Greenhouse gas emission factors for an unmanaged degraded tropical peatland in Southeast Asia

Abstract:

Tropical peatlands have sequestered and preserved gigatons of carbon in the past thousands of years. More than one-third of tropical peatlands are situated in Southeast Asia, of which approximately 20% is considered unmanaged degraded. This peatland degradation generally involves lowering of groundwater level, as well as modification of vegetation cover, both of which potentially influence carbon dioxide (CO₂) and methane (CH₄) emissions. Notably, research related to ecosystem-scale CO₂ and CH₄ exchange remains limited. Yet, published data display a large range of carbon emission estimates and, hence, highlight knowledge gaps in our science on tropical peatland carbon cycling.

We measured the net ecosystem CO₂ and CH₄ exchanges between unmanaged degraded peatland and the atmosphere using the eddy covariance technique in Sumatra, Indonesia over 3 years (Oct 2016-Sep 2019). The site has been disturbed by logging and fire in the 2000s, resulted in vegetation cover loss. Comparing with natural peat swamp forest, more than 50% of the large trees have been logged or fallen and many remaining now lean. Further, the site was disturbed by drainage; consequently groundwater level was always below the surface (i.e. no occurrence of surface inundation).

Our measurements indicate that unmanaged degraded tropical peatland functioned as a large source of CO₂ (1062±66 g CO₂-C m^-2 year^-1) and CH₄ (5.3±0.9 g CH₄ 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 to ~5% of the combined GHG emissions. The total carbon emissions (due to decomposition of peat, litterfall and coarse wood debris) account for ~60% of the observed subsidence rates of 4.2 cm year^-1.

Net CO₂ emissions increased as groundwater level declined. Lower groundwater level enhances peat aeration and potentially increases oxidative peat decomposition, which results in higher CO₂ emissions. CH₄ emissions decreased exponentially as groundwater level declined. Further, the CH₄ emissions showed diurnal variability with significant higher emissions during the daytime as compared to the nighttime. This can be attributed to the vegetation-mediated CH₄ emissions as dissolved CH₄ can be taken up by root system and emitted to the atmosphere.

Our results, which are among the first eddy covariance exchange data reported for any tropical peatland, should help to reduce uncertainty in the estimation of CO₂ and CH₄ emissions from a globally important ecosystem, and develop science-based peatland management practices to reduce carbon emissions.