Assessment of Soil Degradation for Biomass Production in Tanjungpinang, Indonesia

Land management, especially land that is not in accordance with the rules can cause damage/change in soil properties which in turn impacts the disruption of biomass production. The lack of available data regarding soil degradation, especially in Tanjungpinang City, has resulted in less than optimal supervision and control over land management. Mapping the condition of soil degradation is necessary to exercise control over land management. The purpose of this research is to assess soil degradation in Tanjungpinang City, Riau Archipelago for biomass production. Soil degradation can be modeled by scoring and overlaying four parameters, namely land use, slope, soil type, and rainfall, producing a tentative map of potential soil degradation. Next, a tentative map of soil degradation is corrected using a field survey. Field surveys are used to measure the standard criteria for soil degradation according to Government Regulation No. 150 of 2000. The results show that the criteria for soil degradation in Tanjungpinang City are light damage and moderate damage. The criteria for moderate damage have the largest area spread over Dompak Sub-District, Batu Sembilan, Melayu City Plate, Bukit Cermin, West Tanjungpinang, East Tanjungpinang, Kampung Baru, and Tanjung Ayun Sakti. The main limiting factors that limit land use in Tanjungpinang City are permeability, surface rocks, and redox.


Introduction
Soil as one of the natural resources, living areas, environmental media, and factors of production, including the production of biomass that supports human life and other living things, must be preserved and maintained (1).Soil is essential in human life to be used as a medium for plant food needs.Foods can be met through the planting and maintenance of food crops on soil resources, known as biomass production.
Uncontrolled biomass production activities can result in soil degradation for biomass production, thereby reducing its quality and function.In the end, it can threaten the survival of humans and other living things (2,3).Soil degradation for biomass production is a change in the basic properties of the soil that exceeds the standard criteria for soil degradation.Soil degradation due to biomass production can change the soil's basic properties, namely the soil's basic physical, chemical, and biological properties (4).Natural and human factors can cause changes in soil properties for biomass production.However, the human factor is the main factor causing the damage to the land (5).Soil degradation can occur due to excessive tillage, excessive use of chemical fertilizers, continuous use of pesticides and herbicides at doses that exceed the limit, and due to other activities that enter the land for biomass production (6)(7)(8).The exploitation of land resources occurs because of population pressure on the need for land resources.The increase in population from year to year causes an increase in food needs.This condition has occurred in the Tanjungpinang City area, Riau Archipelago, where in the last ten years, there has been an increase in population by 21% from 187,359 to 227,663 people (9).On the other hand, agricultural land has decreased due to the conversion of agricultural land into built-up land.This condition causes over-exploitation of soil resources on agricultural land, which can cause soil degradation.
The phenomenon of soil degradation due to biomass production can cause the stagnant growth of food products and even reduce future productivity (10).On the other hand, this soil degradation will increase additional costs for soil maintenance when planting (11).It needs to be anticipated by making efforts to improve soil resources.Improvement of soil resources can be made by changing cropping patterns and planning land use according to the carrying capacity and capacity of the soil.Soil improvement efforts can be made by first conducting a study of the level of soil degradation.
The level of soil degradation can be determined by observing in the field about several soil properties, including the thickness of the soil's solum, the condition of the rocks on the soil surface, and analyzing specific soil properties as stated in the soil degradation criteria for biomass production on soil with the potential for damage (12).Soil degradation potential can be measrured by combining field measurement and geographic information system (GIS) analysis.GIS can help to analyze and create model by using spatial data in large area such as in regency or broarder scale (13)(14)(15).By using GIS, all of soil properties paramater map can be analyze to produce a soil degradation map used theoverlay tool.
The current problem is the unavailability of data on soil conditions and the status of soil degradation, both in area and distribution in various regions, one of which is in Tanjungpinang City, Riau Archipelago, Indonesia.In addition, based on data on the critical level of land in Tanjungpinang City, it is also very high at 9,119 hectares or around 62% of the research area (16).Therefore, it is important to conduct a research on soil degradation assessment for biomass production so that the control and efforts to repair soil degradation can be effective and on target.This research aims to analyze the spatial variation of soil degradation in Tanjungpinang City.

Methodology
Soil degradation map was carried out in three stages, namely (1) making a tentative map of potential soil degradation for biomass production; (2) field activities for measuring and taking soil samples; (3) preparation of soil degradation maps for biomass production.

