Why in the Kyoto agreement did NZ not sign up to the inclusion of soil carbon?

New Zealand did not sign up to the inclusion of soil carbon under the Kyoto Protocol. There are a number of reasons that make it difficult to include soil carbon in the ETS at present:

It can be difficult and expensive to measure changes to soil carbon
Soil carbon in pastoral soils tend to be in a steady state unless there are significant changes in management/land-use
The methodology for accounting for soil carbon is complicated. New Zealand does not have data to establish 1990 baseline soil carbon levels
There are technical barriers related to measurement and monitoring.

Once a country starts accounting for soil carbon they are obliged to continue to do so. There is a risk of increasing our liability in the event that science shows that our soils are actually losing carbon. While it is generally accepted that soil carbon in New Zealand pastoral soils is in balance, some studies are already showing that our soils are in fact losing carbon. The soils that have been shown to be losing carbon include dairy farms on both volcanic and non-volcanic soils as well as sheep/beef farms on flat volcanic soils. Sheep/beef farming soils on hills have however been shown to be gaining carbon. Scientists are not sure whether these losses and gains have occurred recently, have been occurring gradually over time, and are on-going, or whether they occur uniformly across the country. Such uncertainties make the inclusion of soil carbon difficult.

It is likely that for farmers to gain carbon credits they have to prove what the soil carbon was for a given baseline and that they are sequestering carbon at a faster rate than "business as usual". That being the case, it is likely that farmers on soils that are low in organic matter stand a bigger chance of gaining credits if they changed their management system to build up carbon. On the contrary, farmers on carbon-rich soils will likely be disadvantaged since any change in management system would not improve soil carbon significantly.

There is an economic argument to this debate as well. Enhancing the soil organic matter content of the soil is accompanied by huge input of other minerals such as nitrogen, phosphorous, potassium, sulphur etc. If the sole purpose of increasing soil carbon is to gain credits, it is likely the cost of other inputs will outweigh the benefits to be gained depending on the prices set.

It is possible that soil carbon may one day play a part in meeting targets in New Zealand as rules relating to soil carbon for the second commitment period have yet to be negotiated. This will only be possible if and when economically and technically reliable and efficient methods of determining the quantities of CO^₂ removed from the atmosphere and stored in soil have been developed as well as clear benefits are shown.