to the information provided in the IPCC guidelines (1996) for the Oceania region, as New Zealand scientists and Ministry of Agriculture and Forestry officials considered these were applicable to New Zealand farming practices. For dairy cattle, New Zealand-specific data from Ledgard and Brier (2004) were used.

The “pasture, range and paddock” AWMS is the predominant regime for animal waste in New Zealand. All sheep, goats, deer and non-dairy cattle excreta are allocated to the pasture, range and paddock AWMS. For dairy cattle, 95 per cent of excreta is allocated to pasture, range and paddock and 5 per cent is allocated to anaerobic lagoons (Ledgard and Brier, 2004). Emissions from the “pasture, range and paddock” AWMS are reported in the “agricultural soils” category.

The calculation for the quantity of nitrogen in each animal waste management system is shown in the agricultural worksheets in Annex 8. A time-series of nitrogen excreta (Nex) values used for calculating animal production N2O emissions is also shown in Annex 8. The Nex values show an increase over time reflecting the increases in animal production.

Nex is calculated from:

Nex = N intake – N in products

where, N intake = Feed intake * N content of feed

and, N in products = Animal productivity * N content of products

Feed intake and animal productivity values are the same as used in the Tier 2 model for determining methane emissions (Clark et al, 2003). Nitrogen content of feed is estimated from a review of over 6,000 pasture samples of dairy and sheep and beef systems (Ledgard et al, 2003). Nitrogen contents of products are derived from the values used in the model OVERSEER® or from industry data. For lactating cattle the nitrogen content of milk is derived from the protein content of milk (Nitrogen = protein/6.25) published annually by the Livestock Improvement Corporation. The nitrogen content of sheep meat, beef and wool and the nitrogen retained in deer velvet are taken from OVERSEER®.

6.3.3 Uncertainties and time-series consistency

Emission factors from manure and manure management systems, the livestock population, nitrogen excretion rates and the usage of the various manure management systems are the main factors causing uncertainty in N2O emissions from manure management (IPCC, 2000). New Zealand uses the IPCC default values for EF3 (direct emissions from waste) for all AWMS except for EF3 (PR&P) (manure deposited on pasture, range and paddock). The value of EF3 (PR&P) which is a country-specific factor is 0.01 kg N2O–N/kg N (further details in section 6.5.2). The IPCC default values have uncertainties of –50 per cent to +100 per cent (IPCC, 2000).

The overall inventory uncertainty analysis shown in Annex 7 (A.7.1) demonstrates that the effect of uncertainty in annual emissions from manure management is relatively minor compared to the effect from CH4 emissions from enteric fermentation and N2O from agricultural soils.

6.3.4 Source-specific QA/QC and verification

Methane from manure management was identified as a key category (level assessment) for New Zealand in both the 1990 and 2005 inventories. In preparation for this inventory, the data for this category underwent Tier 1 quality checks.

6.3.5 Source-specific recalculations

There are minor revisions in data due to updated data precision and correcting minor transcription errors.

6.3.6 Source-specific planned improvements, if applicable

No source-specific improvements are planned.

6.4 Rice cultivation (CRF 4C)

6.4.1 Description

There is no rice cultivation in New Zealand. The “NO” notation is reported in the common reporting format.

6.5 Agricultural soils (CRF 4D)

6.5.1 Description

The “agricultural soils” category produces the majority of N2O emissions in New Zealand comprising 12,707.0 Gg CO2-e in 2005. Emissions are 2,673.3 Gg CO2-e (26.6 per cent) above the level in 1990. The category comprises three subcategories:

  • direct N2O emissions from animal production (the pasture, range and paddock AWMS)

  • indirect N2O from nitrogen lost from the field as NO3, NH3 or NOx

  • direct N2O emissions from agricultural soils as a result of adding nitrogen in the form of synthetic fertilisers, animal waste, biological fixation, inputs from crop residues and sewage sludge.

