Skip to main content.

3. Developing the 1990 Baseline for Soils

3.1 Conceptual framework

1990 is accepted as a baseline against which to measure changes in carbon stocks in the future. Therefore determining this baseline for both soil and indigenous forests is an important starting point for this project.  

The work carried out in developing a baseline carbon stock was built on previously published work (IPCC 1996; Daly & Wilde 1997; Newsome & Willoughby 1997; Leathwick et al. 1997; Scott et al. 1997; Tate et al. 1997a), the main limitation of which was that the estimates of soil carbon took no account of land use. As land use, together with soil type and climate, is a key driver of changes in soil carbon, this project, based on the IPCC default methodology, used these three factors to stratify New Zealand for determining changes in soil carbon attributable to land-use changes.

The approach taken was to:

  • establish how the New Zealand soil classification relates to that required by the IPCC

  • determine land-use (vegetation) information for 1990

  • determine the major climate layers for the country to be incorporated into a spatial framework

  • supplement soil carbon (soil C) information for major soil groups not well represented in the initial inventory

  • synthesise all available data in a GIS (geographical information system) format into major land use, climate, and soil types to provide a matrix of cells for the country.

Figure 1 illustrates the approach to determining the 1990 baseline and updating the carbon estimates for the monitoring system.

3.2 Reclassification of soil categories

New Zealand soils were reclassified into categories matched the IPCC guidelines, by cross-referencing with the National Soils Database.  These were then used to produce a new soil data GIS layer. The following IPPC categories were included and Podsols was added.

  • soils with high clay activity

  • soils with low clay activity

  • sandy soils

  • Andisols

  • Aquic soils

  • Histosols

Fig. 1 Framework for assessing the contribution of soils to a national carbon budget

Thumbnail of image. See figure at its full size.

The IPCC guidelines specified the depth range of 0–0.3 m for reporting on changes in soil carbon. We have separated this depth range into 0–0.1 and 0.1–0.3 m to enhance our ability to detect soil C changes. Soil C in depth range 0.3–1 m is also reported when this is available.

3.3 Land information

Soil disturbance through land use can have a marked effect on soil C.  Land-use practices can enhance soil C but, in general, many farming practices cause soil C to decline.  A steady state eventually occurs when soil reflects its final level under the management practices in place.  Land-use information for this project was originally derived from the 47 Vegetation Cover Classes in the Vegetative Cover Map of New Zealand (VCM) (Newsome 1987). The categories were reallocated into those appropriate for soil C monitoring, and include:

    AlpineArable cropland

  • Bare ground

  • Exotic forestry

  • Horticultural land

  • Indigenous forest (conifer)

  • Indigenous forest (broadleaf)

  • Indigenous forest (mixed broadleaf/conifer)

  • Improved pasture

  • Unimproved pasture

    • Scrub

As the VCM will not be updated, future carbon monitoring will need to be updated through an ongoing land-use database such as the Land Cover Database (LCDB).

3.4 Climate data

A climate GIS layer used interpolated climate data (Hutchison 1995), which combined data on precipitation and soil class to give soil moisture classes, which were combined with temperature data to give the following temperature/moisture classes:

  • Boreal

  • Humid boreal

  • Very dry temperate

  • Dry temperate

  • Moist temperate

  • Humid temperate

  • Aquic

Soil type and climate combined gave 49 categories, which were reduced to 18 and combined with land use to give 198 categories, only 147 of which existed in New Zealand.

By amalgamating very minor categories and removing those combinations that did not exist, the number of cells was reduced to 39, which accurately categorise about 93% of the New Zealand landscape.  

3.5 Results

At the end of the 1998/1999 year, once all the new data had been incorporated into the database, the soil C figures at the three depths were revised.  Only one climate/soil/land-use class that exceeds 10 000 km2 remains unsampled.  As this class contributes 35% to the error variance, it remains a high priority for future sampling.

The revised 1990 national soil C estimates are now 1148 + 49.5 Mt for 0-0.1 m, 1413 + 80.0 Mt for 0.1-0.3 m and 1631  193.0 Mt for 0.3-1.0 m layers.

 

[ |