Monitoring soil fertility to mitigate soil degradation in reclamation land after bauxite mining

Bauxite is one of the minerals mined in an open pit system. Mining with this system will initially cause soil degradation because the soil layer covering the bauxite ore will be removed first from the mining location. The top soil will be returned after the bauxite ore extraction is complete. Returning soil to the mining site causes quite severe soil degradation because the process of taking and returning the top soil is carried out by an excavator which mixes the soil. As a result of this process, the soil in the reclamation area changes its chemical, physical and biological properties. To mitigate soil degradation in the reclamation area after bauxite mining, soil fertility status monitoring is carried out. This monitoring activity was expected to be able to identify and reduce the level of soil damage in post-bauxite mining reclamation land.


Introduction
Bauxite is the main mineral source of aluminum metal, and one of the important metals in modern industry which is a source of state revenue [1].One of the areas with abundant bauxite mineral deposits is West Kalimantan Province.Bauxite mining in West Kalimantan is spread over Mempawah, Bengkayang, Sanggau, Porcupine, Ketapang, Sekadau, Kubu Raya and North Kayong Regencies.One of the bauxite mining locations in West Kalimantan which has large reserves is in the Tayan area and is included in the IUP of PT ANTAM UBPB West Kalimantan.The indicated potential bauxite resource is around 104 million cubic tons, with an average grade of T-Al2O3 = 46%, T-SiO2 = 13%, R-SiO2 = 4%, Fe2O3 = 12% and TiO2 = 0.9 % [2].
The potential and benefits of bauxite as a source of state revenue must also be accompanied by good and sustainable management efforts.The government must ensure that bauxite mining activities are carried out by taking into account sustainable environmental, social and economic aspects in order to protect the long-term interests of the state and society [3].Mining activities have two opposing sides, namely as an economic resource and environmental destroyer [4].The ex-bauxite mining land has experienced severe soil degradation, this is evident in the decline in some of the physical, chemical and biological properties of the soil.
The physical properties of the degraded soil are the subsidence of the soil surface due to mining activities, which will disrupt the pattern of water flow in the surrounding area [5].In addition, during the rainy season the potential for erosion will be greater because the soil is unable to absorb and store 1314 (2024) 012011 IOP Publishing doi:10.1088/1755-1315/1314/1/012011 2 water.Changes in the chemical properties of the soil that occur as a result of mining include decreasing the soil's ability to store and provide nutrients for plants, decreasing soil pH, while biologically there will be a decrease in the soil microbial population which is important for various processes and reactions in the soil [6].
The decrease in soil properties in post-bauxite mining land causes a low ability to support optimal plant growth [7].Therefore, monitoring soil fertility status is very important in optimizing the productivity of reclamation land.Locations for monitoring soil fertility status in reclamation areas on post-bauxite mining land can be seen in Figure 1.

Figure 1. Map of Soil Fertility Monitoring Location Point
Monitoring soil fertility status can enable reclamation land managers to refer to real soil conditions.This will provide information about the levels of available elements in the soil and the level of soil fertility.Knowledge of these conditions can be used to devise steps for improvement of soil conditions and fertility can be carried out in an appropriate, effective and efficient manner.Monitoring soil fertility status on reclamation lands can also help in reducing financial losses caused by the inability of the soil to support plant growth.Knowledge of the nature and condition of soil fertility can be used to compile adequate and balanced doses of inorganic fertilizers, organic fertilizers and soil amendments according to soil conditions and plant needs.This will increase the efficiency of using fertilizers and reduce the negative impacts that may arise, such as environmental pollution [8].
In addition, monitoring soil fertility can also affect the sustainability of the ecosystem as a whole.Considering that post-bauxite mining reclamation land is often part of a sensitive ecosystem, monitoring soil fertility can assist in maintaining the balance of the ecosystem.Soil conditions that are known in detail, restoration efforts can be made related to vegetation and other living things, so as to increase biodiversity and maintain the quality of environmental life [9].Monitoring the level of soil fertility in the reclamation area after bauxite mining is important for related parties, such as the government, scientists, reclamation land developers, and mining companies to understand the importance of monitoring soil fertility in reclamation land, especially after bauxite mining.Good cooperation is needed in carrying out regular monitoring to ensure the success of the development of reclamation land and sustainable restoration of ecosystems.Periodic monitoring of soil fertility can be used as a basis for improving post-bauxite mining reclamation land that was previously unproductive and can be transformed into fertile land and contribute positively to human life and the surrounding environment.