Each of these subcategories has been identified as key categories for New Zealand (tables 1.5.2 and 1.5.3). Direct soil emissions from animal production contributed 7,559.5 Gg CO2-e, indirect N2O from nitrogen used in agriculture contributed 3,384.6 Gg CO2-e and direct N2O emissions from agricultural soils contributed 1,762.9 Gg CO2-e.

Carbon dioxide emissions from limed soils are reported in the LULUCF sector.

6.5.2 Methodological issues

Nitrous oxide emissions are determined using the IPCC (1996) approach where emission factors dictate the fraction of nitrogen deposited on the soils that is emitted into the atmosphere as N2O. The two main inputs in New Zealand are from nitrogen fertiliser and the excreta deposited during animal grazing.

The worksheets for the agricultural sector document the emission factors and other parameters used in New Zealand’s calculations. Three New Zealand-specific factors/parameters have been used: EF1, EF3 (PR&P) and FracLEACH. The EF3 (PR&P) emission factor and FracLEACH were extensively reviewed for the 2001 submission, and a new value for FracLEACH was used from the 2001 inventory onwards and back-calculated to 1990. Data on EF1 was reviewed during 2006 and the recommendation by Kelliher and de Klein (2006) to use a country-specific factor of 1 per cent has been adopted.

Animal production (N2O)

Direct soil emissions from animal production refers to the N2O produced from the pasture, range and paddock AWMS. This AWMS is the predominant regime for animal waste in New Zealand as 95 per cent of dairy cattle excreta and 100 per cent of sheep, deer and non-dairy cattle excreta are allocated to it. The emissions calculation is based on the livestock population multiplied by nitrogen excretion (Nex) values and the percentage of the population on the pasture, range and paddock AWMS. The Nex and allocation to AWMS are discussed in section 6.3.2. The Nex values have been calculated based on the same animal intake and animal productivity values used for calculating CH4 emissions for the different animal classes and species. This ensures that the same base values are used for both CH4 and N2O emission calculations.

New Zealand uses a country-specific emission factor for EF33 (PR&P) of 0.01 (Carran et al, 1995; Muller et al, 1995; de Klein et al, 2003; Kelliher et al, 2003). Considerable research effort has gone into establishing a country-specific value for EF3 (PR&P)). Field studies have been performed as part of a collaborative research effort called NzOnet. The parameter EF3 (PR&P) has been measured by NzOnet researchers in the Waikato (Hamilton), Canterbury (Lincoln) and Otago (Invermay) regions for pastoral soils of different drainage classes (de Klein et al, 2003). These regional data are comparable because the same measurement methods were used at the three locations. The percentage of applied nitrogen (as urine or dung) emitted as N2O, and environmental variables, were measured in four separate trials that began in autumn 2000, summer 2002, spring 2002 and winter 2003. Measurements were carried out for up to 250 days or until urine-treated pasture measurements dropped back to background emission levels.

Kelliher et al, (2003, 2005) assessed all available EF3 (PR&P) data and its distribution to pastoral soil drainage class, to determine an appropriate national annual mean value. The complete EF3 (PR&P) data set of NzOnet was synthesised using the national assessment of pastoral soils drainage classes. These studies recognise that: (1) environmental (climate) data are not used to estimate N2O emissions using the IPCC (1996) methodology; (2) the N2O emission rate can be strongly governed by soil water content; (3) soil water content depends on drainage that can moderate the effects of rainfall and drought; and (4) as a surrogate for soil water content, drainage classes of pastoral soils can be assessed nationally using a geographic information system. In New Zealand, earlier analysis showed the distribution of drainage classes for pasture land is highly skewed with 74 per cent well-drained, 17 per cent imperfectly drained and 9 per cent poorly drained (Sherlock et al, 2001).

The research and analysis to date indicates that if excreta is separated into urine and dung components, EF3 for urine and dung could be set to 0.007 and 0.003, respectively. However, it is recognised that the dung EF3 data are limited. Combining urine and dung EF3 values, the dairy cattle total excreta EF3 is 0.006. Conservatively rounding the total excreta EF3 of 0.006 provides a country-specific value of 0.01 for EF3 (PR&P). The IPCC default value of EF3 (PR&P) is 0.02.

