Tropical peatlands hold about 15%-19% of the global peat carbon (C) pool of which 77% is stored in the peat swamp forests (PSFs) of Southeast Asia. Nonetheless, these PSFs have been drained, exploited for timber and land for agriculture, leading to frequent fires in the region. The physico-chemical characteristics of peat, as well as the hydrology of PSFs are affected after a fire, during which the ecosystem can act as a C source for decades, as C emissions to the atmosphere exceed photosynthesis. In this work, we studied the longer-term impact of fires on C cycling in tropical PSFs, hence we quantified the magnitude and patterns of C loss (CO 2 , CH 4 and dissolved organic carbon) and soil-water quality characteristics in an intact and a degraded burnt PSF in Brunei Darussalam affected by seven fires over the last 40 years. We used natural tracers such as 14 C to investigate the age and sources of C contributing to ecosystem respiration (R eco) and CH 4 , while we continuously monitored soil temperature and water table (WT) level from June 2017 to January 2019. Our results showed a major difference in the physico-chemical parameters, which in turn affected C dynamics, especially CH 4. Methane effluxes were higher in fire-affected areas (7.8 ± 2.2 mg CH 4 m −2 hr −1) compared to the intact PSF (4.0 ± 2.0 mg CH 4 m −2 hr −1) due to prolonged higher WT and more optimal methanogenesis conditions. On the other hand, we did not find significant differences in R eco between burnt (432 ± 83 mg CO 2 m −2 hr −1) and intact PSF (359 ± 76 mg CO 2 m −2 hr −1). Radiocarbon analysis showed overall no significant difference between intact and burnt PSF with a modern signature for both CO 2 and CH 4 fluxes implying a microbial preference for the more labile C fraction in the peat matrix.
<p>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 CH<sub>4</sub> pathway in northern peatlands, however, in the Tropics this is still unknown. Within this context, we adopted an integrated approach using on-site measurements (CH<sub>4</sub>, porewater physicochemical characteristics) with genomics to investigate the role of hydrology, vegetation structure, and microbiome on CH<sub>4</sub> emission from fire-degraded tropical peatland in Brunei.</p><p>&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; We found for the first time that in degraded tropical peatlands of Southeast Asia, sedges transported 70-80% of the total CH<sub>4</sub> emission and significantly varied with values ranging from 1.22&#177;0.13 to 6.15&#177;0.57 mg CH<sub>4</sub> m<sup>-2</sup> hr<sup>-1</sup>, 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.</p><p>&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; 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 CH<sub>4</sub> 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 CH<sub>4</sub> emissions provide an important basis for future studies on CH<sub>4</sub> dynamics in degraded tropical peatland areas.</p>
<p>Despite being an important terrestrial carbon (C) reserve, tropical peatlands (TP) have been heavily degraded through extensive drainage and fire, to an extent where degraded TP occupies one-tenth of the total peatland area in Southeast Asia (as in 2015). Consequently, repeated fires along with frequent flooding can alter the microtopography, vegetation composition as well as higher diurnal temperature variation due to open canopy, where each is known to influence C dynamics. However, assessing the importance of all these variables on-site can be challenging due to difficult site conditions; hence an incubation experiment approach may provide more useful insights in disentangling the complex interplay of these important variables in regulating GHG (CO<sub>2</sub> and CH<sub>4</sub>) production and emissions from fire-degraded tropical peatland areas. Therefore, we conducted an incubation study to investigate the interactions of microtopography (creating water-saturation conditions: mesic, flooded oxic, and anoxic), labile C inputs (in form of root exudate secretion from ferns and sedges), as well as on-site diurnal temperature variation in regulating CO<sub>2</sub> and CH<sub>4</sub> production from fire-degraded tropical peat.</p><p>&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; We found that CO<sub>2</sub> and CH<sub>4</sub> production significantly varied among treatments and were strongly regulated by microtopography, labile C inputs, and temperature variation. Mesic (oxic) treatments acted as a strong source of CO<sub>2</sub> (230.4 &#177; 29 &#181;gCO<sub>2 </sub>g<sup>-1 </sup>hr<sup>-1</sup>) and mild sink for CH<sub>4</sub> (-5.6 &#177; 0.2 ngCH<sub>4 </sub>g<sup>-1 </sup>hr<sup>-1</sup>) compared to anoxic treatments acting as a mild source of CO<sub>2</sub> (61.3 &#177; 6.2 &#181;gCO<sub>2 </sub>g<sup>-1 </sup>hr<sup>-1</sup>) and strong source of CH<sub>4 </sub>(591.9 &#177; 112.1 ngCH<sub>4 </sub>g<sup>-1 </sup>hr<sup>-1</sup>). The addition of labile C enhanced both the CO<sub>2</sub> and CH<sub>4</sub> production irrespective of the treatment conditions, whereas the effect of diurnal temperature variation was clearly pronounced in mesic (for CO<sub>2</sub>) and anoxic (for CH<sub>4</sub>) conditions. Q<sub>10</sub> values for both CO<sub>2</sub> and CH<sub>4</sub> production varied significantly with higher values for CO<sub>2</sub> in mesic treatments (1.21 &#177; 0.28) and higher for CH<sub>4</sub> in anoxic treatments (1.56 &#177; 0.35). We also observed a gradient across conditions, where flooded oxic treatments showed in-between values both for CO<sub>2</sub> and CH<sub>4</sub> production and temperature sensitivity, further reflecting the importance of on-site peat water-saturation in regulating the GHG production and emission from the fire degraded tropical peatland areas.