Post-fire carbon emissions from degraded tropical peat swamp forests in Brunei

Tropical peat swamp forests hold about 15–19% 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.

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 (CO₂, CH_4, 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_eco) and CH₄, we used natural tracers such as ¹⁴C.

We observed a major difference in the physico-chemical parameters, which in turn affected C dynamics, especially CH₄. In burnt areas (7.8±2.2 mg CH₄ m^-2 hr^-1) the CH₄ emission was approximately twice compared to the intact peat swamp forest (4.0±2.0 mg CH₄ m^-2 hr^-1) due to prolonged higher water table creating optimum methanogenesis conditions. On the contrary, R_eco did not show a significant difference between burnt (432±83 mg CO₂ m^-2 hr^-1) and intact areas (359±76 mg CO₂ m^-2 hr^-1). Further, radiocarbon (¹⁴C) analysis showed an overall modern signature for both CO₂ and CH₄ fluxes implying a microbial preference for the more labile C fraction in solution.

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.