Mapping of volcanic soil chemical properties with digital soil mapping after the prolonged eruption of Mt. Sinabung (2013-2020)

Volcanic soils in Indonesia cover an area of about 5.4 million ha and around 2.7 million ha in Sumatra. The prolonged eruptions of Mt. Sinabung in North Sumatra from 2013 to today eject pyroclastic materials, which blanketed and altered the soil surface. As a result, volcanic soil formed from these pyroclastic deposits. This study aims to map the chemical properties of volcanic soils after the prolonged eruption of Mt. Sinabung from 2013-2020. Thirty-four soil samples were collected at a depth of 0-20 cm according to the grid sampling system with an interval of 1x1 km covering an area of about 4,500 ha. Regression kriging (RK) was applied to predict the soil’s chemical properties and distribution spatially. The soil samples were air dried, sieved, and analyzed to determine soil pH (H2O and KCl), available, potential and retention P, organic carbon, total nitrogen, cation exchange capacity (CEC), and exchangeable basic cations. Soil pH (H2O) ranges from very acidic to neutral (4.14-6.52) and very acidic to acidic pH (KCl) (3.89-5.26), low to very high available-P (3.46-382.01 ppm), potential-P very low to very high (3.76-230.26 mg 100g-1), P-retention is categorized as very high (90-99%). The organic Carbon range from low to very high (1.73-13.05%), low to high total nitrogen (0.13-0.60%), low to high cation exchange capacity (11.78-97.71 cmolc kg-1), and exchangeable base cations are categorized as high with K values (1.60-2.98 cmolc kg-1). Na (3.72-7.45 cmolc kg-1) and Mg (5.79-12.15 cmolc kg-1) were categorized as high and Ca very low (0.039-0.157 cmolc kg-1). The estimated area of soils with pH between 4 and 5 is about 424.85 ha, and soils with pH between 5 and 6 covered an area of approximately 3,964.81 ha. Our findings suggest that persistent eruptions of Mt. Sinabung supply new plant nutrition, which can enrich and increase soil fertility in the future.


Introduction
Sinabung volcano is one of Indonesia's active volcanoes in Karo District, North Sumatra Province.This volcano has been dormant for 400 years and was classified as a type B volcano until it erupted in 2010.It turned into a type A volcano with a phreatic eruption type.After that, it erupted again in 2013 and has continued every year until now [1].The result of the eruption released pyroclastic material that enveloped and changed the soil surface, providing long-term benefits to the soil.
Soil that develops from volcanic material has a positive value on soil fertility so that it can be optimally optimized to increase plant productivity [2].For this reason, information is needed on the management and use of post-volcanic land use.Digital soil mapping (DSM) has advantages in predicting the soil and calculating distances from one place to another [3].Therefore, it is necessary to digitally map and estimate the observed parameter values using the Regression kriging (RK) method, which can assist in predicting the chemical properties of the soil and its spatial distribution.
Volcanic eruption activity produces pyroclastic material, a volcanic soil parent material source, and the soils are one of the world's most productive soils [4].In Indonesia, the soils area is about 5.4 million ha, and in Sumatra, it is around 2.7 million ha [2].These soils generally contain high organic matter, high P-retention, and Cation Exchange Capacity (CEC) and C/N [4] [5].The research [6] reported that on soil affected by volcanic material from the activity of Mount Sinabung for six years, the pH value of H2O at a depth of 0-5 cm and 5-20 cm was categorized as very acidic, the value of C-organic, CEC, total N, Ca, Mg, and Fe at a depth of 0-5 cm. a depth of 0-5 cm with low to very low, while at a depth of 5-20 cm total N, Ca, Mg, Fe are also low to very low but C-organic, CEC, total-P, available-P, exchange able K, total S are medium to very high.For this reason, volcanic ash from eruptions has the potential to rejuvenate soils with plant nutrients and increase soil productivity.This study aims to map the chemical properties of volcanic soil after the prolonged eruption of the Sinabung volcano from 2013-2020.

Materials and method 2.1 Study sites
This research was conducted at Sinabung volcano, located in Karo District, North Sumatra Province, Indonesia, at geographical coordinates 3°10'17.11"Nand 98°23'25.81"Ewith an elevation of 2460 m above sea level, which is the highest peak in North Sumatra.Thirty-four soil samples were collected at a depth of 0-20 cm according to the grid sampling system with an interval of 1x1 km covering an area of about 4,500 ha.Research area based on the distribution direction of the pyroclastic material Sinabung volcano eruption in 2010 to March 2020, especially the East, Northeast, Southeast, and South at a radius of 3-7 km (disaster-prone areas) in 2 Mt Sinabung Disaster Prone Areas, namely Disaster-Prone Area I at a radius of 5-7 km and Disaster-Prone Area II at a radius of 3-5 km.

Soil sampling and analysis
Sample analysis was carried out at the Laboratory of Soil Chemistry, Department of Soil Science, Faculty of Agriculture, Universitas Andalas, Padang, West Sumatra.Soil samples were taken in areas affected by the eruption of Mount Sinabung in March 2020 (Figure 1).Then the soil was air-dried, sieved, and analyzed to obtain data on pH (H2O and KCl), potential P, retention P, and available P, C-Organic, Total N, Cation Exchange Capacity (CEC), and Base Cations.Analysis of soil pH (H2O and KCl) using a ratio of 1:2.5, potential-P was measured using 25% HCl, P-retention was measured using Blackmore method [7], available-P with Bray I and II, Total N levels were measured using the Kjeldahl method, exchangeable basic cations were measured using the NH4Oac pH 7 leaching method.Cation exchange capacity was measured using NH4Oac (1 M) with a buffe.Kriging method was applied to create thematic maps and JMP-Pro software was used to statistical analyses.

