Judul: Subsidence and Percentage of CO2 Emission from the Decomposition to Subsidence of Peatland on Palm Oil Plantations
Abstrak: The drainage system on peatlands is aimed to eliminate limiting factors that can inhibit the growth of oil palm plantations. On the other side, peatland drainage practices will cause peat subsidence and it is associated with CO2 emissions from peatlands. Subsidence method is one of the methods commonly used in the approach to calculate CO2 emissions. The peat subsidence process consists of consolidation, compaction, and decomposition. Previous research resulted in emission data of 100 tons CO2/ha/year, of which 92% of the subsidence is considered as decomposed peat, and 8% is the components of compaction, soil respiration (autotroph and heterotroph). The percentage of decomposition to compaction-consolidation in the previous study was overestimated. The research was conducted on the oil palm plantation of Koto Gasib Village, Siak Regency, Riau Province. From January 2019 to January 2020. The study was carried out on oil palm plantation blocks on peatlands that have been controlled drained with a canal system to maintain water levels. Observational data were collected from 6 subsidence stick, 3 dipwells, 3 peat profiles, and LiCor 8100A automatic chamber. The observation interval for the observation of bulk density, subsidence, C-Organic and ash content every 3 months, while for the water level using the monthly average and every 30 minutes for CO2 flux. Peat analysis was conducted at the Laboratory of Chemistry and Soil Fertility, Department of Soil Science and Land Resources, IPB. The purpose of this study were to observe the rate of subsidence of tropical peatlands in oil palm plantations at various ages of oil palm plants, and to determine the percentage of decomposition of peat material from the subsidence rate. Subsidence was measured at adjacent oil palm planting blocks at 9, 12, and 17 years of age, with a peat thickness of about 5 m, and the land had been drained for 10, 13, and 18 years, respectively (one year prior to planting). Peat decomposition (heterotrophic respiration) is considered as actual CO2 emission compared to the calculated emission from subsidence which is considered as potential CO2 emission from the calculation of the subsidence method. Analysis of peat fiber at the research site generally shows that the deeper the surface of the peat, the higher the fiber volume with the average fiber at a depth of 0-10 cm 25.3% and classified into hemic peat. The peat density analysis using the kubiena box showed that the average density of peat at a depth of 0-20 cm in the three blocks observed was 0.134 g/cm3. Peat ash content shows accumulation on the peat surface as a result of the decomposition process of peat material, with an average of 5.83% and 2.31% at depths of 0-10 and 10-20 cm, respectively. Meanwhile, the organic C content of peat showed an average of 55.64% at a depth of 0-20 cm. The average subsidence rate for oil palm plantations on tropical peatlands 10-18 years after controlled drainage is 2.47 ±0.76 cm/year. Peatland subsidence is higher in older oil palms due to higher plant load and root density. There is a bouncing back phenomenon or an increase in the peat
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