Digital mapping of soil organic carbon in volcanic soils after prolonged eruption Mt. Sinabung, Karo Regency, North Sumatra, Indonesia

Mount Sinabung (at Karo Regency, North Sumatra) has been considered as an active volcano since 2010, and after three years of inactivity, the volcano’s activity resumed in 2013. The deposition of volcanic ash buried everything on the soil surface with various thicknesses. Volcanic ash is a valuable inorganic material and consists mostly of primary minerals. But with time, these materials can initiate carbon storage through the revegetation process. This study investigates carbon storage and sequestration in volcanic soils affected by the intermittent eruptions of Mt. Sinabung. A total of 34 soil samples were collected from areas of 3 to 7 km from the eruptive vent. The samples were analyzed to determine labile-C, very labile-C, total-C, Organic-C, non-crystalline-C, and metal complex-C fractions. Regression kriging (RK) was applied to spatially predict the carbon distribution. The results showed that the highest labile-C was 1.65% found in the Southeast slope and the lowest, 1.20%, in the Southern slope. The highest value of very labile-C was in the Southeast (1.20%), and the lowest was in the Northern (0.46%). The highest (11.66%) and lowest values (8.84%) of total C were in the Northeast and South slopes, respectively. The highest Organic-C value was detected 7.42% in the Northeast volcanic soils, and the lowest, 5.74%, is in the South slopes soil. The lowest non-crystalline- C was found in the soils of Northeast slope (0.60%), and the highest was in the Southeast (0.82%) soil. While the highest metal complex-C value was 0.88% in the Southeast, and the lowest is 0.36% in the soil of South slope. These data show that the highest carbon storage is parallel with the direction of the ash distribution, which tends to the Southeast side of Mt. Sinabung. The results of this study demonstrated that volcanic regions have a strong resilience capacity to bounce back after devastating natural hazards. Volcanic deposits interact with atmospheric water to initiate vegetation regrowth and create a carbon pool within their particles.


Introduction
The distribution of volcanic soil, known as fertile soil, is closely related to active and inactive volcanic activity.Around 124 million hectares of the world's total area and around 5.4 million hectares of the total land area in Indonesia are volcanic soils located in volcanic regions [1].There are 129 active and inactive volcanoes across Indonesia [2].One of them is Mt.Sinabung in Karo Regency, North Sumatra, since 2013 to 2020 never stops spewing materials from the volcanic vent and these materials are parent 1306 (2024) 012024 IOP Publishing doi:10.1088/1755-1315/1306/1/012024 2 material for volcanic soil.The Sinabung volcano with the highest peak of 2,460 m a.s.l, experienced dormancy for 400 years from its first eruption and erupted again in 2010.However, its activities have stopped and continued from September 2013 until 2020 [3].
The activity of Mt.Sinabung ejected volcanic material up to hundreds of meters from the top of the eruption or to atmosphere [4].The distribution of ash from the eruption of this volcano reaches 10 km from the center of the eruption with a distance of 0-5 km is a red or dangerous zone.The thickness of the volcanic ash layer between 2-32 cm [5].The material from the eruption contains crystalline and noncrystalline minerals, rock fragments, and other primary minerals and they are potential sources for plant nutrients [6].The eruptions from active volcanoes significantly contribute volcanic material that can enrich the nutrients in the soil.Minerals from volcanic ash also contain macronutrients, especially Ca, Na, K, Mg, P, S, and micronutrients such as Fe, Mn, Cu, and Zn [7].Furthermore, volcanic ash can reduce the supply of CO2 in the atmosphere during an eruption and capture and store carbon in the soil.Minasny et al. [8], reported that fresh volcanic ash has no carbon content but after the volcanic ash weathered for 2 years, the carbon content was increased to 1 to 3%.
Organic carbon in soil is vital for three aspects of soil fertility (physics, chemistry, and biology) [9].If carbon in the soil decreases then can reduce plant productivity [10] and if more organic carbon stored in the soils, then soil quality increase.Nevertheless, the existence and distribution of soil carbon fractions after recent volcanic eruption remains unclear or are limited.Thus, it is necessary to map the spatial distribution of soil carbon fractions.Digital soil mapping (DSM) approach can be used for mapping all the carbon parameters and estimate the observed parameter values using the kriging method.With this method, we can interpolate the values obtained with values at unobserved locations [11].The objective of our study were to determine carbon fractions in volcanic soils affected by the continuous eruptions of Mount Sinabung from 2013 to 2020.

