Relation subsidence and water level of peatland cultivated with oil palm in Riau, Indonesia

This study was aimed to determine the relation changes in the subsidence and the depth of water level of peatlands cultivated with oil palm. The research site is the oil palm plantations in Koto Village, Gasib District, Siak Regency, Riau, Sumatera. The observations were in 4 blocks, namely block L1 (shrubs), Block D1 (oil palm aged 15 years), D8 (oil palm aged 10 years), and D31 (oil palm aged 20 years). Research parameters include peat subsidence and groundwater level. The results showed that the value of peat subsidence was fluctuative. This fluctuation was due to the rise and fall of the groundwater level. The highest rate of decline is shown in block L1 with 0.75 cm y−1 with SD 1.03 and the groundwater level in D1 with 52 cm. The water level is influenced by the condition of the monthly rainfall. The correlation between land subsidence and peat water level shows a close and significant relationship (p <0.01, r = 0.871).


Introduction
Oil palm (Elaeis guineensis Jacq.) is one of the plantation commodities that has an important role in Indonesia as a vegetable oil and a renewable fuel (biodiesel) producer [1]. Currently oil palm plantations plays an important role in the Indonesian economy. In 2017, total area of oil palm plantations in Indonesia reached 12.30 million ha with an average productivity of 2.80 tonnes of CPO (crude palm oil) and 0.56 tonnes of PKO (palm kernel oil) ha -1 year -1 [2]. Oil palm plantations mostly developed on marginal lands including peat due to the limited availability of land suitable for oil palm cultivation. The potential of peatlands for oil palm plantation development is good enough if done properly, which is based on the level of land suitability and management according to land conditions. However, there are many conditions for oil palm planted on peatlands that resulted in low growth and production. This is due to the low or marginal suitability of the peatlands and poor land management [3].
The management of oil palm plantations on peatlands is faced with various problems, such as high acidity, relatively low soil fertility, easy-to-dry nature of peat soils, and physical characteristics of peat soils that have low bulk density [3]. The development of oil palm plantations on peatlands requires good water management. This system must be able to remove water surplus both on the surface and below the surface directly during the rainy season and also be able to hold water at a predetermined level range as long as possible during the dry season. This is important to be managed because logged or excessive water can decrease the development of oil palm fruit, while lack of water will cause peatland degradation [4]. Drainage of peatlands is needed for better condition for plant growth. However, over drainage will lead to dryness of peatlands and cause hydrophobicity or irreversible drying [5]. Several research results suggest that the deep water level fluctuation of the peatlands, apart from having an effect on hydrophobicity, also affects the increase in soil CO 2 emissions [6,7], and peat subsidence [8,9].
Subsidence is a phenomenon that inevitably occurs on peatlands which are used as agricultural land. Subsidence occurs because in its use, it is necessary to drain the soil by making water channels or drainage. Subsidence in peatlands initially occurs mainly as a result of consolidation and compaction of the peat material. Consolidation is the mechanical compaction of a permanently saturated peat layer or under the groundwater level, while compacting is a reduction in the volume of peat material in the layer above the water level [8]. On the other hand, subsidence is also caused by oxidation of the peat material, especially those above the groundwater level. Subsidence is an important issue in the use of peatland as agricultural land. According to [10], subsidence in peat soils is very high at the start of drainage and will gradually decrease over time.
This study was aimed to determine the relation changes in the subsidence and the depth of water level of peatlands cultivated with oil palm. This research is one of the studies on the emission factors of Indonesia's peatlands which are drained for oil palm plantations.

Study area
Research was conducted at oil palm plantation on peatland in Koto Village, Gasib District, Siak Regency, Riau, Sumatra, namely at coordinates 0° 44' 55.89" N / 101° 45' 14.04" E. The determination of the research location area was carried out using purposive sampling method, namely a method of determining the location of the study deliberately which is considered representative. The selected research sites was consisted of 4 oil palm planting blocks, namely block L1 (scrub), Block D1 (oil palm aged 15 years); D8 (oil palm aged 10 years); D31 (oil palm 20 years) ( Figure 1).

Field Measurement and Sampling
This research activity includes observations and measurements in the field and laboratory analysis. The research locations are on the block for oil palm plants taken at 2 sampling points and the scrub blocks taken 1 sampling point. Research parameters include the depth of groundwater level and 3 peatland subsidence. The water level is measured with a piezometer. Peat subsidence is measured by installing a permanent iron rod in the peat soil to reach the peat substratum. Subsidence was determined by measuring the distance between the iron ring on the subsidence rod and the ground surface.This measurement is done by looking at the 4 cardinal points around the iron ring on the subsidence stick. Land subsidence and water level depth are measured in cm every 3 months in 1 year. All data are tabulated and presented in a table with standard deviation (SD).

Peat Characteristics.
Physical and chemical characteristics of peat are presented in Table 1. The thickness of the peat in the 4 observation blocks was varied from 300 to 650 cm. Bulk density (BD) gives an average value of about 0.25 to 0.12 g cm −3 with a SD of 0.056 in the total depth of peat, the average ash content was 5.53 to 2.21% in the upper layer with a SD of 1.68%, fiber content of 20.06 to 24.70 with a maturity level classified as hemic, and C-organic content of 55.02 to 56.73%. The rate of peat subsidence is one of indicators of decreasing function of peat ecosystem. The subsidence of peat soil occurs due to the compaction, decomposition and erosion of the peat on the surface which causes the peat to become drier [11]. The data of peat subsidence based on field observations that have been carried out are presented in Figure 2.

Effect of Rainfall on Water-table Depth.
The dynamics of the rise and fall of water levels in peatlands are influenced by the balance of rainfall, evapotranspiration, and groundwater flows [12]. The data of the groundwater level at the research location is obtained from the data of 7 piezometers installed in each observation block.

Oct-19
Nov -19  Dec-19  Jan-20  Feb-20  Mar-20  Apr-20  May-20  Jun-20  Jul-20  Aug-20  Sep-   When connected with monthly rainfall data, the rise and fall of the groundwater level that occurred in the period October 2019 to October 2020 shows that monthly rainfall can contribute directly to the rise and fall of the groundwater level in the study location. Response to groundwater level fluctuations does not occur directly based on rainfall data. There is a delay response to groundwater level about 1 month compared to variations in rainfall. This is presumably because the water resistance of peat soils is still quite strong, where the water does not immediately disappear after the rain stops.These results of the study are indicated by the results of the correlation analysis of groundwater level with rainfall showing a strong correlation with the value (p <0.01, r = 0.734). This is consistent with the statement [13] that rainfall has a different effect on changes in the water level of peat lands.

Relation of Subsidence and Water Level Dynamics.
The peat subsidence in the study has a fluctuating value that is influenced by fluctuations in groundwater level. The results of the correlation analysis showed that the depth of the groundwater level showed a strong correlation and significantly different to subsidence (p <0.01, r = 0.871). This is consistent with the research [14] which showed a correlation value of p <0.01, r = 0.824. Thus, changes in the water level depth can affect peat soil subsidence and have a linear relationship with the subsidence rate [15]. A study [8] in Sarawak, Malaysia, states that in the average groundwater level of 100 cm, subsidence was up to 8 cm y -1 and for an average groundwater level of 25 cm, subsidence was about 2 cm y -1 . This is presumably because the peat soil undergoes changes in anaerobic to aerobic conditions where in these conditions there is an acceleration of the process of peat decomposition and mineralization resulting in soil compaction [16,17]. Watel-level Dept