- 4.2 Effects on non-CO2 greenhouse gases
- 4.3 Definitional issues
- 4.4 Socio-economic factors influencing the activities considered
- 4.5 Applicability of a land-based accounting system
- 4.6 What data, publications and reviews are available
- 4.7 Sampling techniques and issues
- 4.8 What process models are available to complement data
- 4.9 What regional and national scale inventories are available
- 4.11 Projections for the first commitment period where feasible
- 4.12 Data and research needed to reduce uncertainties
4 Cropland management
4.1 Specific activities considered, and effects not covered
Cropland encompasses cropping systems (including grain crops, fodder, orchards, vineyards, vegetables and flowers), tillage, crop residue management, cover crops, crop rotations, irrigation, and fertilisation. The main grain crops are wheat, barley, oats, maize and field peas (Petrie and Bezar 1998), with small areas of many speciality crops. Maize is grown mainly in the North Island, and oats predominantly in Southland and Otago. The selection of crops is made mainly on their anticipated gross margins for the year. Vegetables are usually grown continuously in the same field for many years. Up to three crops can be grown per year, depending on location. The main vegetable crops are potatoes, green peas, sweet corn, squash and onions (Petrie and Bezar 1998).
The main effect of cropping systems and management on carbon stocks is from changes in soil carbon. Most cropping systems in New Zealand are traditionally in mixed cropping (pasture-arable) rotations with the pastoral and arable phases of similar duration (usually 2-5 years). Soil is mouldboard ploughed to 15-20 cm depth in late autumn, followed by a number of secondary tillage operations to create a suitable seedbed. Where spring crops are grown, soil is often left fallow over the winter.
Recently (probably post-1990), there appear to have been changes in land management practices. However, there is no published regional or national information to support these observations. The main changes are:
- length of cropping phase in rotation has increased and the grazed pasture phase has been replaced with pastoral seed cropping.
- a shift to minimum tillage (reducing the amount of cultivation, by reducing either the tillage depth or the number of passes). The proportion of arable crops established by no-tillage remains very small (<5% of cropping land) (Ross et al 2000a).
- increase in green manuring and cover cropping to reduce winter nitrate leaching losses (Francis 1995).
Residues of maize and vegetables are usually incorporated (sometimes following grazing); those of peas are usually baled and removed. Post-harvest residues of wheat and barley are usually burned. There is no widespread use of any other organic amendments.
Nitrogen, phosphorus, sulphur and potassium are the major nutrients applied through fertilisers. Annual nitrogen application rates, in the range 0 to 250 kg N ha-1, vary between farmers, between crops, and between stages in the rotation. Fertiliser rates for vegetable crops are often greater than for arable crops, especially for winter vegetables (Crush et al 1997).
Not included in this report are changes in greenhouse gas emissions due to changes in energy use associated with these activities or off-site implications such as fertiliser production.
4.2 Effects on non-CO2 greenhouse gases
In the United Kingdom, Robertson et al (2000) studied the net effect of cropping on emissions of CO2, nitrous oxide, and methane over eight years. Wide variations were seen depending on land management practices and no cases of net mitigation were found, although soil carbon accumulation in no-till systems came closest to mitigating all other sources. Nitrous oxide was dominant in GWP weighted emissions and removals.
There is very little information on N2O emissions and CH4 oxidation in New Zealand arable soils. Cumulative N2O emissions were greater following the cultivation of a clover sward (3.8 kg N2O-N ha-1) than following the cultivation of a mixed herb ley (1.6 kg N2O-N ha-1) (van der Weerden et al 2000). No differences in N2O emission between conventional (CT) and no-tillage (NT) systems were observed after 5 years of cultivation (Choudhary et al 2001). CH4 oxidation rates in this soil were unaffected by the preceding sward (van der Weerden et al 1999). Cultivation type (shallow vs deep) had no effect on either N2O emission or CH4 oxidation.
The cropping land management effects on these non-CO2 greenhouse gas emissions are insignificant in comparison with national emission estimates.
4.3 Definitional issues
As land in New Zealand is commonly rotated between pasture and arable use, an accounting issue may arise if separate reporting for these land-use categories is required. It would certainly reduce the accounting overhead and avoid definitional problems if a combined agricultural category could be used.
Horticultural land is treated here as falling within the cropland category. If separate reporting were required for horticulture, that would create definitional questions concerning the boundary between cropping and horticulture.
