It also suggested that rivers which originate in the north-east of the South Island and east and north of the North Island will decline, and wildfires will become more prevalent. This month, it released another report on agriculture, forestry and other land use (AFOLU) mitigation options.
Putting the two together, the dangers of including soil carbon in any sort of emissions trading scheme are clear. Yet soil carbon sequestration still seems to be on the political agenda.
The IPCC AFOLU report made particular mention of “land with high carbon stocks not suitable for cropping” in its consideration of whether the world could reduce emissions by altering diet to include more plant-based material.
New Zealand soils have high carbon content in comparison with many elsewhere. Typically pasture soils are about 5% carbon (over 8% organic matter), and cropping soils are about 3% carbon (5% organic matter).
In Waikato, maize crops (for animal feed) are rotated with perennial or annual pasture; if the latter, farm effluent and bedding from shelters is being used to ensure soil organic matter – the skeleton of which is carbon – is maintained. On the Canterbury Plains, urban compost has been used to increase soil organic matter on areas under crops. The soil organic matter holds nutrients and improves soil moisture, but as micro-organisms break it down, thereby releasing nutrients, they also release carbon dioxide which contributes to emissions. In addition, high organic matter has been associated with hydrophobicity – water repellence.
Of note is that the addition of carbon creates a pulse of activity, which is maintained if inputs are maintained. If inputs decrease, then activity decreases while carbon is used up.
Of particular importance given the predictions about ‘warmer and drier’ in some areas of New Zealand, activity is enhanced at warm temperatures (deserts have little organic matter) and decreased when conditions are cold and wet (when peat bogs are created, for instance).
Bringing soil carbon into any sort of trading scheme would be fraught with difficulties in respect of its changeability (particularly as the scenario for many parts of New Zealand is warmer and drier) and the difficulties of measuring it with any meaning.
Alex Tressler, a final year student in management studies (agribusiness) at the University of Waikato, has analysed what other countries are doing about soil carbon and mitigation of emissions.
“Land management practices and land use change can encourage soil carbon sequestration in agricultural systems,” he said.
“But carbon sequestration is a reversible process. In addition, soil scientists in New Zealand have indicated that the potential of soil carbon sequestration as a viable offset is limited by naturally high organic carbon in soils already.”
Despite much talk about soil carbon to mitigate emissions globally, no country has yet brought it into policy. Tressler says that “reasons given were complexity of individual farming systems, and the difficulty of measuring carbon sequestration and carbon stocks on a farm basis.”
Tressler recommends that in order to protect New Zealand’s competitive advantage for agriculture, with all the implications for the economy and hence environment and society’s standard of living, agriculture in general, and soil carbon in particular, should not be brought into any form of ETS.
• Jacqueline Rowarth is professor of agribusiness, The University of Waikato.
