Forest conversion to oil palm and rubber plantations is a common land-use change in Jambi, Sumatra due to the high economic demand of forest border communities. The environmental effects of such conversions have raised global concerns due to the potential to increase nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2) emissions to the atmosphere. To quantify this effect, we conducted a series of monthly N2O, CH4 and CO2 flux measurements between July 2010 and August 2011 using static (for N2O and CH4) and dynamic (for CO2) chamber method in an undisturbed forest, a disturbed forest, a one year old rubber plantation, a twenty year old rubber plantation and an eight year old oil palm plantation. All of the plantations are managed by smallholder farmers and have never been fertilised. In order to understand the effect of management intensification on N2O and CH4 fluxes, we applied nitrogen (N) as urea (33.3 kg N ha-1) in the oil palm plantation in April 2011 and monitored the emissions intensively until 28 days after fertilizer application. Nitrous oxide consumption was important in these weathered soils accounting for 32 and 49% of recorded data during monthly and intensive sampling, respectively. The annual fluxes of N2O emission rates amounted to 1.73 ± 0.48, 1.22 ± 0.27, 1.34 ± 0.36, 1.02 ± 0.27 and 1.04 ± 0.39 kg N ha-1 y-1, for CH4 were -1.43 ± 0.99, 0.43 ± 0.89, -1.73 ± 0.66, -0.24 ± 0.34, 0.24 ± 0.73 kg ha-1 y-1 CH4-C and for CO2 were 13.90 ± 1.20, 13.09 ± 1.37, 15.92 ± 1.74, 14.06 ± 1.02, 17.14 ± 1.86 Mg ha-1 y-1 CO2-C for undisturbed forest, disturbed forest, one year rubber plantation, twenty year rubber plantation and eight year oil palm plantation, respectively. Forest disturbance and conversion to rubber and oil palm plantation did not significantly affect annual N2O, CH4 and CO2 emission rates. However in the oil palm plantation, the amount of N emitted as N2O was high (3.1 ± 1.2% of the fertilizer N applied), so at a typical fertilizer application rate of 141 kg N ha-1 y-1, annual emissions would have amounted to 4.4 ± 1.6 kg N ha-1 y-1, more than twice the emission rate in the undisturbed forest. Dry mass and nitrogen mass in standing litter, distance to the nearest termite nest, rainfall on the day of measurement and air temperature were the key factors that predicted annual N2O fluxes across the land-use change transitions. On the other hand, the annual CH4 fluxes strongly related to the percentage of clay and the annual CO2 fluxes tend to relate with the decrease of soil C:N ratio and termite nest population. This work underlines that forest conversion smallholder rubber and oil palm plantations with no or low N fertiliser application does not lead to increase of N2O, CH4 and CO2 emissions. In fertilized oil palm plantations soil N2O emissions can be controlled through appropriate management of N inputs however studies monitoring concomitantly greenhouse gases emissions and palm oil production are still critically lacking.
