Assessment of carbon and nitrogen distribution on several soil orders for sustainable management in dryland farming: A Case Study in Aceh Besar Regency, Indonesia

The study was based on the distribution of soil carbon and nitrogen orders in a dryland farming system in Aceh Besar District, Indonesia. Soil samples were taken to represent 6 (six) soil orders in the drylands of Aceh Besar, namely: Andisols, Entisols, Inceptisols, Mollisols, Oxisols, and Ultisols. For each soil order, 5 observation profiles were made, and each soil profile was sampled at 7 (seven) soil depths from 0-100 cm, namely: 0-0.05 m; 0.05-0.10 m; 0.10-0.20m; 0.20-0.30m; 0.03-0.50m; 0.05-0.70 m; and 0.07-1.00 m so that there are 210 soil samples. The Walkley and Black method was used to analyze the soil’s carbon content, while the Kjeldahl method was used to determine the soil’s total nitrogen. The total C and N content of the soil in the dryland of Aceh Besar varies greatly depending on the soil order. The highest distribution of C and N in the soil profile is found at a depth of 0.10-0.30 m. Soil C and N content at a depth of 0-1.0 m in the Andisols order are higher than other soil orders. The C content of the topsoil (0-30 cm) ranged from 0.7-6.9% while the N content varied from 0.07-0.56%. The order of N and C content of soil according to soil order is: Andisols>Mollisols>Entisols>Inceptisols >Oxisols>Ultisols.


Introduction
Indonesia is a tropical country with a climate that is particularly susceptible to land degradation due to its high rainfall and hilly and mountainous terrain nature.Many drylands in Indonesia have been classed as marginal drylands, which are distinguished by acidic soil characteristics, due to the extremely high intensity of soil weathering assisted by mild temperatures [1].Marginal dryland or sub-optimal land are terms frequently used to describe this dryland region.Only roughly 56.0 million ha of the entire 102.8 million ha of acid soil are appropriate for agricultural development; the remainder is land that is not suitable and is designated as a special area, a protected area, or a conservation area [2].Because these barriers can be removed with the use of fertilization technologies, the chemical qualities of the soil in this dryland are typically not employed as a limiting factor [3].
Soil erosion is a problem that is deemed serious, particularly on land that has been converted from forest to cultivation and has hilly, mountainous, or steep terrain.Even though the majority of this property is unsuitable for farming, the community is quite active in managing it, particularly for the planting of perennials or plantation crops.The main factor contributing to soil degradation in this region 1230 (2023) 012062 IOP Publishing doi:10.1088/1755-1315/1230/1/012062 2 is erosion [4], as the process of losing the fertile top layer of soil leads to a loss in soil productivity, soil carrying capacity, soil quality, and the environment [5].Because of this erosion, the topsoil, which is nutrient-and SOM-rich, is lost to surface runoff and is replaced with discolored, poor-quality soil [6].
When farmers who cultivate land in this dryland system disregard the rules of soil conservation, the issue is worsened.Such agricultural methods are widely used throughout Indonesia, notably in the Aceh area.According to the estimation by Umar et al. in 2021 [7], the sub-optimal dryland in Aceh Besar District, which has an area of 2,974.12km 2 , is approximately 239,484.63 ha.The soil orders Entisols, Inceptisols, Ultisols, and Oxisols, as well as the orders Andisols and Mollisols, are all present in this sub-optimal dryland [9,10,11].According to the findings of earlier research, the majority of the soils that predominate in Aceh Besar's dryland region have poor fertility levels [12] and a low to moderate soil quality index [13].Recent research findings suggest that several soil quality indicators, including low soil C [14], low CEC, low base saturation [15], and limited availability of nutrients for plants [9], might be the cause of soil infertility in some drylands.The availability of cation nutrients like Ca, Mg, and K are also reduced due to the low SOM content, which is shown by the low negative charge on the surface of soil colloids [16,10].As a result, they are less available to plants [17] so leaching is easy.
Maintaining adequate SOM levels and ensuring effective nutrient biological cycles are crucial for successful soil management and strategies to increase productivity in dryland agriculture [18].These strategies include fertilizer applications, agricultural practice conservation, and maximizing the efficiency of agronomic use of nutrients.A study is required to ascertain the amount of soil organic matter present, particularly C and N distributions in each soil order.To manage soil in dryland farming systems in Aceh Besar District, Indonesia, our research will evaluate the amount and distribution of C and N in various soil orders.

Description of the Study Area
This study was carried out on a dryland area measuring 239,387.91 acres in Aceh Besar Regency, Aceh Province.To examine the amounts of C and N in the soil, six soil samples from sub-optimal dryland in Aceh Besar District were collected.In the drylands of Aceh Besar, six soil orders have been studied: Andisols, Entisols, Inceptisols, Mollisols, Oxisols, and Ultisols.Inceptisols were obtained from Seulimum and Jantho Subdistricts, Andisols were obtained from Saree, Lembah Seulawah Subdistrict, Entisols were obtained from Jantho Subdistrict, Mollisols were obtained from Mesjid Raya District (Lamreh, Krueng Raya), Oxisols were obtained from Lembah Seulawah District, and Ultisols were obtained from Cucum Village

Soil Sampling and Analysis
Based on the findings of observations and observations of the soil profile at the sites plotted by a working GIS map, the soil order in Aceh Besar's dryland is determined.At seven different depths, namely 0-0.05 m, 0.05-0.10m, 0.10-0.20 m, 0.20-0.30m, 0.03-0.50m, 0.05-0.70m, and 0.07-1.00m, soil samples were collected.After being air-dried and pulverized, the soil samples were employed as analytical samples.By oxidizing the soil with a solution of 1N potassium bichromate and H2SO4, the Walkley-Black method for measuring soil carbon was used to analyze soil carbon levels.Titration with FeSO4 was used to measure the soil's C content.Using the Kjeldahl method, which involves structuring the soil with sulfuric acid, the total N in the soil was determined.Distillation was used to calculate the N content of the digesting solution [19].