Materials
The main material in this research is soil samples taken from reclamation areas after bauxite mining during several revegetation periods.Other ingredients are chemicals used to analyse several chemical properties of soil such as distilled water, ethanol, ammonium fluoride, hydrochloric acid, titrisol, ascorbic acid, acetic acid, sodium chloride, sulfuric acid, potassium dichromate and others.

Method
Monitoring the level of soil fertility in the PT ANTAM UBPB West Kalimantan IUP reclamation area was carried out in several stages.The activity begins with determining the sampling point of the soil which is carried out by means of purposive sampling of 12 observation points with the consideration that all reclamation periods are represented.Details of the location points for monitoring soil fertility levels can be seen in Table 1.Soil sampling was carried out using a soil drill according to the coordinates in Table 1.In the first stage, the soil was drilled until the drill was no longer able to penetrate the soil layer.The layers of each soil that can be extracted with a drill are placed sequentially in the prepared place, in the form of a banner with a depth scale written in centimetres to observe some soil properties.Some of the soil properties observed after the layers of soil resulting from drilling were composed, including soil depth (solum), thickness of each soil layer and soil colour.Soil colour is determined based on the colour of the soil contained in the Soil Munsell Book.
The second stage, at each observation point, a soil sample was taken in the tilled layer at a depth of between 0 -20 cm in a composite manner, i.e., soil samples were taken at a distance of ± 25 m each in the direction of the wind, then mixed and stirred evenly (composited), then taken as much as approximately 1 kg to be analysed in the laboratory for some soil chemical properties which are then used to determine the level of soil fertility.Several soil properties were measured, including: Cation Exchange Capacity (CEC), Base Saturation (BS), N-total, P-total, K-total, and C-organic.The five parameters of soil chemical properties are then graded according to the Centre for Soil Research [10].Soil fertility status is based on the Soil Fertility Evaluation Technical Guidelines [11].

Results and Discussion
Soil degradation in bauxite mining areas cannot be prevented at the beginning of the mining process because bauxite mineral extraction is carried out in an open system [12].Bauxite mining begins with clearing all vegetation, removing the top layer of soil 10-15 cm thick and removing the overburden layer ± 40 cm thick [13].The final stage in the mining process is reclamation activities.Post-mining land reclamation is intended to prevent or mitigate soil degradation and restore soil fertility so that it can provide sufficient and balanced nutrients and water for plant growth.Soil fertility monitoring can be used to determine the condition and properties of the soil periodically every year and mitigate the level of soil damage in post-bauxite mining reclamation areas.

