Significant carbon loss from a natural tropical peatland under current climate

Southeast Asian peatlands, one-third of global tropical peatlands, have sequestered and preserved gigatons of carbon in the past thousands of year. Rainfall fluctuation on yearly and even hourly timescales plays an important role that defines peat carbon accumulation or loss from tropical peatlands. Notably, research related to the ecosystem-scale carbon exchange, including methane (CH₄), over tropical peatland ecosystems remains limited. Given their significant carbon stocks, the fate of natural tropical peatlands under current and future climate is unknown.

We performed a study in Kampar Peninsula, a coastal tropical peatland of around 700,000 ha, in Sumatra, Indonesia. This ombrotrophic (acidic and nutrient-poor) peatland largely formed within the past 8000 years. The peninsula is characterized by a large, relatively intact central forest area surrounded by a mosaic of smallholder agricultural land, and industrial fiber wood plantation, smaller secondary forest areas, and undeveloped open and degraded land. We measured the net ecosystem CO₂ and CH₄ exchanges between natural peatland and the atmosphere using the eddy covariance technique over two years (June 2017-May 2019). In addition, peat subsidence rates were measured using polyvinyl chloride poles at every 1 km along 35 km long transect across the natural forest in the peninsula. In the natural forest, groundwater level shows periodic sharp rises and steady decreases corresponding to rain events. The groundwater level can rise up to 20 cm above the peat surface in the wet season, and then in the late dry season can reach -70 cm.

Our measurements indicate that the natural tropical peatland functioned as a significant source of CO₂ (410±60 g CO₂-C m^-2 year^-1) and CH₄ (6.8±0.7 g CH₄-C m^-2 year^-1) to the atmosphere. If we follow IPCC global warming potential (GWP) accounting methodology and apply a 100-year GWP of 34 for CH₄, this implies that CH₄ emissions contributed ~35% of the 100-year net warming impact. Carbon emissions (due to oxidation of peat, litterfall and coarse wood debris) contributed ~30-35% of the observed subsidence rates. The CO₂ exchanges increased linearly as groundwater level declined. Lower groundwater level enhances peat aeration and potentially increases oxidative peat decomposition, which results in higher CO₂ emissions. The CH₄ exchanges decreased exponentially as groundwater level declined.

The results indicate that tropical peatland ecosystems are no longer a carbon sink under the current climate. Our results, which are among the first eddy covariance exchange data reported for any tropical peatland, should help to reduce the uncertainty in the estimation of CO₂ and CH₄ emissions from a globally important ecosystem and improve our understanding of the role of natural tropical peatlands under current and future climate.