The study of soil organic carbon rate in Gunungsewu Karst Area, Pracimantoro District

Karst landscape has a notable role in carbon cycles. The role isn’t merely through the karstification (solutional) process but also on soil since Soil Organic Carbon (SOC) is the biggest terrestrial organic carbon stock. The purpose of this research was to discover characteristic and calculate organic carbon within rainfed paddy, dry land seasonal crops, shrubland, and homogenic forest in Gunungsewu Karst, Pracimantoro District. The soil sampling method was determined by purposive, then computed through a laboratory test to figure out the soil texture, permeability, bulk density, particle density, porosity, and organic carbon percentage. C-organic assessment refers to the calculation by National Standardization Agency. Permeability is the obvious parameter that was discovered. Soil permeability in the agricultural land uses has a higher value than in non-agriculture land uses. This matter is caused by soil tillage as a human agriculture activity. The result of the SOC assessment is that non-agriculture land use has a higher rate than agriculture land use. SOC within the homogenic forest was the highest rate (30,03 tonnes ha-1), and the dry land seasonal crops was the lowest rate (18,9 tonnes ha-1), whereas the rainfed paddy and shrubland, respectively 21,63 tonnes ha-1 and 23,62 tonnes ha-1. It caused by the litter on the agriculture land use was slightly than the litter that piled up on the non-agriculture land use because the land was tillage and cleaned as an agriculture activity. Research like this should be undertaken in order to gain information about soil or karst roles within carbon cycles as a mitigation to climate change.


Introduction
Karst is the term used to describe a special type of landscape that caves and complex underground water system that is developed on soluble rock [1,2].Karst landscapes have a well-developed secondary porosity caused to rock dissolution by rainfall.
Soil has an important role as a carbon reservoir.Soil organic carbon (SOC) is the largest carbon reservoir in terrestrial ecosystems [3,4] and may play an important role in biosphere feedback with increased atmospheric carbon dioxide (CO2), which makes the world warmer [5].Carbon dioxide is the gas that contributes the most to global warming.Global warming and climate change are caused by the greenhouse effect [6,7].Greenhouse gas enhancement, especially carbon dioxide, is the main factor that contributes to and has big impact on climate change in the 21 st century [8].Control of emissible carbon in the atmosphere needs to be a concern for all parties as a mitigation act for disasters caused by climate change.For instance, maintaining the storage carbon stock in the soil.
The soil in karst areas is one that needs to be examined regarding its role as a carbon store.The role of karst areas in Indonesia in absorbing carbon dioxide has a strategic function related to the ITCZ 1314 (2024) 012024 IOP Publishing doi:10.1088/1755-1315/1314/1/012024 2 (Intertropic Convergence Zone) phenomenon that causes the flow of air masses towards the tropics [9,10].Land use changes will affect the soil characteristics and its organic carbon potency.
Pracimantoro District is a region with most of the area is karst landscape.Gunungsewu karst area, which is located in Pracimantoro District, has 12.022 hectares of area, with the result that it should have a high potential for SOC content.However, the wide area of agricultural land may affect the content of SOC which should be stored in the soil.Interaction between the biotic, abiotic, and also cultural elements causes the occurrence of the process of carbon transfer and storage [11,12].The continued of population growth tends to utilise the vulnerable landslide as agricultural land [13,14].Therefore, is important to measure the soil organic carbon stock for each land use so that it can be used as a reference for decision-making in the context of mitigation climate disasters.

Material and Methods
This research uses direct measurements and collects soil samples from four different types of land use (rainfed paddy, dryland seasonal crops, shrubland, and homogenic forest) at Gunungsewu Karst Area in Pracimantoro District, Wonogiri Regency.The Surakarta-Giritontro Sheet of Java's Geological Map, is used to identify the region of the karst landscape.The sampling method is purposive, with land use as a differentiating aspect that is the kind of vegetation covers, namely rainfed paddy, dryland seasonal crops, shrubland, homogenic forest.A soil sample has been collected using disturbed and undisturbed techniques at 10 cm depth.

Figure 1. Soil Sample Location Map
The sampling location can be seen in Figure 1.Sample was collected in two different areas.The first area is located in the dry valley of the Bengawan Solo Ancient River, and the second area is located in the karst valley and hills.The coordinates and land use type of soil samples are listed in the Table 1.Direct mensuration of all soil sample was made to measure the coordinates, elevation, slope, land use and land cover, and pH.Laboratory test were made to find out the texture, bulk density, particle density, porosity, permeability, and SOC percentage.The measurement method for each parameter is listed in Table 2.

Coordinate and elevation
Coordinates and elevations were taken using a Global Positioning System (GPS) with WGS 1984 Universal Transverse Mercator (UTM) zone 49S system coordinates.

