Describing soils

IPCC soil categories

New Zealand system for quantifying changes in the soil carbon reservoir

Under the Framework Convention for Climate Change, New Zealand is obliged to provide accurate reporting of its CO2 and other greenhouse gas emissions, including data on major sources and sinks of carbon. Soils are the largest reservoir of carbon in the terrestrial biosphere and small changes in the reservoir size having major implications for atmospheric CO2 concentrations and mitigation strategies. New Zealand therefore needed a system for quantifying changes in the soil carbon reservoir.

We did this by subdividing all New Zealand soils into the six IPCC Soil categories outlined in IPCC Guidelines for National Greenhouse Gas Inventories Draft Reference manual, July 1996, and adding an extra class (Podzol). This was done to accommodate medium- and fine-textured acid soils of temperate regions having neither high-activity clays or impeded drainage, yet accumulate organic matter that is protected from decomposition by acidity and the formation of iron and aluminium complexes. The six IPCC categories are:

  • Soils with high clay activity
  • Soils with low clay activity
  • Sandy soils
  • Volcanic-derived soils
  • Aquic soils and Organic soils

These catergories are necessarily broad and intended to be used at country scale rather than regional or local scale.

Soils with high clay activity (HCA) have appreciable amounts of high-activity clays (e.g. 2:1 layer clays) that promote long-term stabilisation of organic matter, especially in many carbon-rich temperate soils. In contrast, soils with low-activity clays (e.g. kaolinite) (LCA) have a much lower ability to stabilise carbon, and respond more rapidly to changes in the soil’s carbon balance, and include highly-weathered acid soils. Sandy soils (defined as having more than 70% sand and less than 8% clay) generally have a low capacity to stabilise carbon. The Andisol group of soils derived from volcanic ash (VOL) have allophane as the primary colloidal material and are generally carbon-rich and fertile. Aquic soils (AQU) are mineral soils developed in poorly drained, wet environments and have reduced decomposition rates and high organic matter content. Peat soils, or Histosols, are organic soils (ORG) that form under water-saturated conditions where decomposition is greatly reduced. If these soils are drained and cultivated they can lose massive amounts of carbon over time.