Description of the study area
The research sample was taken in the areas affected by the eruption of Mt.Sinabung, district Payung, Namanteran, Merdeka, and Simpang Empat districts at a radius of 3-5 km and 5-7 km.Based on the geography of Mt.Sinabung, located at coordinates 3° 10' 16.7" North Latitude and 98° 23' 24.66", 34 soil samples were taken based on a map of disaster-prone areas with an area of 4,517.25 Ha.The volcano with the highest peak of 2,460 meters above sea level has been recorded as experiencing dormancy for 400 years; Mt.Sinabung erupted again in August 2010, emitting volcanic smoke and ash, and in September, Mt.Sinabung erupted again by releasing lava.This activity is the largest eruption since the volcano was active.Activity Mt.Sinabung stopped and active again from September 2013 until now [3] The eruption of the volcano emitted thick black smoke accompanied by a rain of sand and volcanic ash.Volcanic ash is volcanic material ejected into the air during an eruption.Volcanic ash resulting from the eruption of Mt.Sinabung resulted in many plants on the slopes of the volcano dying and being damaged [12].In addition, the material from the volcanic eruption can also enrich the nutrients in the soil.The statement of [7] stated that volcanic ash from the eruption of Mt.Merapi contains macro and micro nutrients needed in the soil.

Sampling and Soil Analysis
Sampling in this study used a grid with a distance between samples of ±1 km (Figure 1).For a predetermined sample point, several points are taken and composited.The samples taken were disturbed soil samples.First, disturbed samples were taken using a Belgian drill at a depth of 0-20 cm; then, samples were air-dried.Based on the survey of field conditions, several land uses were obtained in the research area, namely agricultural land, forest, and open land.
IOP Publishing doi:10.1088/1755-1315/1306/1/0120243 Samples analysis was carried out in the soil chemistry laboratory, Department of Soil Science, Faculty of Agriculture, Andalas University.This analysis was conducted to determine the carbon storage in each sample.The analysis carried out is of Organic-C, Label-C, Very labile-C, Humus metal complex-C, Non-crystalline-C, and Total-C.Each method was sequential (Walkley and Black, washing with HCL, hot water extraction, Na-pyrophosphate, Ammonium oxalate, and Furnance), and all samples used were disturbed soil samples.
After laboratory analysis, statistical data processing of the soil chemical properties was carried out using software, namely JMP-Pro; while mapping the soil chemical properties, this study uses ArcGIS software by interpreting it to a map using the kriging interpolation method.The presentation of data is grouped based on the cardinal directions of the sampling point, namely East, Northeast, South, and Southeast.

Organic Carbon
The average value of organic carbon based on research results in the Mt Sinabung area is high in the Northeast slope, namely 7.42%, and low in the South slope, namely 5.74 (Figure 2).Several factors, including climate and rainfall, can influence the high and low values of Organic-C.pH can also affect the carbon value in the soil.Based on the results of [13], the same research area has a high pH value that ranges from 5.11 to 6.18.In addition, the high and low levels of Organic-C in volcanic soils are also influenced by the presence of volcanic ash, according to the statement of [7] that the result of weathering of volcanic ash has a high capacity to bind Organic-C.Generally, the distribution of hot volcanic ash causes plants to die, but this can be one of the high organic sources of volcanic ash.At the time of combustion, carbon stored in aboveground biomass is released back into the atmosphere and then resumed in the process of carbon exchange of land and the atmosphere [14].The t-test Organic-C in Table 1 showed that the slopes in the northeast, east, Southeast and South had non-significantly different organic-C-values because all slopes of the research area have been covered by volcanic ash.Based on the research [15], the presence and level of volcanic ash thickness greatly determine how to process and rehabilitate land in disaster areas.

Labile Carbon
Figure 3 shows that the unstable Carbon (C) analysis has an average value of 1.65%-1.20%,where the highest value is in the distribution of the Southeastern part, and the lowest is in the South.The high and low values of labile-C can also be influenced by temperature and soil moisture.Low air temperature can inhibit the physiological activity of soil microorganisms which causes decomposition and respiration of the soil to be slow so that there is little labile-C in the soil [16].
The distribution of volcanic ash during the eruption of Mt.Sinabung annually tends to the Southeast and slightly towards the South and East.Therefore, the influence of volcanic ash deposits can continuously affect air and soil temperature.The increase in air temperature and soil temperature will then affect the activity of microorganisms in their decomposition.On the other hand, stable and fairly low air temperatures in areas that are slightly affected by fly ash (east and south) cause the activity of organisms to run more slowly [17].Based on the statistical analysis of unstable carbon, between all slopes, there is no significant difference in value (Table 2).This result can be influenced by the activity of microorganisms which tend to be the same because they are in the same climatic conditions (rainfall), and the majority come from the same ash source.