Modelling a Tentative Map of Soil Degradation Potential
The data needed for making a tentative map of potential soil degradation is a base map containing basic information such as roads, settlements, land cover, administrative areas, spatial planning maps (RTRW), soil maps, slope maps, rainfall maps, and maps of land cover.All maps were obtained from secondary data from the study area's city government.Furthermore, the data is processed using the scoring method in accordance with government regulation Number 150 of 2000.The variables used for scoring are soil conditions, slope, land cover, and rainfall.Next, an overlay process is carried out on the four variables used so that the level of soil degradation can be classified based on Table 1.

Field Measurement
Based on the tentative map of the potential for soil degradation, sample points can be determined for measuring and taking soil samples in the field.The determination of sample points is based on differences in the level of soil degradation and variations in land cover.The location of the research sample can be seen in Figure 1.

Figure 1. Tentative map of potential soil degradation and field sample locations
Field activities were used to obtain data: soil depth, percentage of rock on the soil surface, percentage of sand fraction, soil bulk density, soil porosity, permeability, pH, soil conductivity, redox reactions, and microbial counts.

Modelling Soil Degradation Map
The soil degradation status map can be modeled by matching the results of measurements of the parameters of the soil's physical, chemical, and biological properties in section 2.2. with standard criteria for soil degradation.The standard criteria for soil degradation were in accordance with Government Regulation Number 150, 2000.The soil degradation status can be determined for each soil parameter based on the criteria.Soil was categorized as damaged if the value does not match the standard criteria for soil degradation in each parameter measured.

. Soil Depths
The soil solum is to be damaged when the value is less than 20 cm.Based on the results of measurements on 40 field samples in Tanjungpinang City, it can be seen that there are no soils that fall into the damaged category.All soil samples taken at 40 locations had a soil solum depth of more than 40 cm, so all were undamaged.Spatially, the condition of the soil solum throughout the City of Tanjungpinang is undamaged.The condition of the soil solum in the Tanjungpinang City area based on the results of measurements in the field is suitable for biomass production because the average depth of the soil solum is around 60 cm.The condition of the soil solum in the study area can be improved so that it can be directly used as productive land.

Percentage of Rock in Soil Surface
The results of laboratory measurements and analysis on 40 samples in Tanjungpinang City showed that the surface rock values in Tanjungpinang City varied relatively from a value range of 0 -90%.It shows that the condition of the land or land in Tanjungpinang City is in the damaged and undamaged category.The soil condition in the study area in terms of surface rock categorized as undamaged is only 10% of the four samples, while the others are categorized as damaged.In more detail, the condition of surface rock values in Tanjungpinang City can be seen in Figure 2. Soils included in the undamaged criteria are scattered in the areas of Air Raja and Dompak Sub-Districts.Therefore, if productive land is to be made for biomass, management efforts need to be made to reduce surface rock phenomena, one of which is to minimize the rate of sheet erosion.

Percentage of Sand Fraction
The critical threshold limit of the soil is said to be damaged if the content of the sand fraction in the soil texture is more than 80%.If the sand content is too high, no part of the soil can absorb water and various elements needed by the soil, such as nutrients and soil organic matter.The lack of water stored in the soil along with nutrients and soil organic matter can inhibit the growth rate of plants on it.The results of field measurements in Tanjungpinang City at 40 sample points indicate that soils fall into the damaged category because the value of the sand fraction exceeds 80%.The number of soil samples included in the damaged category was 3 of the total soil samples (40 samples).The three soil samples that fall into the damaged category have a sand fraction value of 90%.However, on average, the soil condition in Tanjungpinang City is still in the undamaged category according to the percentage of the sand fraction because most of the soil samples have a value of less than 80%.More clearly, the condition of the measurement results of the sand fraction in Tanjungpinang City can be seen in Figure 3.The critical value limit for soil bulk density is more than 1.4 g/cm3.If this value is exceeded, the soil is said to be damaged due to the weight of its contents.Heavily damaged soil indicates that the soil tends to be dense, so it is difficult for water, nutrients, and organic matter to be stored in the soil, which can disrupt plant growth.The soil bulk density measurements on 40 soil samples in Tanjungpinang City showed that soils were categorized as damaged and undamaged.Three soil samples were in the damaged category, while 37 were in the undamaged category.The value of the soil unit weight in 3 soil samples that are categorized as damaged is close to 1.6 g/cm3.However, overall the average unit weight value of soil in Tanjungpinang City is still below the critical value, so it can be said that the soil in this area is still in good condition in terms of bulk density.The results of measuring the soil unit weight of 40 samples in Tanjungpinang City are presented in Figure 4. Spatially, the distribution of soil categorized as damaged based on the condition of the soil unit weight is in Air Raja, Batu Sembilan, and Dompak Sub-Districts.Soils categorized as damaged based on the weight of their contents are found in areas with dominant soil textures of clay and on fairly steep slopes in the study area.Meanwhile, the land distribution categorized as undamaged spreads to other areas in Tanjungpinang City.Therefore it can be determined that, on average, the soil conditions in Tanjungpinang City can still be utilized for productive land as a biomass producer.