Indirect N2O from nitrogen used in agriculture

The N2O emitted indirectly from nitrogen lost from agricultural soils through leaching and run-off is shown in the agricultural worksheets in Annex 8. This nitrogen enters water systems and eventually the sea, with quantities of N2O being emitted along the way. The amount of nitrogen that leaches is a fraction of that deposited or spread on land (FracLEACH).

Research studies and a literature review in New Zealand have shown lower rates of nitrogen leaching than are suggested in the IPCC guidelines. In inventories reported before 2003, a New Zealand parameter for FracLEACH of 0.15 was used. However, using a FracLEACH of 0.15, IPCC-based estimates for different farm systems were found on average to be 50 per cent higher than those estimated using the OVERSEER® (Wheeler et al, 2003) nutrient budgeting model. The model provides average estimates of the fate of nitrogen for a range of pastoral, arable and horticultural systems. In pastoral systems, nitrogen leaching is determined by the amount of nitrogen in fertiliser, dairy farm effluent and that excreted in urine and dung by grazing animals. The latter is calculated from the difference between nitrogen intake by grazing animals and nitrogen output in animal products, based on user inputs of stocking rate or production and an internal database with information on the nitrogen content of pasture and animal products.

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. When the IPCC methodology 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 FracLEACH more closely followed actual field emissions (Thomas et al, 2005). This value was adopted and used for all years as it reflects New Zealand’s national circumstances.

New Zealand uses the IPCC default EF4 emission factor for indirect emissions from volatilisation of nitrogen in the form of NH3 and NOx.

Direct N2O emissions from agricultural soils

The N2O emissions from “direct N2O emissions from agricultural soils” category arise from synthetic fertiliser use, spreading animal waste as fertiliser, nitrogen fixing in soils by crops and decomposition of crop residues left on fields. All of the nitrogen inputs are collected together and an emissions factor applied to calculate total direct emissions from non-organic soils.

Nitrogen fertiliser use is determined by the Zealand Fertiliser Manufacturers’ Research Association (FertResearch) from sales records for 1990 to 2005. A rolling three-year average is used to calculate inventory data. There has been a six-fold increase in elemental nitrogen applied through nitrogen-based fertiliser over the time-series, from 51,787 tonnes in 1990 to 308,406 tonnes in 2005. The calculation of N2O that is emitted indirectly through synthetic fertiliser and animal waste being spread on agricultural soils is shown in the agricultural worksheets in Annex 8. Some of the nitrogen contained in these compounds is emitted into the atmosphere as ammonia (NH3) and nitrogen oxides (NOx) through volatilisation, which returns to the ground during rainfall and is then re-emitted as N2O. This is shown as an indirect emission of N2O.

The calculation for animal waste includes all manure that is spread on agricultural soils irrespective of which animal waste management system it was initially stored in. This includes all agricultural waste in New Zealand except for emissions from the pasture range and paddock animal waste management system. New Zealand uses a country-specific value for EF1 of 0.01 kg N2O–N/kg N (Kelliher and de Klein, 2006).

Direct N2O emissions from organic soils are calculated by multiplying the area of cultivated organic soils by an emission factor. Recent analysis identified 202,181 hectares of organic soils, of which it is estimated that 5 per cent (ie, 10,109 ha) are cultivated on an annual basis (Kelliher et al, 2002). New Zealand uses the IPCC default emissions factor (EF2 equal to 8 kg N2O–N/kg N) for all years of the time-series.

Direct emissions from agricultural soils are calculated in the six tables shown in the worksheets in Annex 8.

6.5.3 Uncertainties and time-series consistency

Uncertainties in N2O emissions from agricultural soils are assessed for the 1990, 2001 and 2002 inventory using a Monte Carlo simulation of 5,000 scenarios with the @RISK software (Kelliher et al, 2003) (table 6.5.1). The emissions distributions are strongly skewed, reflecting pastoral soil drainage whereby 74 per cent of soils are classified as well-drained, whereas only 9 per cent are classified as poorly drained. For the 2005 inventory, the uncertainty in the annual estimate was calculated using the 95 per cent confidence interval determined from the Monte Carlo simulation as a percentage of the mean value, ie, in 2002, the uncertainty in annual emissions was +74 per cent and –42 per cent.

