Large Greenhouse Gas Emissions From Drained Peatlands in New Zealand, and the Climate Mitigation Potential of Rewetting

Drained peatlands are a substantial contributor to global anthropogenic greenhouse gases despite the relatively small areas they occupy. In New Zealand, drained Organic Soils (former peatlands) under cropland and grazed grasslands disproportionately contribute to emissions, but outdated mapping and limited measurements result in large uncertainties. To progress our understanding of the magnitude of these emissions and potential for mitigation, we compiled existing measurements from New Zealand and default emission factors from the IPCC along with updated soil mapping to develop a comprehensive national assessment. Using the IPCC 2013 Wetland Supplement (WS), Tier 1 default emission factors, total CO 2 equivalent (CO 2 e) drained Organic Soil emissions were 3.86 Mt yr⁻¹, equivalent to 6.5 % of the country’s net emissions. This was similar to the estimate derived from the mean of New Zealand-specific annual CO 2 e emissions (3.61 Mt yr⁻¹). However, there are insufficient measurements from which to develop robust Tier 2 emission factors, and the range of CO 2 e estimates based on existing data (3.06–4.69 Mt yr⁻¹) reinforces the need for more distributed measurements. The estimated CO 2 e contribution to national emissions increased to 6.34 Mt yr⁻¹ (or 11 % of net emissions) when peaty-mineral soils were included in the calculations. Carbon-rich soils with a peaty surface horizon that do not meet the technical definition of Organic Soils are increasingly recognised as a potential source, but we have even less data quantifying their emissions to determine appropriate emission factors. We evaluated the potential for emissions mitigation through rewetting Organic Soils by running scenarios representing different areal rates of annual rewetting over the next ten years using WS default emission factors. Cumulative emissions avoided by rewetting 500–3000 hectares per year from 2026 until 2035 ranged from 0.53 to 1.25 Mt of CO 2 e. Despite the barriers to practical implementation of peatland rewetting in New Zealand, the opportunity for emissions reductions is substantial. Further work is needed to refine emission factors, improve spatial activity data, and develop operational water table management capability to realise the potential in climate mitigation.