Eddy Covariance Measurements of Methane Flux at a Tropical Peat Forest in Sarawak, Malaysian Borneo

Abstract

Tropical biogenic sources are a likely cause of the recent increase in global atmospheric methane concentration. To improve our understanding of tropical methane sources, we used the eddy covariance technique to measure CH 4 flux (F CH4 ) between a tropical peat forest ecosystem and the atmosphere in Malaysian Borneo over a 2-month period during the wet season. Mean daily F CH4 during the measurement period, on the order of 0.024 g C-CH 4 ·m À2 ·day À1 , was similar to eddy covariance F CH4 measurements from tropical rice agroecosystems and boreal fen ecosystems. A linear modeling analysis demonstrated that air temperature (T air ) was critical for modeling F CH4 before the water table breached the surface and that water table alone explained some 20% of observed F CH4 variability once standing water emerged. Future research should measure F CH4 on an annual basis from multiple tropical ecosystems to better constrain tropical biogenic methane sources.Plain Language Summary Methane (CH 4 ) is the third most potent greenhouse gas, and its reduction is seen as an effective method for meeting global temperature targets, but the global growth rate of atmospheric CH 4 concentration has risen to 10.3 ± 2.1 ppb/year from 2014 to 2016 after a period of relative stagnation from 2000 to 2006. Recent research has pointed to tropical biogenic sources as a likely cause. However, no studies to our knowledge have measured whole-ecosystem CH 4 flux (F CH4 ) from a tropical peat forested wetland to date despite the importance of tropical wetlands to global CH 4 budget. To improve our understanding of tropical methane sources, we measured F CH4 between a tropical peat forest ecosystem in Malaysian Borneo and the atmosphere over a 2-month period during the dry to wet season transition. Mean daily F CH4 during the measurement period, on the order of 0.024 g C-CH 4 ·m À2 ·day À1 , are similar to eddy covariance measurements from tropical rice agroecosystems and boreal fen ecosystems. A linear modeling analysis demonstrated the important role of air temperature (T air ) during unsaturated conditions and water table during saturated conditions and further emphasizes the critical role of simulating temperature and water table accurately for accurate modeled ecosystem scale F CH4 estimates.

Nitrous oxide (N<sub>2</sub>O) and methane (CH<sub>4</sub>) in rivers and estuaries of northwestern Borneo

“…The corresponding CH 4 flux densities are higher than the CH 4 flux densities reported for the Lupar and Saribas Rivers but much lower than the flux densities from drainage canals in Central Kalimantan and Sumatra (Table 4). Our CH 4 flux densities are, however, 330 comparable to recently published CH 4 eddy covariance measurements (Tang et al, 2018) in the Maludam National Park, which is drained by the Maludam River, and measurements of the CH 4 release from peat soils when the water table is high and CH 4 from rice paddies (Couwenberg et al, 2010), see Table 4. The mean annual N 2 O and CH 4 emissions for the individual rivers were calculated by multiplying the mean flux density, F, for each river (Table 4) with the river surface area given in 335 Table 2.…”

Impact of forest plantation on methane emissions from tropical peatland

“…When the root zone is inundated, changes in biological processes in vegetation driven by solar energy input might be the most important factors controlling diurnal variation in measured NEE-CH 4 (Figure 5a-d), as reported in northern peatlands (Chanton, Whiting, Happell, & Gerard, 1993;Garnet, Megonigal, Litchfield, & Taylor, 2005;Kim et al, 1998;Long et al, 2010;Whiting & Chanton, 1996) and recently reported over tropical peatland (Tang et al, 2018) and flooded forest (Dalmagro et al, 2019). At the natural forest, the observed positive correlation between NEE-CH 4 and canopy conductance to water vapor suggests that CH 4 could be dissolved in the water, absorbed by the roots, transported with sap flow, and emitted through the stem by effervescence (Garnet et al, 2005;Nisbet et al, 2009).…”

“…Our annual NEE-CH 4 over the natural forest are in the same range as those measured using the eddy covariance technique above a tropical peatland in the presence of CH 4 -transporting trees (10.0-14.4 g m −2 year −1 ; Tang et al, 2018;Wong et al, 2018). In the absence of CH 4 -transporting trees, a study in a tropical peatland reported no significant diurnal pattern in NEE-CH 4 (Sakabe et al, 2018) and far lower annual CH 4 emissions (0.12-0.23 g m −2 year −1 ), despite similar GWLs to our forest site.…”

“…We applied two gap-filling approaches (a) mean diurnal course (MDC; Dengel, Levy, Grace, Jones, & Skiba, 2011;Sakabe et al, 2018;Wong et al, 2018) and (b) marginal distribution sampling (MDS; Alberto et al, 2014;Dalmagro et al, 2019;Tang et al, 2018) using the REddyProc package (Wutzler et al, 2018). The MDC is a simple interpolation technique where the missing value is replaced with the averaged value of the adjacent days at exactly that time of day (Falge et al, 2001).…”

“…Knowledge of the magnitude of CH 4 exchanges including all existing sources and removals in tropical peatland ecosystems is limited (Pangala et al, 2013;Sakabe, Itoh, Hirano, & Kusin, 2018;Tang et al, 2018;Wong et al, 2018). Many process-based CH 4 models lack sufficient details in their treatment and parameterization of transport pathways to derive reliable emissions estimates (Gedney, Huntingford, Comyn-Platt, & Wiltshire, 2019;Parker et al, 2018).…”