Table 6.5.1 Uncertainties in N2O emissions from agricultural soils for 1990, 2002 and 2005 estimated using Monte Carlo simulation (1990, 2002) and the 95 per cent CI (2005)

Year N2O emissions from agricultural soils (Gg/annum) 95% CI min 95% CI max

1990

31.9

17.2

58.2

2002

40.6

23.4

70.4

2005

41.0

23.6

71.1

The overall inventory uncertainty analysis shown in Annex 7 (Good Practice table 6.1) demonstrates that the uncertainty in annual emissions from agricultural soils is a major contributor to uncertainty in the total estimate and trend from 1990. The uncertainty between years is assumed to be correlated, therefore the uncertainty is mostly in the emission factors and the uncertainty in the trend is much lower than uncertainty for an annual estimate. Uncertainty in the N2O emissions from agricultural soils contributes 8.9 per cent of the uncertainty in New Zealand’s total emissions and removals in 2005 and 1.1 per cent to the trend in emissions and removals from 1990 to 2005.

The Monte Carlo numerical assessment was also used to determine the effects of variability in the nine most influential parameters on uncertainty of the calculated N2O emissions in 1990 and 2001. These parameters are shown in table 6.5.2 together with their percentage contributions to the uncertainty. There was no recalculation of the influence of parameters for the 2005 inventory. The Monte Carlo analysis confirmed that uncertainty in parameter EF3 (PR&P) has the most influence on total uncertainty, accounting for 91 per cent of the uncertainty in total N2O emissions in 1990. This broad uncertainty reflects natural variance in EF3 determined largely by the vagaries of the weather and soil type.

Table 6.5.2 percentage contribution of the nine most influential parameters on the uncertainty of total N2O emissions inventories for 1990 and 2001

Parameter 1990 2001
  % contribution to uncertainty % contribution to uncertainty

EF3(PR&P)

90.8

88.0

EF4

2.9

3.3

Sheep Nex

2.5

1.8

EF5

2.2

2.8

Dairy Nex

0.5

0.7

FracGASM

0.5

0.5

EF1

0.3

2.4

Beef Nex

0.2

0.3

FracLEACH

0.1

0.2

6.5.4 Source-specific QA/QC and verification

Nitrous oxide emissions from “direct soil emissions” and “pasture, range and paddock manure” are key categories for both 1990 and 2005 (level and trend assessment). Nitrous oxide from “indirect emissions” is a key category for both 1990 and 2005 (level assessment). In preparation for this inventory, the data for these categories underwent Tier 1 quality checks.

The nitrogen fertiliser data obtained from FertResearch are corroborated by the Ministry of Agriculture and Forestry using nitrogen imports and exports, urea production figures and industrial applications (including resin manufacture for timber processing) data.

6.5.5 Source-specific recalculations

There are minor revisions in data due to updated data precision and correcting minor transcription errors.

6.5.6 Source-specific planned improvements

Research is continuing by New Zealand scientists to better quantify N2O emission factors for New Zealand’s pastoral soils.

6.6 Prescribed burning of savanna (CRF 4E)

6.6.1 Description

Prescribed burning of savanna is not a key category for New Zealand. The New Zealand inventory includes burning of tussock (Chionochloa) grassland in the South Island for pasture renewal and weed control. The amount of burning has been steadily decreasing since 1959 as a result of changes in lease tenure and a reduction in grazing pressure. In 2005, total emissions accounted for 1.0 GgCO2-e, a 2.3 Gg CO2-e (70.6 per cent) reduction from the 3.3 Gg CO2-e reported in 1990.

The IPCC Guidelines (1996) state that in agricultural burning, the CO2 released is not considered to be a net emission as the biomass burned is generally replaced by regrowth over the subsequent year. Therefore the long-term net emissions of CO2 are considered to be zero. However the by-products of incomplete combustion, CH4, CO, N2O and NOx, are net transfers from the biosphere to the atmosphere.