4.4 Socio-economic factors influencing the activities considered
Commodity prices and irrigation are driving this land use. With increased profits in dairying, the area of arable cropping in Canterbury is declining slightly as more land comes under irrigation, and farmers convert to dairying. Increase in dairy farms will also result in a greater proportion of feed and fodder being grown on neighbouring arable farms to overcome problems of dairy feed supply at certain times of the year.
Arable cropping around major towns and cities is also declining, as land is sub-divided for lifestyle blocks (Balks 1999; Singleton 1999). These blocks are too small to support arable cropping and are mainly converted to pasture.
Encouragement of renewable energy sources could lead to development of biofuels in New Zealand. Depending on the type of vegetation chosen and the precise definitions adopted for forests within the Protocol, the establishment of biofuel plantations may be covered by Article 3.3 as afforestation, or may be covered under Article 3.4 as a change in land-use.
4.5 Applicability of a land-based accounting system
Agribase farm surveys, e.g. Situation and Outlook for New Zealand Agriculture (SONZA) and Statistics New Zealand (MAFStats), give areas for each crop type, but there is no associated information on the location within farms or regions of various land-use activities.
The Land Cover Database (LCDB) and other spatially explicit methods cannot estimate cropping area accurately at present. This leads to potential difficulties for estimation of soil carbon changes in the Soil Carbon Monitoring System (Soil CMS). Thus the present information base may limit New Zealand's ability to claim verifiable credits under Article 3.4
Carbon sequestration resulting from changes in land management, such as no-till agriculture, are not permanent and depend on long-term maintenance of agricultural practices. This means that credible accounting would require tracking of land-use and management regularly and effectively in perpetuity to ensure that carbon stock changes could be assessed.
4.6 What data, publications and reviews are available
Soil organic carbon contents do not measurably change during the course of pastoral-cropping rotation (Haynes and Francis 1990; Haynes and Swift 1990; Haynes et al 1991; Haynes 2000a). However, when soils under permanent pasture are brought into cultivation there is a rapid loss of organic carbon during the first few years, and continuous cropping results in further carbon loss (Sparling et al 1992; Aslam et al 1999; Haynes and Tregurtha 1999; Shepherd et al 2000, 2001; Saggar et al 2001b). The size of this decline is greater when crops are grown under conventional tillage (CT) than no-tillage (NT) (Francis and Knight 1993; Ross et al 2000a). An increasing proportion of arable cropping in the rotation also reduces soil organic carbon contents (Haynes and Swift 1990; Francis et al 2000a; Haynes 2000b).
The decline in organic carbon varies with soil type (Fig. 4.1). The loss in soil carbon is faster in light (sandy) soils than heavy (clay) soils; faster in calcareous soils than in volcanic soils; and peaty soils lose carbon rapidly when drained and cultivated (Fig. 4.1). Reversion back to pasture restores soil carbon levels. Long-term cultivated soils would require a much longer pasture phase to restore organic carbon levels (Fig. 4.2) than short-term cultivated soils. Long-term application of fertiliser may have very little effect on the changes in soil organic carbon contents (Haynes and Beare 1996).
Figure 4.1. Changes in total organic carbon under increasing cropping time, in the topsoil depth (0 - 0.2 m).
Figure 4.2. Changes in total carbon in the topsoil (0 - 0.2 m) of the Kairanga silty clay loam under increasing cropping time and conversion back to pasture.
There are very few New Zealand data on post-harvest residue management effects on soil carbon contents. International data suggest soil carbon contents increase where residues are incorporated rather than burned (Kumar and Goh 2000). But no differences were obtained after 6 years of contrasting residue management practices under New Zealand conditions (Curtin and Fraser 2001).
4.7 Sampling techniques and issues
Several issues arise in relation to the adequacy of current data and sampling methodologies:
- Most soil carbon data are available only to 20 cm soil depth, while the IPCC guidelines require data to 30 cm depth.
- Appropriate sampling techniques, to account for changes in soil bulk density, need to be considered to avoid confounding effects of soil disturbance on soil carbon (Saggar and Tate 2001).
- A spatially explicit representation of arable land by crop type is required to include crop type/soil type/climate relationships.
- The LCDB data do not "see" cropland due to its small dispersed area and ephemeral nature.
