Coupled abiotic-biotic cycling of nitrous oxide in tropical peatlands

Atmospheric nitrous oxide (N2O) is a potent greenhouse gas thought to be mainly derived from microbial metabolism as part of the denitrification pathway. Here, we report that in unexplored peat soils of Central and South America, N2O production can be driven by abiotic reactions (> 98 %) highly competitive to their enzymatic counterparts. Extracted soil iron positively correlated with in-situ abiotic N2O production determined by isotopic tracers. Moreover, we found that microbial N2O reduction accompanied abiotic production, essentially closing a coupled abiotic-biotic N2O cycle. Anaerobic N2O consumption occurred ubiquitously (pH 6.4-3.7), with proportions of diverse clade II N2O-reducers increasing with consumption rates. Our findings show denitrification in tropical peat soils is not a purely biological process, but rather a 'mosaic' of abiotic and biotic reduction reactions. We predict hydrological and temperature fluctuations differentially affect abiotic and biotic drivers and further contribute to the high N2O flux variation in the region.

Seasonal litter contribution to total peat respiration from drained tropical peat under mature oil palm plantation

The amount of CO2 gas emissions in drained peatland for oil palm cultivation has been widely reported. However, the research addressing the contribution of litter respiration to peat and total respiration and its relationship with several environmental factors is found rare. The aim of this study was to measure peat and heterogeneous litter respiration of drained tropical peat in one year at a distance of 2.25 m and 4.50 m from mature oil palm trees of 14 years using the chamber method (Licor Li-830). In addition to CO2 efflux, we measured other environmental parameters, including peat temperature (10 cm depth), air temperature, groundwater table (GWL), and rainfall. Results showed that the mean total peat respiration (Rt) was 12.06 g CO2 m-2day-1, which consisted of 68% (8.24 g CO2 m-2day-1) peat (Rp) and root (Rr) respiration and 32% (3.84 g CO2 m-2day-1) of litter respiration (Rl) at the distance of 2.25 m from the palm tree. Meanwhile, at a farther distance, the Rt was 12.49 g CO2m-2day-1, the contribution of Rp was 56% (6.78 g CO2 m-2day-1), and Rl was higher than the closest distance (46%; 5.71 g CO2 m-2day-1). Thus, one-year observation resulting the mean Rt and Rr was 0.07–0.08 Mg CO2 ha-1 day-1, while Rl was 0.04–0.06 Mg CO2 ha-1 day-1. The means of Rt, Rp, and Rl were significantly different in the dry season than those recorded in the rainy season. The climatic-related variable such as peat and air temperature were chiefly governing respiration in peat under mature oil palm plantation, whereas the importance of other variables present at particular conditions. This paper provides valuable information concerning respiration in peat, especially for litter contribution and its relationship with environmental factors in peatland, contributing to global CO2 emission.

The Potency of Cratoxylum arborescens Blume (Geronggang) and Combrecarpus rotundatus Dans (Tumih) as Natural Regeneration in Degraded Tropical Peat Swamp Forest

The massive forest fire disasters have left an enormous area of ​​degraded peatland. This study aims to analyze the performance of two species, namely C. arborescens and C. rotundatus, as the natural regeneration post forest fires. This research was conducted in 5 different locations that experienced severe fires in 2006. We made a total of 25 plots for each location to measure biodiversity at four growth levels. We analyzed the data with vegetation analysis formulas from Magurran. The results show that at the tree growth level, C. rotundatus can withstand the fires in 2006 and is currently still growing in more significant numbers than C. arborescens. At the pole, sapling, and seedling growth levels, these species perform well as natural regeneration species with many individuals, but C. arborescens is a bit more dominant. Both species are suitable for natural regeneration after fires in degraded peat swamp forests based on survived and existing individuals. On the other hand, both species could not improve the vegetation diversity in the whole ecosystem. These two species can be the option for natural regeneration if there a limited budget and the degraded areas are in a very remote location.