Soil Porosity
The critical value limit of soil porosity is less than 30% and more than 70%.A less than 30% porosity indicates that the soil pores for storing water are assumed to be too few, so they cannot meet plant water needs.Meanwhile, a more than 70% permeability value indicates that there are many soil pores so that water will only flow past so that minimal water is stored in the soil.
Based on the measurements and laboratory analysis results on 40 samples in Tanjungpinang City, the soil porosity values in the study area ranged from 40 -90%.The porosity value indicates that there is land or soil in Tanjungpinang City that is in the damaged and undamaged criteria.In total, undamaged soil conditions, judging from the porosity value, dominate the study area.The number of soil conditions categorized as damaged is 20% or at 8 sample points.The 8 sample points have values exceeding the soil porosity's upper limit of 70%.The results of measurements on 40 soil samples in Tanjungpinang City can be seen in Figure 5.

Permeability
The results of observing the permeability value of the soil in Tanjungpinang City at 40 sample points showed a value of more than 10 cm/hour; even soils had a value of 80 cm/hour.It shows that the soil in Tanjungpinang City can pass high water; however, it is rather difficult to store/hold a certain amount of water in the soil.

pH
The results of measuring the pH value of the soil in Tanjungpinang City on 40 soil samples showed that most of the soil had a very acidic pH value below 4.5.The low soil pH measurement results make the soil in Tanjungpinang City fall into the damaged category.The number of soils categorized as damaged based on the pH value was 36 soil samples from a total of 40 soil samples.Based on this value, the soil condition in Tanjungpinang City is average in the damaged category.The results of measuring soil pH in Tanjungpinang City can be seen in Figure 6.

Reduction-Oxidation (Redox) Reactions
The results of measuring redox values at all 40 sample points in Tanjungpinang City showed values below 100 mV at all locations.This value indicates that the soil in Tanjungpinang City is in the damaged category regarding its redox value.Spatially, the redox values in Tanjungpinang City have the same pattern in all areas because all measurement results show a damaged category.It shows that the land in Tanjungpinang City experiences more frequent inundation within a certain period.Therefore it is necessary to make land or soil improvement efforts to increase the redox value of the soil in Tanjungpinang City before the land is used for biomass production.

Number of Microbes
The results of soil microbial measurements on 40 samples in Tanjungpinang City showed that all samples were included in the undamaged category.The number of microbes in the 40 soil samples measured was more than 104 cfu/g.This value is far above the critical threshold value for the number of microbes in the soil.This condition indicates that soil chemical processes are taking place intensively in Tanjungpinang City so that the land in Tanjungpinang City can be more productive regarding its number of microbes.

Soil Conductivity (DHL)
The critical limit for the DHL value contained in the soil is more than 4.0 mS/cm.If the soil has a value of more than 4.0 mS/cm, it is categorized as damaged.The results of measuring the DHL value on 40 soil samples in Tanjungpinang City yielded two categories: damaged and undamaged.The number of soils included in the damaged category was two samples, while the remaining 38 were included in the undamaged category.The average soil condition in Tanjungpinang City is in the undamaged category because the DHL value is only in the range of 0.5 -1 mS/cm.The results of measuring soil DHL conditions in Tanjungpinang City are presented in Figure 7.

