Impact of land conversion on environmental conditions and methane emissions from a tropical peatland.

Tropical peatlands are significant sources of methane (CH₄), but their contribution to the global CH₄ budget remains poorly quantified due to the lack of long-term, continuous and high-frequency flux measurements. To address this gap, we measured net ecosystem CH₄ exchange (NEE-CH₄) using eddy covariance technique throughout the conversion of a tropical peat swamp forest to an oil palm plantation. This encompassed the periods before, during and after conversion periods from 2014 to 2020, during which substantial environmental shifts were observed. Draining the peatland substantially lowered mean monthly groundwater levels from -20.0 ± 14.2 cm before conversion to -102.3 ± 31.6 cm during conversion and increased slightly to -96.5 ± 19.3 cm after conversion. Forest removal increased mean monthly soil temperature by 2.3 to 3.1 °C, reducing net radiation (R_n) and raising vapor pressure deficit (VPD). Following the tree removal, controlled burning temporarily warmed air temperature by 8 °C, increased VPD and significantly attenuated R_n, resulting in negative values owing to radiation interception by smoke and increased surface warming. Contrary to expectations that drainage would lower CH₄ emissions, the site remained a consistent net source, with even higher emissions observed during and after conversion. The mean monthly NEE-CH₄ during conversion (23.3 ± 8.6 mg C m^-2 d^-1) was about 2-times higher than before conversion (12.1 ± 5.3 mg C m^-2 d^-1) and about 1.5-times higher than after conversion (16.3 ± 4.1 mg C m^-2 d^-1). The heightened CH₄ release is likely attributable to emissions from drainage ditches, underscoring their significant role in post-conversion CH₄ dynamics. Despite its short duration, controlled burning substantially elevated NEE-CH₄, ranging from 0.04 to 0.91 mg C m^-2 s^-1. Our findings highlight the substantial impact of land conversion on peatland CH₄ dynamics, emphasizing the need for accurate flux measurements across various conversion stages to refine global CH₄ budgets.