Tropical peatlands are a known source of methane (CH4) to the atmosphere, but their contribution to atmospheric CH4 is poorly constrained. Since the 1980s, extensive areas of the peatlands in Southeast Asia have experienced land‐cover change to smallholder agriculture and forest plantations. This land‐cover change generally involves lowering of groundwater level (GWL), as well as modification of vegetation type, both of which potentially influence CH4 emissions. We measured CH4 exchanges at the landscape scale using eddy covariance towers over two land‐cover types in tropical peatland in Sumatra, Indonesia: (a) a natural forest and (b) an Acacia crassicarpa plantation. Annual CH4 exchanges over the natural forest (9.1 ± 0.9 g CH4 m−2 year−1) were around twice as high as those of the Acacia plantation (4.7 ± 1.5 g CH4 m−2 year−1). Results highlight that tropical peatlands are significant CH4 sources, and probably have a greater impact on global atmospheric CH4 concentrations than previously thought. Observations showed a clear diurnal variation in CH4 exchange over the natural forest where the GWL was higher than 40 cm below the ground surface. The diurnal variation in CH4 exchanges was strongly correlated with associated changes in the canopy conductance to water vapor, photosynthetic photon flux density, vapor pressure deficit, and air temperature. The absence of a comparable diurnal pattern in CH4 exchange over the Acacia plantation may be the result of the GWL being consistently below the root zone. Our results, which are among the first eddy covariance CH4 exchange data reported for any tropical peatland, should help to reduce the uncertainty in the estimation of CH4 emissions from a globally important ecosystem, provide a more complete estimate of the impact of land‐cover change on tropical peat, and develop science‐based peatland management practices that help to minimize greenhouse gas emissions.

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<p>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 (CH<sub>4</sub>), over tropical peatland ecosystems remains limited. Given their significant carbon stocks, the fate of natural tropical peatlands under current and future climate is unknown.</p><p>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 CO<sub>2</sub> and CH<sub>4</sub> 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.</p><p>Our measurements indicate that the natural tropical peatland functioned as a significant source of CO<sub>2</sub> (410±60 g CO<sub>2</sub>-C m<sup>-2</sup> year<sup>-1</sup>) and CH<sub>4</sub> (6.8±0.7 g CH<sub>4</sub>-C m<sup>-2 </sup>year<sup>-1</sup>) to the atmosphere. If we follow IPCC global warming potential (GWP) accounting methodology and apply a 100-year GWP of 34 for CH<sub>4</sub>, this implies that CH<sub>4</sub> 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 CO<sub>2</sub> exchanges increased linearly as groundwater level declined. Lower groundwater level enhances peat aeration and potentially increases oxidative peat decomposition, which results in higher CO<sub>2</sub> emissions. The CH<sub>4</sub> exchanges decreased exponentially as groundwater level declined.</p><p>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 CO<sub>2</sub> and CH<sub>4</sub> emissions from a globally important ecosystem and improve our understanding of the role of natural tropical peatlands under current and future climate.</p>

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