Monitoring Soil Properties in Several Reclamation Areas
Monitoring soil fertility on reclaimed land during several planting periods is carried out by measuring changes in several soil properties.A comparison of soil fertility level data from monitoring in 2022 and this year ( 2023) is expected to result in changes in soil fertility levels which are reflected in the values of nutrient content and soil properties, for example the nutrient content of Nitrogen, Phosphorus, Potassium, Aluminium, Iron.Several soil properties that can be compared from two periods of monitoring soil fertility levels include: pH, CEC, and BS.
The results of soil pH analysis during soil fertility monitoring in 2023 ranged from 4.96 -6.28 with acid to slightly acid (Table 2).The soil pH value this semester has increased from the results of soil fertility monitoring in 2022 which also shows that the pH value in the reclamation area is very acid to acid with a soil pH value range of 4.08 -4.64 respectively [14].Changes in soil pH can affect the interaction of nutrients with soil particles, complex bonds, and soil microorganisms.When soil pH increases, heavy metal ions such as aluminium (Al) and iron (Fe) will be reduced or decrease their solubility in the soil, thereby reducing toxicity which affects nutrient availability and plant growth.Apart from that, increasing soil pH can also provide more optimal conditions for the activity of soil microorganisms which play an important role in the cycle of plant nutrients [15].The presence of soil microorganisms will directly or indirectly increase soil fertility, especially in providing the nutrient element phosphorus (P) and fixing nitrogen elements from the atmosphere into a form that is available for plants in the reclamation area [16].
The lowest pH value was found at the 2016 reclaimed land location at Bukit 7b (B7b-R2016), while the highest was at the 2016 reclaimed land at Bukit 2 (B2-R2016).This is different from the results of monitoring soil fertility levels in 2022 which showed that the lowest soil pH was found in the 2015 reclaimed land in Bukit 7 (B2-R2015), while the highest was in the 2015 reclaimed land in Bukit 15 (B15-R2015) with pH values land of 4.92.The soil pH value and its value can be seen in Table 2. Another chemical property of soil that determines soil fertility is the CEC of the soil.Cation exchange capacity is a description of the soil's retention or binding capacity for nutrients, especially positively charged nutrients.The CEC value for soil in soil fertility monitoring in 2023 ranges from 20.01 -35.82 cmol(+)kg -1 with a medium -high value.The CEC value from monitoring soil fertility levels this year is higher than the results from monitoring in 2022, which is very low (Sulakhudin et al., 2022).The increase in the CEC value in soil fertility monitoring activities this year was due to an increase in soil organic matter content.[17] stated that soils with high organic matter content tend to have high water content and CEC.This is because organic materials, especially humus, have a large specific surface area (> 800 m 2 /g) so they are able to absorb and store more water and nutrients.
The soil's ability to bind and store nutrients is moderate to high at all locations monitoring soil fertility levels in 2023, with CEC values between 20.01 -35.82 cmol(+)kg-1.Locations that have moderate CEC scores are B2-R2014, B19-R2017 and B7a-R2018, nine other locations are included in high grades.The soil CEC value will directly influence the soil base saturation value.
Base saturation is a soil property that reflects the percentage of the total cation exchange capacity (CEC) occupied by base cations such as potassium, calcium, magnesium and sodium.Soils in the PT ANTAM reclamation area in various revegetation periods have very low BS values, ranging from 3.74 -9.53%.The soil BS value in the 2023 soil fertility level monitoring activity is classified as very low, when compared with the range of BS values in the previous year's monitoring, it has decreased, namely very low to moderate with values ranging from 10.51-40.92%.The decrease in soil BS values in soil fertility monitoring in 2023 is thought to be due to erosion due to very high rainfall (>3500 mm/year).Soil erosion can reduce the BS value of soil because erosion can remove the top layer of soil which is rich in nutrients.When erosion occurs, layers of soil containing organic material and important minerals such as calcium, magnesium, potassium and sodium can be transported and lost [18].
Low soil base saturation reflects the lower amounts of basic cations compared to acidic cations such as Al and Fe.These two elements are micro elements that will cause plants to become poisoned if presence in the soil is high enough.According to [19] stated that base saturation describes the relative proportion of exchangeable bases in soil colloids.In soil in areas that have experienced advanced weathering, most of the exchange complexes on the colloid surface are occupied by Al cations.The Bs value is closely related to the pH and level of soil fertility.Acidity will decrease and fertility will increase with increasing soil Bs.The rate of release of adsorbed cations for plants depends on the level of base saturation of the soil.Soil base saturation ranging from 50% -80% is classified as having moderate fertility and is said to be infertile if it is less than 50% [20].

