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Anthropogenic activities, and in particular the use of synthetic nitrogen (N) fertilizer, have a significant influence on soil nitrous oxide (N2O) emission from oil palm plantation on tropical peatland. Finding a suitable N rate for optimum N uptake efficiency and yield with low environmental impact and production cost is crucial for the economic growth of Malaysia’s oil palm sector. However, studies on the impact of N fertilizers on N2O emissions from tropical peatland are limited. Thus, long-term monitoring was conducted to investigate the effects of N fertilization on soil N2O emissions. This study was conducted in an oil palm (Elaeis guineensis Jacq.) plantation located in a tropical peatland in Sarawak, Malaysia. Monthly soil N2O fluxes were measured using the closed-chamber method in a control (T1, without N fertilization), and under three different N treatments: low N (T2, 31.1 kg N ha−1), moderate N (recommended rate) (T3, 62.2 kg N ha−1), and high N (T4, 124.3 kg N ha−1), from January 2010 to December 2013 and from January 2016 to December 2017. The only N fertiliser rate to significantly increase (p2O emissions was 124.3 kg N ha-1 (T4). Increased in water-filled pore space (WFPS) (>70%) with a decrease in both N2O flux and nitrate (NO3−) implies that complete denitrification has taken place. Increased in NO3- uptake by oil palm with an increase in WFPS decreased NO3- concentration in soil, resulting in the reduction of N2O emission. This study highlights the importance of WFPS on denitrification and N uptake by oil palm in tropical peatland. This needs to be taken into account for the accurate assessment of N dynamics in oil palm plantations on tropical peatland in order to enhance N fertilization management strategies and counteract anthropogenic activities that produce greenhouse gases.Keywords: WFPS, oil palm yield, NO3-, N uptake

Fires and drainage are common disturbance factors in tropical peatlands (TP) in Southeast Asia. These disturbances alter the hydrology, vegetation composition, and peat biogeochemistry; thereby affecting the microbiome where microbial communities reside. Studies from northern peatlands have well established the role of vegetation composition in regulating the labile C, in the form of plant root exudates, and microbial community composition affecting the peat decomposition; however, for tropics, it remains unexplored. Recent studies have also established how these fire-degraded TP areas become a hot spot of sedge-mediated CH4 emission. To further our understanding of control mechanisms regulating CH4 dynamics, we investigated the composition of plant root exudates (n=3 per plant species) from sedges (Scleria sumatrensis) and ferns (Blechnum indicum, Nephrolepis hirsutula), the most commonly occurring plant species at our fire-degraded tropical peatland site in Brunei, Northwest Borneo, as well as microbial community composition in plant (n=9 for S. sumatrensis, and B. indicum, and n=5 for N. hirsutula) rhizo-compartments (rhizosphere, rhizoplane, endosphere). Using a targeted analysis, we found that the root exudates compounds secreted from sedge (Scleria sumatrensis) and one species of fern (Blechnum indicum) were significantly different (p4 emissions from fire-degraded TP. Our results provide fresh insights into the effects of post-fire vegetation composition in regulating the labile C and microbial community composition, and hence affecting CH4 emissions from fire-degraded TP. Further, our results can form an important basis for future CH4 dynamics studies as the emissions might increase with the expansion of degraded TPs as a consequence of frequent fire episodes and flooding

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.

Transpiration is a key process in the terrestrial ecosystems linking water, carbon, and energy exchanges between the vegetation and the atmosphere. However, the understanding of transpiration rate, its spatiotemporal dynamics, and the controlling factors in tropical peatlands are still constrained by limited measurements. This study aims to investigate the transpiration rates at the stand level of Acacia plantation under different groundwater levels. The measurements were performed at two large-scale lysimeter plots with groundwater level of 40 and 80 cm below the ground surface. The transpiration rate was quantified based on sap flow measurements from 16 trees with different diameters at breast height using heat ratio method. The initial results indicate that the transpiration rate was closely correlated to the meteorological parameters, including atmospheric vapor pressure deficit and solar radiation. The two plots with different groundwater level regimes exhibit the same diurnal pattern of transpiration rate yet shows differences in their magnitude. The findings from this study will improve the understanding about relative contribution of transpiration to the total water balance under different groundwater levels. Further, an ongoing measurement of above and below-ground biomass growth and hydrological modeling work will advance the knowledge on plant-water interaction from this ecosystem.

Changes in hydrological regime associated with land cover change may result in crucial implications to tropical peatland landscape since hydrology strongly controls peatland geomorphology, ecology, and biogeochemical cycle. Therefore, improved understanding of the land cover change impacts to the water balance is of significant importance in order to formulate responsible peatland management strategies. In this study, we investigated the water balance under historical land cover change within Padang Island, Indonesia, an ombrotropic tropical peatland landscape with heterogeneous land covers. For this purpose, we established a model setup using a coupled MIKE SHE and MIKE Hydro River. The model was calibrated and validated against comprehensive data set from field measurements. Land cover change impacts were evaluated by comparing the water balance under current and past condition. The past land cover distribution was derived from historical satellite imagery analysis covering the period of 25 years before the current condition. Meanwhile, the past topography data was generated following long-term subsidence monitoring data. Here, we will present the impacts of land cover change to water balance at the landscape level and their implications for management of tropical peatlands.