4.8 What process models are available to complement data
A number of crop-growth simulation models have been developed for wheat (Jamieson et al 1998), maize (Wilson et al 1995), peas (Wilson et al 2000), potatoes (Stone et al 2000), tomatoes (Reid et al 1998) and carrots (Reid and English 2000). In these models, crop potential yield is initially calculated from photosynthetically active radiation and temperature. Actual yield is then calculated with known limitations for either N supply, drought, pest or disease pressure. These models do not predict changes in soil carbon associated with crop growth. Soil models (RothC, Socrates, Century) are available to predict soil carbon changes but have not yet been coupled to crop growth models in New Zealand.
4.9 What regional and national scale inventories are available
The main sources of data are:
- Agribased farm surveys: SONZA and MAF surveys.
- the National Soil carbon monitoring system (CMS) with limitations on spatially explicit cropping data.
and as noted earlier these have inadequacies for tracking change in carbon stocks that can be attributed to land-use management.
There is only a limited amount of systematic data for a few cropping soils in New Zealand. The New Zealand Soils Database (NZSD) mainly consists of pastoral soils. For most cropping soils on the NZSD, land management history is not well documented (e.g., length of arable cropping, method of cultivation, residue management, fertiliser history, etc.). Data for a number of important cropping soil types (500 soils) to 0.1 m depth have recently been collected as part of national and regional environmental monitoring systems (Francis et al 1998; 2000b). More detailed information has also been recently collected from up to nine profiles for each of eight Canterbury soils (Webb 2000).
4.10 What is known of 1990 carbon stocks and current trends
Above-ground carbon stocks have been estimated at 2.5 TgC (Tate et al1997). In addition soil carbon associated with cropland is estimated as: 26 TgC for arable land to 1 m depth, or 17 TgC to 0.3 m depth; and 28 TgC for horticultural land to 1 m depth, or 9.1 TgC to 0.3 m depth. These estimates are based on SONZA areas and carbon density values from Davis et al (1999).
There appears to be a slight increase in area under horticulture (3 Kha y-1). If this increase in horticultural land is significant and represents conversion from pasture, then a ~0.05 TgC release from soil carbon stocks is calculated based on the Carbon Monitoring System factor for arable land (Tate et al 2000b).
Table 4.1. Estimated 1990 Carbon stocks and Recent Trends for Cropland Estimated for 1990
Estimated for 1990 |
Recent rates of change per yeara |
||||||
| Area (Mha) | Vegetative C stock (Tg C) | Soil C stock (Tg C) | Total C stock (Tg C) | Area (Kha) | Veg C Stock (Tg y-1) | Soil C stock (Tg y-1) | Total C stock (Tg y-1) |
| 0.31 | 2.5 | 25.7 | 28 | +3 | +0.025 | -0.052? | -0.025 ? |
a is the average change during 1990 and 1999.
1 0.2 is grain crops; remaining 0.1 is horticultural (SONZA, 1990).
2 Calculation based on change in cultivated area multiplied by CMS factor for
pasture to arable conversion (Tate et al 1997). Highly uncertain.
The total carbon stocks and recent trends for croplands are summarised in Table 4.1. As noted above, some uncertainty derives from the use of spatial differences between sites with different histories to infer temporal differences at a single site.
4.11 Projections for the first commitment period where feasible
The best estimates for the first commitment period are a simple extrapolation of the figures shown in Table 4.1 - viz a small stock depletion of ca -0.025 TgC y-1. Overall the magnitude of changes in cropland carbon is estimated to be negligible due to small areal coverage (1.2% of total land area), and small rate of land conversion.
It is unlikely that there will be any significant development of new biofuel or bioenergy crops in the near term, but if this were to occur then there may be related changes in terrestrial carbon stocks that might be considered under Article 3.4 of the Protocol.
4.12 Data and research needed to reduce uncertainties
As noted earlier, improved spatial representation of cropping and management types will be required for accurate reporting. Considerable sampling of soil under a variety of crop types will be required to verify models and sampling to greater depth is required to meet IPCC guidelines.
Good crop simulation models exist for some major grain crops and vegetables, but these models need to be expanded to treat changes in soil carbon stocks.
Management of organic or peat soils (Daverin, 1978) is an aspect that needs further study. These soils can lose considerable amounts of carbon under agricultural management (Schothurst, 1982). In the Waikato, decomposition of peat under agriculture could release 0.5 TgC y-1 although there is insufficient specific data to substantiate this estimate. The total area of land covered by such soils in New Zealand is about 1.8 Mha containing about 608 MgC ha-1
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