Importance of CO2 production in subsoil layers of drained tropical peatland under mature oil palm plantation

Expansion of oil palm (Elaeis guineensis Jacq.) plantations in tropical peatlands is always followed by a lowering groundwater level, which exposes peat to oxidic condition and releases large amounts of carbon dioxide (CO2) to atmosphere. This study is concerned with decomposition of organic matter in the soil profile of tropical peatland under oil palm plantation. For 2 years, CO2 fluxes were measured from the soil surface (CO2-efflux) and from the surface where peat was removed to the depths of 10, 30 and 50 cm (CO2 efflux potential/CO2-EP). CO2-EP is a potential estimation of organic matter decomposition from subsoil. Groundwater level, soil moisture and soil temperature were measured as soil environmental factors. The annual CO2 efflux was 139 mg m–2 h–1 while CO2-EP increased with depth which were 1032, 1209, 2304 mg m–2 h–1 from 10, 30, 50 cm depths, respectively. CO2 efflux did not significantly correlated with soil environmental factors. CO2-EP had a significant negative correlation with groundwater level and soil moisture at depths of 30 cm and 50 cm. There are significant correlations among CO2 released from 0, 10, 30, and 50 cm depths. These results indicated that the formation of CO2 in the deeper peat contributes greatly to the emission of CO2, and that keeping the groundwater level at a high level will greatly contribute to the suppression of CO2 emissions from peatland. The subsoil of cultivated peatland is considered to be vulnerable, and its exploitation should be avoided.