Monitoring changes in soil organic carbon stocks in New Zealand’s managed grasslands
Manaaki Whenua – Landcare Research has obtained Sustainable Land Management & Climate Change funding from the Ministry for Primary Industries to develop a method to monitor soil organic carbon stocks and stock changes in New Zealand’s managed grasslands, which occupy more than half the country’s total land area.
Countries worldwide place great emphasis on maintaining, or preferably increasing, soil organic carbon stocks for sustainable development and to address the impacts of climate change, and so we are developing a method to monitor any changes in soil organic carbon stocks at the national scale. An increase in soil organic carbon is a direct removal of carbon dioxide from the atmosphere and, therefore, one way to alleviate climate change impacts. If farm management practices exist that can reduce the loss or increase the rate of organic carbon sequestration into soils, governments will encourage these practices by providing financial incentives in carbon trading schemes. However, for this to happen, management methods must be auditable. Delegates from 111 countries, including New Zealand, recently met at an FAO meeting in Europe to review the role of soil organic carbon in the context of climate change and sustainable development. Top on their recommendations was the need to ensure countries are able to measure, map, monitor, and report on soil organic carbon stocks and stock changes to support management decisions.
Soil organic carbon stocks are inherently spatially and temporally variable, so sampling at field sites is problematic. Therefore, we are designing a fit-for-purpose soil organic carbon monitoring protocol, suitable for use in New Zealand’s managed grasslands. In this SLMACC project, the protocol will be implemented in hill country, where there is potential to increase the value of production, and where information is lacking on any potential impacts of increased intensification on soil organic carbon stocks. Hill country occupies 38% (9.9 million hectares) of all managed grasslands, and is broadly defined as the more rugged parts of our landscape, with slopes >15° and located below an altitude of 1,000 m. The method is being designed to be implemented over upcoming decades, and to be appropriate for national reporting, i.e. accurate, consistent, transparent, and compliant with international IPCC (International Panel on Climate Change) standards. The important contribution of an appropriate sampling design (i.e. how to select field sampling positions) cannot be overemphasised in this exercise, as this determines the quality of the end result. The sampling design is determined using prior knowledge of the spatial distribution of soil organic carbon stocks (i.e. existing measurements) and other factors that influence its accumulation (see Table 1), which are available to this project as digital map layers.
Table 1. Datasets that relate to the distribution of soil organic carbon stocks
| Variable | Data-layer examples |
| Soil factors | Soil depth, clay content, drainage class |
| Climate | Rainfall, temperature |
| Biota (including human activity) | Land use |
| Relief | Elevation, slope, aspect |
| Parent material | Geology, inferred from soil order |
A dataset of soil organic carbon stocks collected previously from 3,010 sites has been assembled for the target area and this, together with the digital map layers, provides the resources needed to select sampling positions objectively. A statistical approach is being used to do this, and once the positions are selected, new soil samples will be collected in a field campaign, due to begin this year. The new soil organic carbon data will be spatially modelled to derive a mean estimate of soil organic carbon stock within defined confidence intervals (e.g. ±10%) for New Zealand’s hill country in 2017. These confidence intervals can be narrowed by collecting more samples, and future campaigns will repeat this method to assess for any changes.
In summary, a fit-for-purpose method is being designed to monitor soil carbon stocks at approximate 5-year intervals over upcoming decades. This monitoring will support New Zealand’s commitment to the Paris Agreement to reduce greenhouse gas emissions by 30% below 2005 levels by 2030. If soil carbon sequestration is occurring, this can then be audited and used as an offset to greenhouse gas emissions in national accounting systems.
CAROLYN HEDLEY, STEPHEN MCNEILL, PIERRE ROUDIER
Manaaki Whenua – Landcare Research
E: HedleyC@landcareresearch.co.nz