Slope
Abney level and range poles are used to measure in degrees the rate of slope at each section of the sample spot area.The measurement result was converted to percent and classified based on slope class, according to Chow [15] (Table 3).

Land use and land cover
Land use and land cover is recorded by means of observation.

Texture
Texture was tested using the hydrometer method which generates three percentages of texture fraction (silt, sand, and clay).The test result will be calibrated by the soil texture triangle (Figure 2).

Permeability
Soil permeability was measured using the De Boodt method based on Darcy's law.The measurement results will be classified based on the soil permeability classification according to Uhland and O'Neil [16] (Table 4).

Bulk density and particle density
Bulk density is known by the dry weight of soil divided by its volume including voids.Particle density defined as the dry, solid mass of soil particles per unit volume.Particle density is measured using pycnometer method.Both of bulk density and particle density be avowed in gram per cubic centimeter.

Porosity
The porosity calculation can be formulated based on Kurnia et al., 2006 [17] as:

Soil organic carbon
Soil organic carbon rate is measured using the Walkley and Black method [18].Organic carbon is oxidized in dichromatic acid (H2Cr2O7) by a back titration of the remaining dichromic acid with an appropriate indicator [19].The calculation of soil organic carbon content is obtained by multiplying soil depth, bulk density, and SOC rate, based on the Indonesian National Standards Bureau [20].the equation is as follows: • SOC rate in gram per square centimetres (Cr) equation: Cr = d ×  × % C − organic 1 d is the depth of soil sample in centimetre, ρ is bulk density value in gram per cubic centimetres, and % C-organic is percentage of carbon organic based on the laboratory test result.
• SOC content in tonnes per hectare (Cs) equation: Cs = Ct × 100 2 Cr is the SOC rate in gram per square centimetres and 100 is a conversion factor from grammes per square centimetres to tonnes per hectare.
The results of measurements of soil organic carbon content were then grouped based on the classification of soil chemical characteristics (Table 5).The results of all measurements will be analyzed through descriptive comparative analysis and spatial analysis to explain the characteristics and content of SOC in each area of land use.

Results and discussion
3.1.Soil characteristics 3.1.1.Land use, slope, land cover, and type of soil.The result in Figure 3 showed a few differences in each land use at the first research area.Based on the topographic approach, homogenic forest is located at the crest section, rainfed paddy at the foot or bottom section, and dry land seasonal crops and shrubland are located at the slope section.All of the land uses have the same type of soil, namely Mediterranean soil.This condition related with previous research that Gunungsewu Karst was dominated by Mediterranean Soil [22].Homogenic forests have a very steep slope and are covered by teak trees.Dry land seasonal crops (moor) and shrubland have a very steep slope, and the moor is covered by maize and peanuts, while shrubland is covered by shrubs.Rainfed paddy has a flat slope with paddy and maize coverings.The result in Figure 5 showed a few differences in the second research area.Homogenic forest is located at the crest section, rainfed paddy and shrubland are at the foot or bottom section, and dry land seasonal crops is located at the slope section.Mediterranean soil is the type of those four land use types.Red soil is allegedly formed by recombination of weathering result of primary minerals from limestone residues as well as volcanic material that reaches research area [22].Homogenic forests have a very steep slope and are covered by acacia trees.Dry land seasonal crops (moor) have a very steep slope and is covered by maize and peanuts.Rainfed paddy has a flat slope with paddy and maize coverings, while shrubland has a flat slope too, it covered by shrubs.3.1.2.Texture.The texture examination results of all soil samples show similarities, i.e., they all have a high percentage of clay (Figure 7).The ratio of the three fractions indicated that entire sample had a clay texture, as shown in Figure 8.  3.1.3.pH, C-organic, porosity, particle density, bulk density, and permeability.Figure 9 summarizes the characteristics of the soil based on land use in graphical form.Soil characteristics in the four land uses are relatively uniform without any significant differences.Soil pH in the four land uses is slightly acidic, with the lowest value of 6.35 and the highest value of 6.9.The lowest porosity is 51.935% volume, and the highest is 53.94% volume.The highest particle density is 2.51 g/cc, and the lowest is 2.34 g/cc.The highest bulk density is 1.185 g/cc, and the lowest is 1.16 g/cc.The soil permeability of each type of land use is different according to the factors that influence it [23].The clear difference is in the permeability of the soil, where shrubs and homogenic forest have permeability levels that are not too far apart, while the other two land uses show quite clear differences in values, namely rainfed paddy and moor.Dry land seasonal crops have a permeability value of 3.495 cm/hour, rainfed paddy has a value of 1.915 cm/hour, and the use of homogenic forest land and shrubland are in the range of 1.45 cm/hour.Differences in soil characteristics tend to be clearly seen in agricultural land (rainfed paddy and dry land seasonal crops) and non-agricultural land (homogenic forest and shrubland).This trend is caused by agricultural activities, for instance soil tillage, which makes the agricultural land crumblier.