El Niño-like conditions and seasonal aridity in the Indo-Pacific Warm Pool during the Younger Dryas

Island South-East Asia (ISEA) is a highly humid region and hosts the world’s largest tropical peat deposits. Most of this peat accumulated relatively recently during the Holocene, suggesting a generally drier and/or more seasonal climate during earlier times. Although there is evidence for savanna expansion and drier conditions during the Last Glacial Maximum (LGM, 21 ka BP), the mechanisms behind hydroclimatic changes during the ensuing deglacial period has received much less attention and are poorly understood. Here we use CESM1 climate model simulations to investigate the key drivers behind ISEA climate at the very end of the last deglacial period, at 12 ka BP. A transient simulation (TRACE) is used to track the climate seasonality and orbitally driven change over time during the deglaciation into the Holocene. In agreement with proxy-evidence, CESM1 simulates overall drier conditions at 12 ka BP. More importantly, ISEA experienced extreme seasonal aridity, in stark contrast to the ever-wet modern climate. We identify that the simulated drying and enhanced seasonality at 12 ka BP is mainly the result of a combination of three factors: 1) large orbital insolation difference between summer and winter in contrast to the LGM and the present day; 2) a stronger winter monsoon caused by a larger interhemispheric thermal gradient in boreal winters; and 3) a major reorganization of the Walker Circulation with an inverted land-sea circulation with a complete breakdown of deep convection over ISEA. The altered atmospheric circulation mean state during winters led to conditions resembling extreme El Niño events in the modern climate and a dissolution of the Inter-Tropical Convergence Zone (ITCZ) over the region. From these results we infer that terrestrial cooling of ISEA and at least a seasonal reversal of land-sea circulation likely played a major role in delaying tropical peat formation until at least the onset of the Holocene period.

Thermal Imaging Study on The Effect of Permeability on Smouldering Behaviour of a Tropical Peat

Smouldering is a slow-burning, low-temperature, flameless combustion, and frequently happens in peatland fires. The smouldering spread occurs because of the parameter achievement in oxygen supply, generated heat, and heat released to the environment. The condition of porous and fibrous peat soils makes oxygen supply easily happens. The difficulty of getting to the location of the burning peatland is one of the problems to extinguish the fire. This study aims to observe with thermal imaging study the effect of peat permeability on smouldering behaviour of a tropical peat sample. Mechanical compaction was applied to reduce permeability and pore value in the central of the peat soil. Then, peat soil is ignited to create the smouldering propagation through the compacted peat area. The combustion process that occurs on the surface is observed by a visual camera and an Infrared FLIR Thermal Camera. The initial results showed a reduction in the smouldering spread rate on the compacted soil region as compared to the undisturbed peat smouldering region. Nevertheless, smouldering combustion of peat still occurred in all regions of the reactor, once the smouldering front could penetrate the compacted region.

Assessing Impact of Multiple Fires on a Tropical Peat Swamp Forest Using High and Very High-Resolution Satellite Images

Tropical peat swamp forests, found mainly in Southeast Asia, have been threatened by recurring El Niño fires. Repeated burnings form a complex and heterogeneous landscape comprising a mosaic of burned patches of different fire frequencies, requiring fine-scale assessment to understand their impact. We examined the impact of the El Niño fires of 1998 and 2003 on a tropical peat swamp forest in northern Borneo, with the combined use of high and very high-resolution satellite images. Object-based and pixel-based classifications were compared to classify a QuickBird image. Burned patches of different fire frequencies were derived based on unsupervised classification of the principal components of multitemporal Normalized Difference Water Index (NDWI) data. The results show that the object-based classification was more accurate than the pixel-based classification for generating a detailed land cover map. Fire frequency had a severe impact on the number of burned patches and the residual forest cover. Larger patch area retained more residual forest cover for the burned patches. Forest structure of burned-twice patches was more severely altered compared to burned-once patches. Two burned-once patches had a relatively promising recovery potential by natural regeneration due to higher residual forest cover, a vast number of large trees, and aboveground biomass. Except for the largest patch, rehabilitation seemed inevitable for burned-twice patches. This approach can be applied to assess the impact of multiple fires on other forest types for better post-fire forest management.