Fluvial organic carbon losses from tropical peatland oil palm plantations in Sarawak, Malaysia

Currently, there are only very limited estimates of carbon loss from oil palm plantations (OPPs) on tropical peat, with the aquatic fluxes largely unquantified. This thesis presents an annual estimate of exported dissolved (DOC) and particulate (POC) organic carbon from the drainage waters of four OPPs and nearby stands of tropical peat swamp forest (PSF) in Sarawak, Malaysia, subjected to varying degrees of anthropogenic disturbance. Annual total organic carbon (TOC) fluxes (104 ± 19.3 g C m-2 yr-1) from the OPPs explored here are one third larger than those from intact PSFs (63 g C m-2 yr-1) and comparable to fluxes from degraded PSFs (97 g C m-2 yr-1) as reported in the literature. Forest fluxes measured in this project were of a similar magnitude (71.2 ± 11.0 to 84.5 ± 13.1 g C m-2 yr-1), likely as a result of hydrological disturbance from the adjacent plantation drainage system. Qualitative analysis (14C and spectrophotometric analysis) of DOC derived from the OPP land cover revealed that the majority (> 50 %) originates from aged carbon sources (100 – 499 years, BP) and is both labile and highly oxidised, suggesting loss of carbon from long-term stable storage. Extrapolation of the plantation sub-catchment fluxes indicates that industrial peatland OPPs across Peninsular Malaysia, Borneo and Sumatra contribute a combined TOC flux of 3.2 Tg C yr-1. This represents one third of the regional TOC flux (10.4 Tg C yr-1; PSF intact + degraded + industrial OPPs) and suggests a more than two-fold increase in TOC losses since 1990. Overall, this investigation reinforces the importance of considering alternative fluvial carbon loss pathways when assessing the on-going collapse of tropical peatland carbon stores in response to their continued anthropogenic exploitation.