Very unstuble carbon
Very unstable carbon describes the amount of carbon lost more quickly because its small molecules can easily be carried away by water.The average value of carbon is very unstable at a depth of 0-20 cm in this study area; the highest value is in the Southeast slope, namely 1.20%, and the lowest is 0.46% in the northern part of Table 4.The C-very unstable value at this research location is considered low because it is easily lost to the air.The very labile-C has small molecules so it is easily carried away by water, especially at the top layer of 0-20 cm.from the t-test performed, it can be seen that the data obtained between slopes are not significantly different (Figure 4).From the t-test, the value is less stable carbon in the western part, which is higher but not much different from the other parts of the region.Research [18] showed that the plant biomass in the area was not degraded well and affected the activity of microorganisms.

Carbon bound to non-crystalline clay minerals
Carbon bound to non-crystalline clay minerals is carbon bound to non-crystalline minerals so that it is more stable to decompose.This research area is an area of volcanic soil resulting from the eruption of Mt Sinabung, such as volcanic glass.Volcanic glass is an amorphous (non-crystalline) material derived from the remains of magma that has undergone incomplete crystallization [19].The content of volcanic glass in the soil reflects the level of weathering and mineral reserves in the soil. .The average value for carbon bound to non-crystalline clay minerals is highest in the Southeast and the lowest in the Northeast slopes, respectively 0.82% and 0.60% (Figure 5).No significance difference found in all samples (Table 4).Factors that affect the value of carbon bound to non-crystalline clay minerals are pH, temperature, rainfall, and non-crystalline mineral containing allophane and imogolite because, at pH > 5, weathering of rocks by carbonic acid will release Si into the soil.The soil allows Al to polymerize and precipitate together with Si to form allophane formations, whereas at pH <5, the formation of metal complexes (Al and Fe)-humus will be easier to form [20].

Metal-humus complexes carbon
The carbon humus metal complex (HMC) is a stable carbon, so it is difficult to decompose.HMC carbon analysis using sodium pyrophosphate solution, which can release active compounds Al, Fe, and Si dissolved with organic acids The high and low percentages of HMC content can be influenced by the number of minerals in the soil, which can also affect the mechanism of soil organic carbon against biological oxidation [21].The average value of humus metal complex Carbon is from 0.88% -0.59%, with the highest average value being in the Southeast slope (Figure 6).The amount of HMC also describes the organic carbon content in the soil; this is because HMC carbon is carbon associated with Al and Fe minerals that occur in an environment rich in organic carbon and low pH but limited by non-crystalline allophane minerals at pH > 5, from The t-test that was carried out showed that there was no significant difference between the Metal-humus complexes carbon slope in the study area (Table 5) [22].The average total-C value at a depth of 0-20 cm in the study area ranged from 11.66% -8.84%.The highest percentage value is in the Northeast slope, and the lowest is in the Southern slope (Figure 7 and 8).Total-C in the soil is the sum of all the carbon present in the soil.The total-C value in the soil in the research area is classified as very high because the sampling location is a volcanic parent soil with high organic C levels.Based on the t-test of the Total-C content at depths of 0-20 cm in each part of the slope, the results were not significantly different for each value obtained (Table 6).

Conclusion
The results of this study indicate that the carbon content in the study area of Mt.Sinabung, especially in areas affected by the eruption, differed.Still, the carbon value obtained was higher in the southeastern area because the distribution of volcanic ash is mostly towards the Southeast.In addition to being a carbon source, volcanic material from volcanic eruptions can also contribute macro and micronutrients to the soil so that the soil becomes sluggish.Another factor that affects the amount of carbon in the soil is temperature.Low temperatures in volcanic areas also hamper the metabolism of microorganisms, thus affecting the soil's carbon decomposition process.

Figure 2 .
Figure 2. Average C-Organik.The t-test Organic-C in Table1showed that the slopes in the northeast, east, Southeast and South had non-significantly different organic-C-values because all slopes of the research area have been covered by volcanic ash.Based on the research[15], the presence and level of volcanic ash thickness greatly determine how to process and rehabilitate land in disaster areas.

Figure 5 .
Figure 5. Average carbon bound to non-crystalline clay minerals.

Figure 6 .
Figure 6.The Average value of C-humus metal complex.

Table 3 .
Data C-Very unstable soil, very unstable.

Table 4 .
Carbon data bonded non-crystalline clay minerals soil