Subsidence - a progressive problem

Peatland land use change, for example, in Indonesia and Malaysia to oil palm and pulp wood plantations or in the UK for agricultural production, increases the flood risk in low lying peatland areas. Deforestation and drainage of peatlands in order to prepare the land for plantation crops leads to soil subsidence. This is the lowering of the soil surface as the result of compaction, consolidation and loss of volume due to deforestation, drainage, oxidation and erosion (Fig. 1).

Fig. 1_S Drainage Processes

Fig.1: Schematic of changing carbon dynamics of peatlands throughout stages of draining. Image: Sue Page, University of Leicester.

Peat soils constitute approximately 10% accumulated organic material (carbon) and 90% water. When the water is removed through drainage, the carbon in the peat soil is exposed to aerobic conditions, which leads to decomposition of the peat and the emission of CO2 into the atmosphere. The process continues as long as drainage continues and until all peat above the drainage level is lost. If the area is low lying the soil surface can subside below river or sea levels, leading to increasingly frequent and prolonged flooding. All over the world, peatland subsidence has led to drainage problems, salt intrusion in coastal peatlands and eventually to the loss of productive land. It has been estimated that drained peatlands in the temperate zone can lose between 1 to 2 cm in height per year, but with year round high temperatures that drive higher rates of peat decomposition, the rate can be as high as 3 to 5 cm per year. Subsidence rates are rapid in the first one to two years following drainage, as the peat loses water and consolidates owing to increased overburden resulting from a loss of buoyancy; this phase can result in initial subsidence rates of more than 0.5 m yr-1 (Fig. 2).

Fig. 2_S Subsidence pole

Fig. 2: Subsidence pole in peatland in a peatland in Johor, peninsular Malaysia. The pole was inserted beside an oil palm plantation, in 1978 and at the time of this photograph (2007), 2.3 m of subsidence had occurred. Image: Sue Page, University of Leicester.

Following this primary stage of subsidence, a secondary phase of irreversible shrinkage and compaction of the peat together with rapid rates of peat decomposition leads to a slower but constant rate of subsidence. The processes of consolidation, shrinkage, and compaction are entirely physical, and no carbon is actually lost in the process, but peat bulk density (and carbon concentration) increases with time since drainage. Any subsidence, after the secondary phase described above, is likely to be caused through oxidation, which inevitably leads to a loss in carbon. Large-scale subsidence studies in Acacia and oil palm plantations in SE Asia show that in the first 5 years after drainage peat surface subsidence is found to be 142 cm, of which 75 cm occurs in the first year. After about 5 years, the subsidence rate remains constant at a lower rate of ~5 cm yr−1, which is caused entirely by peat decomposition. Furthermore, several other studies in the region are comparable, and show that peat subsidence rates stabilize between 4 cm yr−1 and 5.5 cm yr−1 at average water table depths around 0.7 m, after an initial phase of more rapid subsidence.

In addition to carbon storage, peatlands play an important role in protecting adjacent or downstream areas against floods after heavy rainfall and in ensuring a supply of clean water throughout the year. If subsidence occurs this capacity to produce and provide for local communities (human and non-human) is lost, and can, ultimately, cause ecosystem disruption. For example, within the Rajang River Delta along the coast of Sarawak, Borneo, the cover of industrial-scale oil palm plantations on peatland has increased from 6 % to 47 % between 2000 and 2014, with most of the remaining non-plantation peatland also affected by drainage. This has already and will increasingly result in the coastal lowlands being affected by subsidence and an increasing risk and duration of flooding. Flood models generated by Deltares (https://www.youtube.com/watch?v=SRwrAczShUc), show that a plantation flood risk estimated to be 29 % in 2009, will increase to 42 % in 25 years, 56 % in 50 years, and 82 % in 100 years. It is predicted that as flood conditions intensify in terms of frequency and impact, oil palm production may eventually have to be abandoned in most of the area

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