pH H20 and KCl
From the area affected by the eruption of Mount Sinabung, the soil pH value (H2O) (4.14-6.52)was found to be very acidic to neutral, with an average of radius 3 (4.14),radius 5 (5.6) and radius 7 (5.37)(Figure 2a).The pH value of KCl (3.89-5.26) is very acidic to acidic with an average pH of KCl at a radius of 3 (3.93), a radius of 5 (4.41), and a radius of 7 (4.31)(Figure . 2b).Based on these values, it shows that the closer to the top of the mountain, the more acidic the soil pH.It is mainly due to the presence of volcanic ash covering the soil surface resulting from the previous eruption of Mount Sinabung.Research results from [8] show the pH value of volcanic ash is 4.54 with acid criteria.Acidic volcanic ash will produce acidic soil, which shows that volcanic ash significantly affects soil acidity [9].
The sulfur content of pyroclastic material also affects soil acidity; high sulfur (S) content in volcanic ash will produce very acidic to acidic soil pH.Sulfur is one of the factors that affect the soil's acidity in the surrounding soil due to the formation of sulfuric acid [8].  ) with low to very high criteria (1.73-13.05%)with an average radius of 3 (12.43%), a radius of 5 (7.18 %) and radius of 7 (5.91 %) (Figure 3a) have very high criteria.This high carbon content can be caused by the eruption of Mount Sinabung, which emits volcanic ash and hot clouds, causing many plants to die and the high sulfur content of the ash to become toxic to plants [10].In addition, volcanic ash also affects the level of organic Carbon; according to the statement [11], the result of weathering volcanic ash has a high capacity to bind organic C.
Meanwhile, the available P-content is low to very high (3.46-382.01), the value increase from soils at radius 3 km (36.37 ppm) < radius 5 km (92.4 ppm) and radius 7 (109.42ppm) (Figure 3b).The P value is low because P is easily leach, easily adsorbed, very mobile and retain by non-crystalline minerals such as allophane, imogolite and ferrihydrite.The non crystalline minerals are dominant component in volcanic soils.The availability of nutrients in the soil, such as phosphorus and nitrogen, is inversely proportional to the presence of allophane.According to the statement [12], the low content of available P as a result of fixation of P is thought to be caused by the content of allophane which is able to retain P up to 97.8%, and the presence of Al and Fe in amorphous form also has the ability to bind P.
For the high content of P are attributed from the release or leach of P from volcanic ash from the previous eruption of Mount Sinabung to surface soil resulting an increasing P content in the soil.The pH value of the soil also affects the availability of P in the soil.The higher the pH value, the availability of P will also be reduced by the fixation of Ca and Mg.P is very susceptible to being bound in acidic and alkaline conditions [8].Research from [13] The potential phosphate content is very high and tends to decrease with washing.In this study, radius 3 has a low P-potential value because there is still material from the eruption above the ground surface.
Soil P-retention content is categorized as very high (90-99%), more P retain at radius 3 (99.8%),then at radius 5 (97.0 %), and radius 7 (98.98 %) (Figure 4b).Higher P retention (≥85%) are common in volcanic soils and it is one of the requirements for soils to be classified as Andisols.Noncrystalline minerals are well known to retain P. The higher the allophane mineral content in the soils, the higher the P-retention.The pH value of the soil influences the high P-retention value and the content of free Al and Fe.In this study, the pH value obtained was relatively low.

Total Nitrogen and Cation Exchange Capacity (CEC)
The total nitrogen content is low to high, range from 0.13 to 0.60%, no trend found for nitrogen content in soil at radius 3 km (0.29 %), radius 5 km (0.27 %) and radius 7 km (0.35 %) (Figure 5a).Total nitrogen content is directly proportional to organic carbon because nitrogen comes from soil organic matter [10].The thickness of the ash deposits of the top layer of the soil can determine the availability of nitrogen in the soil.The thicker the ash deposits, the more plants cannot utilize the N in the soil.
Soil cation exchange capacity (CEC) is considered low to very high (11.78-97.71cmolc kg -1 ), no different exist in soils at radius of 3km (99.8 cmolc kg -1 ), radius 5 km (97.03 cmolc kg -1 ) and radius 7 km (98.98 cmolc kg -1 ) (Figure 5b).The value of the cation exchange capacity (CEC) of the soil also relates to soil pH.If the soil pH is low, then the colloid adsorption is filled with a positive charge which causes a low CEC; while the pH value is close to neutral, then the colloid adsorption is filled with a negative charge so that the cation exchange capacity (CEC) is high.When the pH is low, the basic cations are replaced by H and Al [8].On the other hand, when the pH is low resulting low CEC.Regarding the exchangeable basic cations in volcanic soils of Mt.Sinabung, they are considered high and have beneficial effects to the growth and production of crops.

Conclusion
The prolonged eruption of the Sinabung volcano from 2013 to 2020 has produced volcanic material that covers the soil surface, providing new plant nutrients that can enrich and increase soil fertility.Changes in soil chemical properties occurred after volcanic ash inundated the soil surface such decreasing soil pH, increasing the available nutrients such as P, Ca, Mg.K and S. Spatial distribution of affected soil chemical properties were estimated with regression kriging.The study provides an effective guide to monitor changes in soil properties aftermath of volcanic eruption.

Figures 7 , 8 ,
Figures 7, 8, and 9 are spatial distribution of soil chemical properties on soils affected by the prolonged eruption of the Sinabung volcano from 2013 to 2020.This study used the Kriging method, a geostatistical technique for interpolating values for unobserved locations from the observed values and values from nearby locations [16].

Figure 7 .
Figure 7. Spatial Distribution of Soil pH (H2O and KCl) in the Studied Area