A seven-year study in Sarawak found soil CO₂ emissions peaked during the first three years of oil palm cultivation on peatland before declining in the fourth and fifth years.
PALMOILMAGAZINE, KUCHING — Soil carbon dioxide (CO₂) emissions rise significantly during the first three years after peatland is converted into an oil palm plantation, before declining in the fourth and fifth years as soil conditions, water-table management, and canopy development change.
The findings come from a study titled “Evaluating soil CO₂ fluxes during the transition from peat swamp forest to an oil palm plantation,” conducted by Nur Azima Busman and researchers from the Sarawak Tropical Peat Research Institute, Malaysia, in collaboration with researchers from Nagoya University, Japan. The study was published in the journal Science of the Total Environment and accepted on August 10, 2026.
According to the study, soil CO₂ emissions were measured monthly for more than seven years, from January 2016 to April 2023, at a site in Betong Division, Sarawak, Malaysia. The researchers monitored three stages of land-use change: peat swamp forest, land preparation, and oil palm plantation.
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Emissions Peak During the Early Plantation Years
The study found that soil CO₂ emissions in the peat swamp forest ranged from 1,402 to 1,430 grams of carbon per square meter per year (g C/m²/year).
During the land-preparation stage, emissions increased to approximately 1,768–2,135 g C/m²/year. However, the increase was not statistically significant compared with the peat swamp forest stage.
A more pronounced increase occurred after oil palm was planted. During the first three years of plantation development, soil CO₂ emissions rose to 2,606–3,119 g C/m²/year.
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Emissions subsequently declined in the fourth and fifth years, falling to approximately 1,858–2,348 g C/m²/year.
The highest emissions were recorded during the first year. Site A reached 3,119 g C/m²/year, while Site B recorded 2,738 g C/m²/year. Emissions remained relatively high during the second and third years before declining in the fourth and fifth years.
The researchers suggested that the high emissions during the early plantation stage were associated with increased heterotrophic respiration. Fertilizer application may stimulate microbial activity and the decomposition of soil organic matter, while residual root biomass from the cleared forest may provide an additional source of CO₂.
However, the study noted that woody residues piled along certain plantation pathways were not directly measured. Therefore, the recorded emissions primarily represent emissions from areas surrounding the palms, or palm circles, rather than the entire plantation area.
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Groundwater Levels Initially Declined
Land conversion also altered the hydrological conditions of the peatland. Drainage lowered the water table from a level close to the soil surface under peat swamp forest conditions to approximately 78 centimeters below the surface during land preparation.
During the first year after oil palm planting, the water table fell further, reaching approximately 98 cm below the surface at Site A and 90 cm at Site B.
By the fifth year, however, the water table had partially recovered, rising to around 49 cm below the surface at Site A and 52 cm at Site B.
These changes occurred alongside shifts in soil temperature. Soil temperature increased from around 26°C in the peat swamp forest to approximately 29.2°C during the second plantation year, before declining to around 28.2°C in the fifth year.
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Why Did Emissions Decline?
The researchers attributed the decline in CO₂ emissions during the fourth and fifth years to several changes within the peatland ecosystem.
One indicator was an increase in the pyrophosphate solubility index (PSI), suggesting more advanced humification and a growing proportion of recalcitrant carbon that is more resistant to decomposition.
As more readily decomposable organic compounds are broken down over time, the proportion of more resistant carbon increases. This can reduce the availability of substrates for microorganisms to generate CO₂ through respiration.
Improved water-table management and the development of an oil palm canopy, which provides greater shade, were also considered potential contributors to the decline in emissions during the later stages.
The study further found that during the plantation phase, CO₂ flux had a negative relationship with water-filled pore space (WFPS) and soil bulk density.
Higher WFPS can restrict oxygen diffusion, while increasing soil bulk density can reduce porosity and limit oxygen penetration into the upper peat layer. Both factors can influence microbial activity and the decomposition processes responsible for CO₂ emissions.
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Importance of Protecting Peat Swamp Forests
The researchers described the study as the first to monitor soil CO₂ fluxes across the full sequence of tropical peatland conversion from peat swamp forest to oil palm plantation at a single location.
Because the study site was previously a secondary peat swamp forest, the researchers cautioned that increases in emissions associated with land-use change could potentially be greater if conversion were to occur in primary peat swamp forests.
The findings therefore reinforce the importance of conserving remaining peat swamp forests to prevent further carbon losses.
Although emissions showed a declining trend after the fourth year, the researchers said it remains uncertain whether the decline will continue as the oil palms mature.
Longer-term monitoring beyond the fifth year, along with measurements across different plantation management zones, is needed to provide a more representative assessment of CO₂ emissions at the plantation scale. (P2)
