The effects of water-table depth on carbon dioxide emissions for oil palm cultivation on peatlands
Globally, around 16.4 million hectares of land is under cultivation by oil palm plantations, which produce a third of the world’s vegetable oil. Most oil palm expansion is in Southeast Asia, an area with vast peatlands. Plantations are increasingly being established on these carbon-rich organic soils. Tropical peatland draining for oil palm cultivation leads to a net release of greenhouse gas emissions to the atmosphere. Recent research identifies a relationship between increasing drainage depth and increasing carbon loss from peatlands. We apply regression analysis to assess carbon loss as a function of water table depth as measured by subsidence or closed chamber methods from oil palm and other perennial tree plantations in Southeast Asia. We limit these studies to those that include drainage depth, report at least nine months of emissions, are measured more than five years after initial drainage, and in the case of closed chamber studies, exclude autotrophic respiration.
Our chosen model, developed from subsidence studies, suggests an emission rate of 68 tons CO2 ha-1 yr-1 at 70 cm water table depth for peatlands that have been drained for >5 years. This rate falls within the range of values suggested by other authors that conducted mass balance studies or compiled subsidence measurement reports. Based on our selected model, reducing water table depth from 70 cm to 50 cm would reduce emissions by 19% per year. Because peat soil emissions are disproportionately high in the first years after peatland drainage, we offer an optional and highly uncertain modification of our base emissions rate by a factor of 2.6 for the first five years after drainage, derived from a study by Hooijer et al. (2012). While our recommended emissions factor is derived from the best currently available data, long-term, large-scale, oil palm-focused studies are needed to more accurately estimate greenhouse gas emissions from oil palm plantations on peatland, as is necessary for policy decisions about sourcing oil palm for everything from cooking oil to biofuels. Moreover, water management techniques serve only to reduce emissions rates; only active protection will reduce total future emissions from tropical peatland ecosystems.