Soil organic carbon
3.2.1.SOC percentages.Figure 10 shows the result of the SOC measurement.Significant differences were found between agricultural land use and non-agricultural land use.Both the values of each soil sample and the average of land use types show the same trend.Homogenic forest has the highest SOC rate at around 2.6% (medium class).According to Diah Auliyani [14] who mentions that "a pioneer such as Acacia mangium is expected to improve the physical, chemical and biological properties of the soil".Dry land seasonal crops (moor) have the lowest rate at around 1,595% (low class), shrubland has a value of around 2.010% (medium), and rainfed paddy at around 1.895% (low class).This trend is caused by agricultural activities, like soil tillage, which make the agricultural land have less organic matter, namely litter, biomass and necromass, than non-agricultural land uses.In this research, we found that soil organic carbon in Pracimantoro Karst was not controlled by topographic condition.Variation of SOC in study area was relatively controlled by type of landuse.It can be seen from the Figure 9 that every type of landuses have different value of SOC.This condition is relatively different with previous study conducted in Gunungsewu Karst especially in Wonosari [10] and Biduk-Biduk Karst [24].The two previous research found that SOC in karst area varied in different topographic condition.SOC in crest, slope, and foot/ bottom have different value due to variation of landuse in every topographic condition.In contrast, in study area which have similar landuse condition in every topographic condition.Therefore, it is possible that SOC in study area was not varied in topographic conditions.
The rates of SOC in grammes per square centimeter Figure 10.Homogenic forest consistently has the highest SOC rate level in grammes per square centimetre.homogenic forest has a value of around 0,3003 gram/cm 2 .Moor or dry land seasonal crops have the lowest rate at around 0,189 gram/cm 2 , shrubland at around 0,2361 gram/cm 2 , and rainfed paddy at around 0,2163 gram/cm 2 .It is caused by the SOC percentage of homogenic forest is also the highest, followed by shrubland, rainfed paddy, then moor as the lowest.The SOC rates affect the value of organic carbon in grammes per square centimetre later.The outcome of conversion to tonnes per hectare (Figure 11) shows that homogenic forest (30 ton/ha) has the highest value constantly, followed by shrubland (23,61 ton/ha), rainfed paddy (21,63 ton/ha), and dry land seasonal crops (18,9 ton/ha) at the lowest rate.

Content of SOC.
The estimation of total SOC content (Table 6) stored in the soil shows that dry land seasonal crops (moor) become the land use type with the biggest soil carbon organic content.This is due to the fact that the area of moor is the largest in comparison to other land uses.Dry land seasonal crops have around 3.838,940 ha, and the SOC content stored within 10 cm of soil depth is around 32.703,18 tonnes.The land use with the second largest SOC content is homogeneous forest with an area of 2.031,968 ha and a SOC content of around 61.020 tonnes, followed by rainfed paddy with an area of 1.511,936 ha and its SOC content of around 32.703,18 tonnes, and shrubland is the land use with the lowest area and SOC content, which is only 233.556 ha and 5.514,25 tonnes of SOC content.The area of land use also influences the SOC content stored in it.Distribution map of potential soil organic carbon conten tbased on land use in Gunungsewu Karst Area Pracimantoro District, Wonogiri, Central Java (Figure 12) shows the possibilities of soil organic carbon content at each land use area.The value has been represented using a graduated symbol, where the size of the symbol indicates the value of the SOC content.

Conclusions
Characteristics of each land use indicate significant differences in land cover, slope, and permeability, while the other parameters show a similar trend.In general, significant differences appear between agricultural land uses and non-agricultural land uses.This trend is caused by agricultural activities, for instance soil tillage, which makes the agricultural land crumblier, and affects the diversity of vegetation.
Soil organic carbon which is stored, also shows the same trend.Agricultural land uses have lower rates than non-agricultural land uses.It is also caused by agricultural activity that reduces the organic matter in the soil (litter, biomass and necromass).whereas organic matter has a role as an important carbon pool in the ecosystem [25].

Figure 8 .
Figure 8.The ratio of each texture fraction has been calibrated on texture triangle.The black dot represents the texture fraction ratio for each sample.

Figure 10 .
Figure 10.SOC percentages of all samples.

Figure 11 .
Figure 11.Bulk density, SOC per-centage, and SOC rate in grammes per square cen-timetre based on each land use.

Figure 12 .
Figure 12.SOC rate in tonnes per hectare.

Figure 13 .
Figure 13.Distribution map of potential SOC content in each land use

Table 1 .
Description of Sample Location

Table 2 .
The test method of soil sample.

Table 6 .
Total SOC content in