Hydrology plays a pivotal role in the geomorphology and carbon balance of tropical peatlands. The alteration of the hydrological processes due to climate and/or land cover change might result in significant impacts to this ecosystem. Therefore, improved understanding of tropical peatland hydrology is critical in order to evaluate their fate under current and future climate and ultimately to develop sustainable peatland management practices. However, due to its complexity related to various flow interactions and anthropogenic interferences, comprehensive hydrological studies based on measured field data on tropical peatlands are still limited. Alternatively, hydrological models have been used to simulate the major components of the hydrological processes and to answer “what-if” questions.In this context, a fully distributed and physically-based MIKE SHE model was used to simulate the water balance within Padang Island in the eastern coast of Sumatra, Indonesia. The island is characterized as a mosaic landscape of natural forest, forest plantation and smallholder agriculture. Comprehensive data set from field measurements including high resolution digital terrain model derived from airborne LiDAR were used for the model development. The model was calibrated and validated against observed groundwater level and stream flow data distributed across the island. The simulation was performed using current climate data that cover a distinct dry and wet year. The subsidence impacts were investigated by simulating the future projection up to 50 years. Further, additional scenario was developed to represent the pre-existing condition without agriculture and forestry practices to evaluate the land cover change impacts.The results show that the water balance is predominantly controlled by climatic variables. The evapotranspiration accounts for the main water loss representing 50 – 80 % of the total annual rainfall. The amount of evapotranspiration remains relatively constant in the temporal basis irrespective to the rainfall, which means that the magnitude and direction of the remaining hydrological flow paths are driven by the balance between rainfall and evapotranspiration. In the dry period with a rainfall deficit, the water storage is depleted in order to meet the evapotranspiration demand. In the wet period, the excess rainfall is transformed into overland flow, base flow and positive storage change which contributes to increased inundation frequency.The future projection indicates that there is a shift in the hydrological flow path, as the overland flow increases and the groundwater flow decreases due to the changing topography from peat subsidence. However, the hydrological flow path of the natural forest in the central part of the island remains relatively intact. The agriculture and forestry practice doesn’t significantly alter the hydrological flow path compared to the pre-existing condition. In addition, the boundary impact to the natural forest is not apparent under the wet period, while it gets more prominent in the dry period (~300 meter under current condition).Our results, which are among the first comprehensive hydrological studies for the tropical peatlands, should help to improve the understanding of landscape scale hydrological processes in tropical peatland, which is relevant for scientists and policymakers to develop science-based peatland management practices.

Over the past few decades, tropical peatlands in Southeast Asia have been heavily degraded for multiple land uses, mainly by employing drainage and fire. More importantly, the extent of these degraded areas, primarily covered with ferns and sedges, have increased to almost 10% of the total peatland area in Southeast Asia. In particular, the role of sedges in plant-mediated gas transport to the atmosphere has been recognized as a significant CH4 pathway in northern peatlands, however, in the Tropics this is still unknown. Within this context, we adopted an integrated approach using on-site measurements (CH4, porewater physicochemical characteristics) with genomics to investigate the role of hydrology, vegetation structure, and microbiome on CH4 emission from fire-degraded tropical peatland in Brunei. We found for the first time that in degraded tropical peatlands of Southeast Asia, sedges transported 70-80% of the total CH4 emission and significantly varied with values ranging from 1.22±0.13 to 6.15±0.57 mg CH4 m-2 hr-1, during dry and wet period, respectively. This variation was mainly attributed to water table position along with changes in sedge cover and porewater properties, which created more optimal methanogenesis conditions. Total emissions via this process might increase in the future as the extent of degraded tropical peatlands expands due to more frequent fire episodes and flooding. Further, we used 16S rRNA high-throughput sequencing to investigate the microbiomes in peat profile (above and below water table) as well as rhizo-compartments (Rhizosphere, Rhizoplane, Endosphere) of sedges. We found that the peat profile as well as rhizo-compartments of sedge harboured a higher number of methanogenic archaea in the order Methanomicrobiales and Methanobacteriales, compared to non-burnt and bulk soil, which further explains our findings of higher CH4 emission from degraded tropical peatland areas covered with sedges. These insights into the impact of fire on hydrology, vegetation structure, and microbial community composition on CH4 emissions provide an important basis for future studies on CH4 dynamics in degraded tropical peatland areas.