</p><p>&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; Overall, these findings highlight how the water-saturation conditions due to microtopographic variation in peat surface, quality, and quantity of labile C secreted from plant communities and temperature variation during a day can influence the GHGs production rates from the fire degraded tropical peat. More importantly, given the current state and extent of degraded tropical peatland areas and future climate and land-use changes as well as frequent fire episodes in the region, our results demonstrate the increasing trend in GHG production from the fire-degraded tropical peatlands in Southeast Asia.</p>
<p>Tropical peat swamp forests hold about 15&#8211;19% of the global organic carbon (C) pool of which 77% is found in Southeast Asia. Nonetheless, these ecosystems have been drained, exploited for timber and land for agriculture, leading to frequent fires in the region. Fire alters the physico-chemical characteristics of peat as well as the hydrology, which may convert these ecosystems into a source of C for decades as C emissions to the atmosphere exceeds photosynthesis.</p><p>To understand the long-term impacts of fire on C cycling, we investigated C emissions in intact and degraded PSFs in Brunei Darussalam, which has experienced 7 fires over the last 40 years. We quantified the magnitude and patterns of C loss (CO<sub>2</sub>, CH<sub>4, </sub>and Dissolved Organic carbon) and soil-water quality characteristics along with continuous monitoring of soil temperature and water table level from June 2017 to January 2019. To investigate the age and sources of C contributing to ecosystem respiration (R<sub>eco</sub>) and CH<sub>4</sub>, we used natural tracers such as <sup>14</sup>C.</p><p>We observed a major difference in the physico-chemical parameters, which in turn affected C dynamics, especially CH<sub>4</sub>. In burnt areas (7.8&#177;2.2 mg CH<sub>4 </sub>m<sup>-2</sup> hr<sup>-1</sup>) the CH<sub>4</sub> emission was approximately twice compared to the intact peat swamp forest (4.0&#177;2.0 mg CH<sub>4 </sub>m<sup>-2</sup> hr<sup>-1</sup>) due to prolonged higher water table creating optimum methanogenesis conditions. On the contrary, R<sub>eco</sub> did not show a significant difference between burnt (432&#177;83 mg CO<sub>2 </sub>m<sup>-2</sup> hr<sup>-1</sup>) and intact areas (359&#177;76 mg CO<sub>2 </sub>m<sup>-2</sup> hr<sup>-1</sup>). Further, radiocarbon (<sup>14</sup>C) analysis showed an overall modern signature for both CO<sub>2 </sub>and CH<sub>4</sub> fluxes implying a microbial preference for the more labile C fraction in solution.</p><p>With frequent fires and more flooding in the future, these degraded tropical peat swamp forests areas may remain a hot spot of C emissions as suggested by our findings.</p>
<p>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<strong>. </strong>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 CH<sub>4</sub> emission. To further our understanding of control mechanisms regulating CH<sub>4</sub> 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).</p><p>&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; 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 (p<0.05) and showed a similar ratio of 2:1 for sugars (glucose, fructose) and organic acids (acetate, formate, lactate, malate, oxalate, succinate, tartrate), which is in contrast to that secreted from trees in intact tropical peatlands (1:2). Further, using 16S rRNA gene amplicon sequencing, we found that the microbial community composition in rhizo-compartments of plant species showed significant differences (p<0.001). Interestingly, the sedge species harboured a relatively higher abundance of methanogens (Thermoplasmata) and lesser methanotrophs (Alphaproteobacteria, Gammaproteobacteria) across all three compartments compared to fern species, which further supports the higher sedge-mediated CH<sub>4</sub> emissions from fire-degraded TP.</p><p>&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; 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 CH<sub>4</sub> emissions from fire-degraded TP. Further, our results can form an important basis for future CH<sub>4</sub> dynamics studies as the emissions might increase with the expansion of degraded TPs as a consequence of frequent fire episodes and flooding</p>
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