Multi-pathway methane and nitrous oxide emissions from Acacia plantations on tropical peatlands

Tropical ecosystems are major contributors to global methane (CH₄) emissions, yet substantial uncertainties remain in both top-down and bottom-up estimates. Such uncertainties partly can be attributed to limited understanding of various emissions and uptake pathways in tropical ecosystem. Most field measurement of CH₄ focused solely on soil-atmosphere exchange, overlooking other exchange pathways. Furthermore, several studies from natural forest ecosystem confirmed significant CH₄ emission from tree stems. However, such quantification remains scarce in tropical forest plantations, which constitute significant proportion of current land use. A better quantitative and process-based understanding of CH₄ emissions, removals, and transport pathways is therefore essential for improving regional and global CH₄ budgets and mitigation strategies, especially under changing climate and land use.

In this study, we measured soil and stem CH₄ fluxes from two managed plantation forests (Acacia and Eucalyptus plantations) and two natural forests ecosystem (peat swamp forests and riparian forests) in Sumatra, Indonesia. We used LI-8200-01S (LICOR, USA) for soil and semi-rigid chambers made with polyethylene terephthalate (PET) plastic sheets for stem measurements. We used LI-7810 (LICOR, USA), connected to the chambers during the measurement period to measure the CH₄ concentration. The fluxes were calculated using a linear function of changes in CH₄ concentration during incubation time.

The preliminary result shows that plantation emits significantly smaller CH₄ from both soil and stem compared to respective natural forested ecosystems, indicating that land-use change substantially alter the methane production, consumption, and transport processes. We observed a clear decreasing stem CH₄ fluxes with increasing stem height in both ecosystems on peat, strongly suggest a soil-originated CH₄ transport mechanism. Interestingly, no significant difference between stem height was detected in Eucalyptus plantations and adjacent riparian forests. Tree stems acted as net CH₄ sources across all ecosystems. Soil surfaces functioned as CH₄ sources in peatland ecosystems but as net CH₄ sinks in Eucalyptus plantations and adjacent riparian forests. These results demonstrate strong contrasts in soil–stem CH₄ dynamics between peatland and non-peatland ecosystems in tropics. Comprehensive, pathway-specific assessments are therefore required to reduce uncertainties in tropical CH₄ budgets.