Current studies also indicate that the transformation of an intact peat swamp area to oil palm plantations leads to a release of Green House Gasses (GHG) to the atmosphere [1], [26], [27] and [28]. Peat land emits greenhouse gases in the forms of CO2, CH4 (methane) and N2O. Of these three gases, CO2 is the most important because it forms the highest amount emitted by peat land, especially converted peat forest to agriculture or settlements. CH4 is measurable in peat forests that are normally saturated or submerged. CO2 emissions dominate drained peat, whereas CH4 emissions decrease significantly or even become undetectable in drained peat land. N2O emissions occur from nitrogen-rich soils. Part of the leached nitrate into the anaerobic layer is reduced into N2O. [3].
Draining inland organic soils lowers the water table and increases the oxygen content of the soil, thus increasing CO2 emissions. CH4 emissions from drained inland organic soils are generally negligible because the soil carbon is then preferentially oxidized to CO2. However, methanogenesis may take place in drainage ditches with a higher water table causing significant sources of CH4 to the atmosphere. Drained organic soils can also emit significant amounts of N2O from nitrogen in the organic matter or nitrogen added by fertilization. Rewetting inland organic soils raises the water table again, decreases CO2 emissions, rapidly decreases N2O emissions to close to zero, and increases CH4 emissions compared to the drained state as the oxygen level in the soil drops and methanogenesis starts again [3].
The ongoing and future expansion of oil palm plantations may, or may not, result in future emissions of CO2, the most significant GHG linked to land use, depending on the type of land cover that is converted for new plantations. For example, if expansion occurs on forest landscapes with high above- and below-ground carbon stocks, then net emissions linked to the sector will be proportionally large. In contrast, if the source of land for new plantations has low C stock value, such as shrub land or agroforest, then future expansion could be considered carbon neutral. In some cases, expansion might actually be carbon positive if the initial carbon stock is less than that of oil palm as is the case with grassland and most types of annual crops. [29].
Peat fires increases CO emissions owing to burning or oxidation of one or a combination of plant biomass, necromass and peat layers [30]. Fires often occur during land-use change from forest to agriculture or other land uses [31]. Fires can also occur during long drought periods. Under traditional farming practices, burning can be done intentionally to reduce soil acidity and improve soil fertility. But, on the other hand, this practice increases the contribution of peat to CO2 emissions [30].
Transformation of forest to agricultural use involves increased management activities such as use of machinery, inputs of fertilizer and mill operations, many of which may promote CH4 emissions such as those from mill effluent and biomass burning in mill boilers. Soil subsidence can cause the peat surface to drop to levels that enable the water table to reach and rise above the new surface level in periods of high rainfall. This may lead to flooding of adjacent land and downstream areas [32]. In addition, because of the soil subsidence and reduced water retention, the freshwater buffer function of the peat swamps decreases, resulting in a decreased buffer against salt water intrusion in the dry seasons [33].