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Soil carbon

Soil sampling in a New Zealand pine forest

Soil sampling in a New Zealand pine forest

A large proportion of the global carbon stock is stored in soil, the top layers of which play a part in the carbon cycle. Living micro-organisms, dead animals and plants, and minerals all contribute to soil carbon levels. For practical purposes, very fine roots of living plants and trees (under 2 mm diameter) are also considered to be part of the soil carbon pool.

It is generally accepted that the amount of carbon in the underlying soil remains constant, or very nearly so in the short term, if land use does not change. Soil type and climate are important factors for soil carbon levels, but these tend to change very little over time.

Any change in land use, however, may cause the soil carbon stock to increase or decrease gradually. This is especially so if that land-use change involves a disturbance of the top soil, as in agriculture and forestry. For New Zealand, soil carbon stocks are estimated to a depth of 30 cm, the same depth as the Good Practice Guidance defaults of the IPCC.

Soil CMS

Recent years have seen the development of a Soil Carbon Monitoring System (Soil CMS) for estimating New Zealand’s soil carbon stocks for all land uses, and likely stock changes with changes in land use. The system has been subject to international scientific scrutiny and formally published in 2004 and 2005.

Pastoral farming has been used as a convenient reference land use, with the effects of land-use change measured relative to grassland (most changes involve changes to or from grassland in New Zealand). Planting Pinus radiata is the most important land-use change, and many supporting studies have focused on the change in soil carbon stocks with afforestation.

The Soil CMS is a statistical model, based on default methodology recommended by the IPCC (in Good Practice Guidance).

The model combines actual soil carbon data from New Zealand soils, with national spatial datasets of soil type, climate, land use and topography. By doing so it can:

  • derive estimates of soil carbon stocks for all land uses
  • model likely changes in soil carbon following changes in land use.
Currently it uses the IPCC default of a linear change over a 20-year period, for the carbon stock under one land use to reach its new stock level under the new land use. Several refinements to the Soil CMS are planned over time.

Sampling

The bulk of the soil data has been extracted from New Zealand's National Soil Database. This covers all of the North Island and the South Island of New Zealand, but not all land-use/soil/climate combinations are represented. Additional data collection has been undertaken to fill in data gaps and more is planned.

This additional data collection and analysis, plus refinements to the Soil CMS model, will strengthen the robustness of the model and reduce uncertainties around stock change estimates. This process will be undertaken throughout the first commitment period (CP1: 2008-2012) of the Kyoto Protocol.


Last updated: 4 August 2009