Carbon dioxide dynamics across three stages of tropical peatland conversion to oil palm plantations

• Tropical peat swamp forest (PSF) changed from carbon (C) neutral to C source following conversion. • CO 2 emissions rose mainly due to a significant GPP reduction, with a small R eco reduction. • PSF conversion raised both soil and below-canopy air temperature. • Increased temperatures can accelerate peat decomposition and alter ecosystem C cycling and water balance. This study represents the first long-term investigation spanning from a tropical peat swamp forest (PSF) to its conversion into an oil palm plantation (OPP), offering valuable data for assessing carbon dioxide (CO 2 ) dynamics across different conversion stages. The conversion of tropical peat swamp forests to oil palm plantations has significant implications for CO 2 dynamics. However, ecosystem-scale studies investigating CO 2 dynamics across different stages of land conversion are lacking. This study used the eddy covariance (EC) technique to measure the net ecosystem exchange (NEE) of CO 2 above a tropical peat swamp forest in Sarawak, Malaysia, from 2011 until it was cleared in 2017 and ultimately converted into an OPP in 2018. Our study found that the removal of forest biomass during land preparation led to a substantial increase in annual NEE from 25 ± 179 (2011 to 2016) to 2732 ± 655 g C m −2 year −1 (2017 to 2019). This increase was attributed to an 83 % reduction in gross primary productivity (GPP) and a 14 % reduction in ecosystem respiration ( R eco ). The near-ground environmental conditions also significantly changed across the conversion stages, inducing drier conditions compared to the forest. These changes were found to affect the controlling factors of nighttime NEE during conversion, resulting in a negative relationship with both air temperature and vapor pressure deficit above canopy, in contrast to the typical relationship with groundwater level observed before conversion. The conversion is also found to cause significant reduction in overall ecosystem photosynthetic activity as evidenced by the reduction in maximum gross photosynthetic rate ( P max ), photosynthetic photon flux density (PPFD), quantum yeild (α), and dark respiration ( RE d ). Although ecosystem-scale assessments of CO 2 dynamics provide insights into how ecosystems respond to changes in relation to land conversion, it is crucial to assess other respiration components, such as soil respiration and aboveground woody debris, for a more comprehensive analysis.