Tropical peatlands of Southeast Asia are a globally important carbon reservoir, storing an enormous amount of soil organic carbon as peat. These ecosystems are complex and poorly understood with large unknown biogeochemical processes. Despite the huge carbon stocks in these ecosystems, data on ecosystem-scale carbon dioxide (CO2) and methane (CH4) fluxes are still limited in comparison with mid- and high-latitude peatland ecosystems. The recent increase in the intensity of climate anomaly such as El Niño may alter the hydrological regime of this ecosystem, thus affects its carbon cycling. It is crucial to quantify the CO2 and CH4 fluxes of the ecosystem and understand their responses to environmental changes to predict the role of peat swamp forest in global carbon cycles. To date, the application of the eddy covariance technique to measure the ecosystem-scale CO2 and CH4 fluxes in tropical peatlands is still limited to few studies in Malaysia and Indonesia.In 2010, we established a long-term greenhouse gas fluxes monitoring using the eddy covariance technique over a peat swamp forest in Sarawak, Malaysia. Here, we present the net ecosystem exchange of CO2 (NEE) and CH4 (FCH4) from February 2014 to January 2017 (3 years). We had quantified the NEE and FCH4, the diurnal and seasonal variations of NEE and FCH4, and the response of NEE and FCH4 to GWL. The FCH4 was determined half-hourly as the sum of eddy CH4 flux and CH4 storage change in an air column below the flux measurement height. We had determined the global warming potential of this ecosystem from annual NEE and FCH4 using sustained-flux global warming potential (SGWP). The annual FCH4 was converted into a CO2 equivalent unit using an SGWP factor of 45 which represents the SGWP for CH4 over a timescale of 100 years. Our preliminary result showed that the CH4 emission potentially offset the CO2 sequestration, which was higher than those reported in other regions in the world.

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.

Tropical peatlands have a specific hydrology that regulates their internal processes and functioning. External disturbances such as drainage, land cover and land use changes, and climate change could disrupt the peat-specific hydrology and convert the immense peatland carbon stocks into strong greenhouse gas (GHG) emitting sources. The need for (more) accurate monitoring of GHG emissions has led to the development of complex biogeochemical models, which highly depend on proper representation of peat-specific land surface hydrology. However, the latter is often inadequately accounted for in global Earth system modeling frameworks.In this research, we leverage the PEATCLSM modules recently developed for the Catchment land surface model (CLSM) of the NASA Goddard Earth Observing System framework (Bechtold et al., 2019). These modules were evaluated for northern peatlands, hereafter referred to as PEATCLSMN. Here, we present an extended version of PEATCLSM for tropical peatlands with literature-based parameter sets for natural (PEATCLSMT,Natural) and drained (PEATCLSMT,Drained) tropical peatlands. A suite of modeling experiments was conducted to compare the performance of PEATCLSMT,Natural, PEATCLSMT,Drained, PEATCLSMN, and the currently operational CLSM version that includes peat parameters but no peat-specific model structure (CLSMO). Simulations over major tropical peatland regions in Southeast Asia, the Congo Basin, and South and Central America were evaluated with a comprehensive and self-compiled dataset of groundwater table depth (WTD) and evapotranspiration (ET). Preliminary results show that the simulated WTD from CLSMO exhibits too much temporal variability and large biases, either positive or negative. The temporal correlation coefficient between simulated and observed WTD for both PEATCLSMT,Natural (over undeveloped peatlands only) and PEATCLSMT,Drained (over drained peatlands only) is similar to that of PEATCLSMN. However, both tropical versions reduce the average absolute bias to a few centimeters. Performance differences across the major tropical peatland regions are discussed.Reference: Bechtold, M., De Lannoy, G. J. M., Koster, R. D., Reichle, R. H., Mahanama, S. P., Bleuten, W., et al. (2019). PEAT‐CLSM: A specific treatment of peatland hydrology in the NASA Catchment Land Surface Model. Journal of Advances in Modeling Earth Systems, 11(7), 2130-2162. doi: 10.1029/2018MS001574

Southeast Asia is a region where forest clearance, drainage of peatlands for agriculture, and ongoing use of fire to ‘manage’ land leads to extensive emissions of greenhouse gases to the atmosphere, and significant disturbance to peatland soils. While recent campaigns investigating tropical peatland fire emissions have improved our knowledge and understanding of ‘direct’ greenhouse gas emissions during fires, there remains a significant gap in our knowledge of the immediate post-fire impacts on peat respiration and methanogenesis. Ongoing research shows that peatland microbial communities (responsible for respiration), including methanogens and methanotrophs (responsible for controlling net methane emissions), are considerably altered following fire disturbance. As such, we hypothesise that peatland fires will lead to significant alterations to GHG emissions, compared to sites that have not burned. Further, we also hypothesise that the magnitude of this post-fire effect will be predictably interrelated to different forms of peatland degradation and land-use history.Here we present results from seven fire locations (recently burnt) and their corresponding neighbouring control sites (not recently burnt), three of our fire locations were associated with forest clearance fires, while the other four locations were slash fires on oil palm plantations. We characterize the post-fire disturbance emissions of carbon dioxide (CO2) and methane (CH4) in situ, in the immediate aftermath of a fire (within days or weeks), and in the subsequent months following a fire at our burn sites. For comparison, we also measure CO2 and CH4 emissions from neighbouring control sites that remained unburnt. We find substantial, significant differences in CH4 emissions between the burn sites and control sites for all seven of our measurement locations. We suggest a number of mechanisms responsible for this post-fire effect, including disturbance to the methanotroph microbial communities at the burn sites, as well as reduced elevation at the burn sites, leading to higher water tables.