Organic soil greenhouse gas emissions accounting for the Waikato region

The Waikato Region has about 84,000 ha of Organic Soils (OS) that formed from the slow accumulation of peat (about 1 mm per year) in wetlands over the past 10,000 – 14,000 years, resulting in organic material more than 10 m deep in some cases. Drainage of OS in local wetlands began in the early 1900s and now about 65,000 ha have been drained, mostly for pastoral agriculture. Some shallow OS that existed in wetlands prior to colonisation have been lost due to drainage and burning and are now represented by other soil orders (e.g., Gley Soils).

While intact peat wetlands are a greenhouse gas (GHG) sink (Goodrich et al. 2017), disturbance and drainage of OS causes loss of organic material to the atmosphere as GHG emissions, thereby contributing to the region’s GHG footprint (Pronger et al. 2021).

Waikato Regional Council seeks to account for GHG losses from drained OS, with the intention that these emissions could be included in a future regional inventory (2024/25). This approach recognises their significant contribution to GHG emissions and aligns with the national inventory (Ministry for the Environment 2023a).

This technical report supports the commentary presented in the latest regional GHG inventory (Waikato Regional Council and EnviroStrat Limited 2023) about eventually including OS, by identifying and recommending accounting methodologies, presenting key calculations and considerations, and providing a pathway for reporting OS emissions at the regional scale. Results from this report will enable OS emissions comparisons across districts and sectors, and benchmark Waikato against national and international inventories.