Availability of Several Nutrients in Reclamation Land
Based on their needs, the nutrients needed by plants in large quantities are called macro nutrients which include: N, P, K, Ca, Mg, and S; while the nutrients that plants need in small quantities are called micro nutrients such as: Fe, Mn, B, Mo, Cu, Zn, and Cl [21].Most of the N nutrients in the soil come from soil organic matter.If soil organic matter is low then the soil total N content will also be low, this can be seen in Table 3 which shows that locations that have low organic C values will also have low soil total N content as in location B19-R2017, and B2-R2014.On the other hand, locations that have moderate total N content usually have moderate C-organic values, such as locations B19-R2013, B2-R2015 and B7b-R2020.The low total N content is in line with or follows the organic C value because most of the total N in the soil is organic N which comes from organic material, the less organic material, the lower the nitrogen nutrient content.[22] explains that soil organic matter is a source of the nutrient's nitrogen, phosphorus and sulphur.
Soil organic matter is the main source of nitrogen elements in the soil.The decomposition process of organic material by organisms such as bacteria, fungi and insects produce complex organic nitrogen compounds.During decomposition, nitrogen is bound in organic compounds such as proteins, amino acids, and nucleic acids.Naturally, these organic nitrogen compounds are then converted into a form that can be used by plants through the mineralization process [23].Bacteria and fungi that form microbes in soil break down organic compounds into ammonia (NH3) in a process called ammonification.Ammonia is then converted into ammonium ions (NH4 + ) in the oxidation process by nitrifying bacteria.Finally, water-soluble nitrate (NO3 2-) is also produced through the nitrification process by other bacteria.Plants can use nitrogen compounds in the form of ammonium and nitrate ions for their growth and development.Therefore, the higher the organic matter content in the soil, the greater the availability of nitrogen elements for plants.Organic matter also helps increase the soil's ability to store and release nitrogen according to plant needs [24].The part of soil organic matter that is important in providing macro nutrients is particulate organic matter.Nitrogen elements as much as 95%, phosphorus and organic sulphur around 20-75% are produced from the mineralization process of soil organic matter and will be converted into a form that can be absorbed by plants [25].A low C/N ratio value indicates that the organic material has undergone weathering/decomposition which has further formed a stable organic material fraction with a low N content, so it is unlikely that the organic material can supply sufficient N nutrients [26].Apart from containing high amounts of N nutrients, soil organic matter is also a source of phosphorus (P) in the soil (Table 4).
In soils that contain high levels of organic matter, phosphorus is usually bound in organic forms, such as organic phosphate.Organic matter functions as a store of phosphorus, and the decomposition process carried out by soil microorganisms' releases phosphorus in a form that can be taken up by plants, namely in the form of H 2 PO 4 -, HPO 4 -2 and PO 4 -3 .When organic remains are decomposed, organic phosphorus compounds are converted into inorganic phosphorus compounds, such as phosphate ions.Inorganic phosphate is more easily taken up by plant roots and used in the growth process [27].The presence of inorganic phosphorus in the soil varies greatly depending on many factors, so that its availability is determined by the most dominant factors in the soil.
Table 4 shows that the available P content in the soil on the PT ANTAM UBPB West Kalimantan reclamation land is included in the high -very high criteria with a range of 12.73 -18.27 ppm.The location with the lowest available P content is the reclamation area on Bukit 7 with code B7a-R2017, while the highest is the reclamation area on Bukit 19 with code B7b-R2019.The available P content in reclaimed land with code B7b-R-2019 is the highest among other locations.This is suspected to mean that the leaching of P elements is lower compared to other locations because this location is quite flat.A flat location will cause less leaching of P nutrients by reducing the intensity of water runoff in the horizontal direction of the soil.The P-available data from soil fertility monitoring this year is higher than the P-available in 2022, while the overall amount of P (P-total) has not changed much.The amount of P-total in monitoring soil fertility levels in 2023 ranges from 7.50 -28.26 mg/100g, and in 2022 the amounts of P-total ranges from 2.82 -24.88 mg/100g [28].The total amount of P elements this year is higher compared to the previous year's monitoring of soil fertility levels, presumably because the soil organic matter content in monitoring soil fertility levels in 2023 is higher than in 2022.Organic matter is one of the sources of P nutrients in soil, the greater the organic matter content in the soil, the greater the total phosphorus [29].Apart from requiring sufficient P nutrients to grow well, plants in reclamation areas also require sufficient amounts of the nutrient potassium (K).
The content and value of the nutrient K soil in the PT ANTAM UBPB West Kalimantan reclamation area in several reclamation periods in 2023 can be seen in Table 5.The exchangeable potassium (K-dd) of soil in monitoring soil fertility levels in 2023 shows only in value very low to low with values ranging from 0.06 -0.31 cmol(+)kg -1 .The K-dd value is not much different from the K-dd value in monitoring soil fertility levels in the previous year which ranged from 0.03 -0.38 cmol(+)kg -1 with the same value, namely very low to low [28].The lowest K-dd value was found at the 2014 reclamation site at Bukit 2 with code B2-R2014, while the highest was at Bukit 7b at the 2019 reclamation site with code B7b-R2019.When compared with natural forests that are around bauxite mining areas and have not been disturbed by bauxite mining activities which range between 0.07 -0.17 cmol(+)kg -1 [30], it can be said that the K nutrients are not affected by mining activities because the exchangeable K content in postmining locations is classified as the same.
The nutrient K in the soil apart from being in exchangeable form (K-dd), is also in the form of fixed K, and K which is still in the mineral matrix, all of which are called soil K-total.Exchangeable potassium reflects the amount of potassium element that is easily or readily absorbed by plant roots, while total K is the total amount of potassium element in the soil.The total soil potassium in the reclamation area is around 9.03 -20.54 mg/100 g with monitoring of soil fertility levels in 2023 with very low to medium levels.The lowest total potassium was found at location B15-R2015, while the highest was at location B7a-R2017.The highest K-total at location B7a-R2017 is thought to be because the source rock found in the fields at Bukit 7 contains a lot of feldspar and mica minerals which contain a lot of K elements and has not experienced much weathering so the total potassium is still within the medium criteria.When compared with the K-total in the reclamation area in the 2022 soil fertility monitoring which ranges between 3.09 -7.93 mg/100 g with a very low value, there has been an increase in the total potassium content in the soil.The increase in soil K-total is thought to be due to an increase in soil organic matter content which will be monitored in soil fertility levels in the reclamation area in 2023.According to [31] organic matter can be a source of the nutrient K. Potassium organic compounds are mainly found in organic material that has not completely decomposed, such as leaf litter, stems, twigs and other fresh organic materials.The potassium content in this organic material is usually relatively low, but can make a significant contribution to the availability of potassium for plants when the organic material breaks down or decomposes.