Soil Degradation Classification
The results of identifying the parameters of soil degradation in each soil sample can be used as a basis for determining soil degradation in Tanjungpinang City.Soil sample points can be superimposed on land units in Tanjungpinang City to produce a distribution of soil degradation status.The results of identification and analysis of field data show that the status of soil degradation in Tanjungpinang City is in the category of slightly damaged and moderately damaged.The condition of the land, categorized as slightly damaged, has an area of 5,624.9hectares, or around 40% of the total area of Tanjungpinang City.While the status of land that is categorized as moderately damaged in Tanjungpinang City has an area of 8,478.1 hectares or around 60% of the total area of Tanjungpinang City.This condition indicates that the soil in Tanjungpinang City is dominated by land status with moderately damaged criteria.
The status of soil degradation in Tanjungpinang City is influenced by several factors, which can be seen from the parameters of soil degradation.Factors influencing soil degradation status are often referred to as limiting factors.Limiting factors are termed as factors of soil properties that can prevent the land from being used productively.The limiting factors for soil degradation in the identified Tanjungpinang City are permeability, surface rock, and redox.
Permeability is the soil's ability to drain a certain amount of water in the soil.The permeability value is the main inhibiting factor because, based on the results of field measurements, the permeability value of the soil in Tanjungpinang City is in the high category or has a value of more than 8 cm/hour.This condition indicates that the soil's ability in Tanjungpinang City is very minimal to absorb or store a certain amount of water in the soil.In addition, the high rate of soil permeability also affects the faster process of washing or leaching plant nutrients in the topsoil.Land that has a limiting factor in the form of permeability is spread over areas with ultisol soil conditions.The presence of a layer of clay in ultisol soil due to weathering and leaching processes causes the rate of soil permeability to slow down (17).This problem was also found in many previous studies where the presence of a layer of clay in the soil reduced the ability of the soil to allow water to pass through (18)(19)(20).
While surface rock is the percentage of rock found above the ground surface, the percentage of surface rocks in Tanjungpinang City shows a relatively low value at almost all sample locations.The surface rock value in Tanjungpinang City is less than 40%, indicating that the soil tends not to experience further erosion.Land with a limiting factor in the form of surface rocks is found in areas with steep slopes (15-25%).The slope of the high slopes causes an intensive erosion process, especially sheet erosion (21)(22)(23).Sheet erosion causes the topsoil to be transported, leaving rock above the soil surface.Previous research stated that areas with high slopes are very prone to erosion, especially sheet erosion which can develop into rill erosion (24)(25)(26)(27)(28).
Another dominant limiting factor is redox.Redox is closely related to the condition of the air contained in the soil.The redox value of all soils in Tanjungpinang City has a very low value of less than 80 mV, so it is on the critical threshold of redox value.It shows that the process of exchanging soil air, especially oxygen, rarely occurs in Tanjungpinang City, so the air content in the soil tends to be static.Static air content conditions can reduce the potential of the air needed by plants to grow and develop.In addition to these three dominant limiting factors, other limiting factors have a minor influence on specific areas in Tanjungpinang City.The limiting factors are bulk density, pH, DHL, percentage of sand fraction, and porosity.The five limiting factors are only found in certain areas.The results of determining the soil degradation status in Tanjungpinang City and its limiting factors can be seen in full in Table 2. Based on Table 2, it can be seen that low damage levels dominate the status of soil degradation in Tanjungpinang City with limiting factors in the form of surface rock and permeability.This damage status has an area of 2,984.65 hectares or around 21% of the total area of Tanjungpinang City.The condition of the status of soil degradation is spread over several sub-districts in Tanjungpinang City, such as Senggarang Sub-District on the north side, Bugis Village in the middle, Pinang Kencana in the east, a small part in Dompak and Batu Sembilan.Meanwhile, in the status of moderate soil degradation, the most significant area is found in moderate soil degradation with limiting factors, namely surface rock, permeability, pH, porosity, and redox, of 2,421.01hectares or 17% of the total area of Tanjungpinang City.The status of this soil degradation is spread across the central region of Tanjungpinang City, to be precise in most of Air Raja and Batu Sembilan Sub-Districts, as well as a small part of Dompak and Pinang Kencana.
The smallest area of soil degradation status is found in the medium soil degradation class with limiting factors, namely surface rock, sand, redox, and permeability.The area of this soil degradation

Figure 2 .
Figure 2. Percentage of Rock in Surface Soil Value in Tanjungpinang City

Figure 4 .
Figure 4. Value of Soil Bulk Density in Tanjungpinang City

Figure 5 .
Figure 5. Value of Soil Porosity in Tanjungpinang City

Figure 6 .
Figure 6.Soil pH conditions in Tanjungpinang City

Figure 7 .
Figure 7. Soil Conductivity in Tanjungpinang City Spatially, two soil samples fall into the damaged category in Sei Jang and Dompak Villages, while others fall into the undamaged category.If the DHL condition is too high, it indicates that the soil has a very high clay content and is found in areas with steep slopes in hilly areas.The difference in DHL values at the two locations can also be due to factors that are endemic to those areas.

Table 2 .
Soil Degradation Status Conditions and Limiting Factors in Tanjungpinang City