6.6.2 Methodological issues

New Zealand has adopted a modified version of the IPCC methodology (IPCC, 1996). The same five equations are used to calculate emissions. Instead of using total grassland and a fraction burnt, New Zealand uses statistics of the total amount of tussock grassland that has been granted a consent (a legal right) under New Zealand’s Resource Management Act (1991) for burning. Only those areas with a consent are legally allowed to be burned. Expert opinion obtained from land managers in local government is that approximately 20 per cent of the area allowed to be burnt is actually burnt in a given year.

Current practice in New Zealand is to burn in damp spring conditions which reduces the amount of biomass consumed in the fire. The composition and burning ratios used in calculations are from New Zealand-specific research (Payton and Pearce, 2001) and the IPCC reference manual (1996).

6.6.3 Uncertainties and time-series consistency

The same sources of data and emission factors are used for all years. This gives confidence in comparing emissions through the time-series from 1990 and 2005. The major sources of uncertainty are the percentage of consented area actually burnt in that season, extrapolation of biomass data from two study sites for all areas of tussock, and that many of the other parameters (ie, the carbon content of the live and dead components, the fraction of the live and dead material that oxidise and the nitrogen to carbon ratio for the tussocks) are the IPCC default values. Uncertainty in the New Zealand biomass data has been quantified at ± 6 per cent (Payton and Pearce, 2001), however many IPCC parameters vary by ± 50 per cent and some parameters lack uncertainty estimates.

6.6.4 Source-specific QA/QC and verification

There are minor revisions in data due to updated data precision and correcting minor transcription errors.

6.6.5 Source-specific recalculations

There were no recalculations for the 2005 inventory.

6.7 Field burning of agricultural residues (CRF 4F)

6.7.1 Description

Burning of agricultural residues produced 14.2 Gg CO2-e in 2005. Emissions are currently 11.1 Gg CO2-e lower (–43.9 per cent) than the level of 25.2 Gg CO2-e in 1990. Burning of agricultural residues is not identified as a key category for New Zealand.

New Zealand reports emissions from burning barley, wheat and oats residue in this category. Maize residue is not burnt in New Zealand. New Zealand uses three-year averages of crop production in combination with the IPCC default emission ratios and residue statistics. Oats are included under the same emission factors as barley.

Burning of crop residues is not considered to be a net source of CO2 because the CO2 released into the atmosphere is reabsorbed during the next growing season. However, the burning is a source of emissions of CH4, CO, N2O and NOx (IPCC, 1996). Burning of residues varies between years due to climatic conditions and is a declining source.

6.7.2 Methodological issues

The emissions from burning of agricultural residues are estimated in accordance with the IPCC guidelines (IPCC, 1996). The calculation uses crop production statistics, the ratio of residue to crop product, the dry matter content of the residue, the fraction of residue actually burned, the fraction of carbon oxidised and the carbon fraction of the residue. These figures are multiplied to calculate the carbon released. The emissions of CH4, CO, N2O and NOx are calculated using the carbon released and an emissions ratio. N2O and NOx emissions calculations also use the nitrogen to carbon ratio.

Good Practice Guidance suggests that an estimate of 10 per cent of residue burnt may be appropriate for developed countries but also notes that the IPCC defaults “are very speculative and should be used with caution. The actual percentage burned varies substantially by country and crop type. This is an area where locally developed, country-specific data are highly desirable”. (IPCC, 2000). For the years 1990 to 2003 it is estimated that 50 per cent of stubble was burnt. For the years 2004 and 2005, experts assessed this to have been 30 per cent. These figures were developed from opinions of the Ministry of Agriculture and Forestry officials working with the arable production sector.

6.7.3 Uncertainties and time-series consistency

No numerical estimates for uncertainty are available for these emissions. The fraction of agricultural residue burned in the field is considered to make the largest contribution to uncertainty in the estimated emissions.

6.7.4 Source-specific QA/QC and verification

There was no source-specific QA/QC for this category.

6.7.5 Source-specific recalculations

There are minor revisions in data due to updated data precision and correcting minor transcription errors.