Soil Fertility Status in Post-Bauxite Mining Reclamation Areas
Soil fertility is the ability of the soil to provide water, air and nutrients in a fairly balanced condition and available according to plant needs.Determination of soil fertility status is based on references made by the Indonesian Soil Research Centre.Soil fertility status is determined by five parameters, namely CEC value, base saturation, organic matter content, P-total and K-total.The status of soil fertility on the PT ANTAM UBPB West Kalimantan reclamation lands as a result of monitoring in 2023 can be seen in Table 6.
Based on the results of soil analysis, it is then linked to the criteria for assessing the fertility status of soil chemical properties, showing that soil fertility in monitoring soil fertility levels in 2023 on the reclaimed lands of PT ANTAM UBPB West Kalimantan is relatively low (Table 6).The level of soil fertility this year has not changed with the results of soil fertility monitoring in 2022.The status of soil fertility in the reclaimed lands of PT ANTAM UBPB West Kalimantan shows that soil fertility is determined by the base saturation value.All locations in the reclamation area have very low base saturation values.Base saturation is always associated as an indication of the fertility of a soil.The ease of releasing adsorbed ions to plants depends on the degree of base saturation.Soil is very fertile if the base saturation is > 80%, moderately fertile if the base saturation is between 50 -80% and infertile if the base saturation is < 50%.This is based on the nature of soil with a base saturation of 80% which will release base cations which can be exchanged more easily than soil with a base saturation of 50%.

Table 1 .
Locations for Monitoring Soil Fertility Status in 2023

Table 2 .
Soil pH, CEC and BS Values in the PT ANTAM UBPB KALBAR Area No.

Table 3 .
C-organic Content, N-total and C/N Ratio of Soil on Reclaimed Land No.

Table 4 .
P-available and P-total Content in the Monitoring Area No.

Table 5 .
K-exchanged and K-total Content in the Monitoring Area