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Monthly measurements of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) fluxes in peat soils were carried out and compared with groundwater level over a year at four sites (drained forest, upland cassava,upland and lowland paddy fields) located in Jambi province, Indonesia. Fluxes from swamp forest soils were also measured once per year as the native state of this investigated area. Land-use change from drained forest to lowland paddy field significantly decreased the CO2 (from 266 to 30 mg C m-2 h-1) and N2O fluxes (from 25.4 to 3.8 microgram N m-2 h-1), but increased the CH4 flux (from 0.1 to 4.2 mg C m-2 h-1) in the soils. Change from drained forest to cassava field significantly increased N2O flux (from 25.4 to 62.2 microgram N m-2 h-1), but had no significant influence on CO2 (from 266 to 200 mg C m-2 h-1) and CH4 fluxes (from 0.1 to 0.3 mg C m-2 h-1) in the soils. Averaged CO2 fluxes in the swamp forests (94 mg C m-2 h-1) were estimated to be one-third of that in the drained forest. Groundwater levels of drained forest and upland crop fields had been lowered by drainage ditches while swamp forest and lowland paddy field were flooded, although groundwater levels were also affected by precipitation. Groundwater levels were negatively related to CO2 flux but positively related to CH4 flux at all investigation sites. The peak of the N2O flux was observed at -20 cm of groundwater level. Lowering the groundwater level by 10 cm from the soil surface resulted in a 50 increase in CO2 emission (from 109.1 to 162.4 mg C m-2 h-1) and a 25% decrease in CH4 emission (from 0.440 to 0.325 mg C m-2 h-1) in this study. These results suggest that lowering of groundwater level by the drainage ditches in the peat lands contributes to global warming and devastation of fields. Swamp forest was probably the best land-use management in peat lands to suppress the carbon loss and greenhouse gas emission. Lowland paddy field was a better agricultural system in the peat lands in terms of C sequestration and greenhouse gas emission. Carbon loss from lowland paddy field was one-eighth of that of the other upland crop systems, although the Global Warming Potential was almost the same level as that of the other upland crop systems because of CH4 emission through rice plants.
