Methane emissions from an unmanaged degraded tropical peatland: Interacting influences of plant-processes and groundwater level

Tropical peatlands are a complex ecosystem with poorly understood biogeochemical regimes. An immense peat carbon stock and waterlogged-anaerobic conditions may possibly favor methane formation in this ecosystem. Methane is released to the atmosphere either from soil/water surface or through vegetation (both herbaceous plant and tree). Using the conventional flux chamber method, assessing spatiotemporal variability and vegetation-mediated methane emissions remains a practical challenge for scientists. Consequently, research related to ecosystem-scale methane exchange remains limited. Yet, published data display a large range of methane emission estimates and, hence, highlight a knowledge gap in our science on tropical peatland methane cycling.

In this context, we set out to measure the net ecosystem methane exchange (NEE-CH₄) from an unmanaged degraded peatland in the east coast of Sumatra, Indonesia. The measurements were conducted using the eddy covariance system, composed of a 3D sonic anemometer coupled with a LI-7700 open-path methane analyzer, above the vegetation canopy at 41 m tall tower for over 4 years period (October 2016-September 2020). Therefore, the measurements incorporated all existing methane sources and sinks within the flux footprint, i.e. soil surface, trunk of living tree, vascular plant, and water surface.

Our measurements indicate that unmanaged degraded tropical peatland emitted 54±12 kg CH₄ ha^-1 year^-1 to the atmosphere. The magnitude of daytime NEE-CH₄ were up to six times larger than those during the nighttime. This cautions that sampling bias (e.g. only daytime measurements) can overestimate the daily NEE-CH₄. The diurnal variation in NEE-CH₄ was correlated with associated changes in the canopy conductance to water vapor. Therefore, it was attributed to the vegetative transport of dissolved methane via transpiration. There was no clear relationship between NEE-CH₄ and soil temperature, while it decreased exponentially with declining groundwater level. Low groundwater level enhances methane oxidation in the upper oxic peat layer. Further, low groundwater level might relocate methane production below the root zone, resulting in insufficient methane in the root zone to be taken and transported 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 methane emissions from a globally important ecosystem, and to better understand how land-use changes affect methane emissions.