Data Analysis
Descriptive statistics and analysis of variance (F test) were used to assess data from field observations and laboratory analysis.

Contents of Soil Carbon
The analysis of soil carbon content in each soil layer from different soil orders in Aceh Besar revealed that while soil C content varied significantly between soil orders, particularly in the upper soil pool, the overall distribution pattern of changes in soil C content tended to decline.with soil depths ranging from 0 to 1 m (Table 1).According to Table 1, the average SOC is higher in the Andisols order's various soil profile levels (0-1.0 m) than it is in other soil orders.In the Andisols layer between 0.0 and 0.05 m, the average SOC content was 51.85±3.26g kg -1 .Again rising to 68.06±4.02g kg -1 at a depth of 0.05-0.1 m, the SOC content dropped from 62.55±3.77g kg -1 to 30.29±3.00 g kg -1 or lower at a depth of 0.10-0.20 m to 0.70-1.0m.The SOC content in the Andisols profile is said to range from extremely high to high criteria based on these data [19].These soils are formed from volcanic ash material [16].After weathering, these materials form amorphous minerals and amorphous imogolites [20,21].As amorphous alumino-silicate minerals, these soil fractions have a very high specific surface area [22].They can absorb organic molecules and water so that the soil is dark in color, especially in the surface layer [23].These organic molecules are not easily degraded because the soil organic matter that makes them up forms complexes with the allophane minerals.This form is known as the humus-allophane complex [24].This high humus content results in a high cation exchange capacity of the soil but does not reflect the number of cations in the soil colloid because this CEC is influenced by soil pH [25].On the other hand, high humus content in Andisols will provide better soil physical properties such as looser soil consistency, easier to retain water, and better soil aeration.
The second order of soils containing high SOC is Mollisols (Table 1).The average SOC content varied from moderate criteria (36.77±3.08 g kg -1 to very low (4.79±0.44 g kg -1 ) in the deeper soil layers (from 0 to 100 cm) The order Mollisols found in the dry land of Aceh Besar is one of the soil orders with a narrow distribution [10].This soil is formed from limestone parent material with a moderate soil development level.This limestone has weathered and becomes clay that can absorb organic molecules so that the topsoil is darker in color.The SOC content is lower in the deeper soil layers.Mollisols are a soil order known as fertile soil because it contains high humus so it is very loose and is generally formed from thick grass vegetation so that it will produce good soil quality [26].
Entisols of Aceh Besar have moderate to low SOC content.This soil order is found in hilly to steep topography with a high slope of >40% and has a shallow solum thickness of ~30-40 cm [10] and belongs to the subgroup Typic Udorthens.Entisols found in dryland agricultural lands in Aceh Besar are generally found in complex forms with other soil orders such as Ultisols, Inceptisols, and Andisols.In the soils of the order Inceptisols, Oxisols, and Ultisols found in the drylands of Aceh Besar, the SOC content in the average soil profile is very low.In Inceptisols, the SOC content varied from 5.34±1.47 to 22.26±4.80g kg -1 .In Oxisols, it varies from 12.69±0.37g kg -1 (layer 0-0.2 m) to 3.62±0.37g kg -1 , and in Ultisols it varies from 12.84±0.65g kg -1 (top layer) to 4.86±0.59g kg -1 at 1.0 m depth.
The data above, these three soil orders (Inceptisols, Oxisols, and Ultisols) have relatively lower SOC content than the soils of the Andisols, Mollisols, and Entisols orders in the drylands of Aceh Besar.This variation results from variations in the soil parent material, slope, and soil genesis intensity.Inceptisols, Oxisols, and Ultisols are those found in Aceh Besar formed from acid clay parent materials that have progressed moderately (Inceptisols) to very advanced (Oxisols and Ultisols), and these soils include soils that are low in organic matter and many chemical constraints.such as acidic soil pH, low organic matter, low CEC, and poor nutrients [9,10].These soils are dominated by mixed clay mineral types between type 1:1 (kaolinite, metahaloysite), and hydrous oxides of Al and Fe so they have a generally low soil CEC so that the uptake of organic fractions lowlands [17].Following a ranking of high to low SOC content, the six soil orders of Aceh Besar's drylands are as follows: Andisols>Mollisols>Entisols>Inceptisols>Oxisols>Unltisols.The comparison of the SOC content at each soil can be seen in Figure 1.In Figure 1, it can be seen that the SOC content that crossed the low limit (dotted lines) was found in the soils of the order Andisols and Mollisols, and slightly on Entisols.In Andisols, in almost all soil Ultisols layers (0-1.0 m) the SOC content is above the low-level line, whereas in Mollisols it is only at 0-0.3 m and 0-0.2 in Entisols.In the soils of the order Inceptisols, Oxisols, and Ultisols, it was found that all soil layers contained SOC below the low-level line.This shows that for Entisols, Inceptisols, Oxisols, and Ultisols soils, addition of organic amendments is required in the form of biochar, compost, or other Ccontaining materials to increase the C content of the soil above the low level or around 3-5%.

SOC Distribution Pattern
In Figure 2, it can be seen that the distribution pattern of C in the soils of Aceh Besar dryland from the topsoil layer to a depth of 1.0 m is the same, namely the lower the soil C content, the lower the C content.This is a general condition of C distribution in the soil because the accumulation of C or SOM starts from the top layer.From the topsoil, it then decomposes into humus compounds and mixes with the soil below (subsoil).Except for particular soil layers, such as the Bh horizon of the Spodosol order, the C content in this subsoil is typically lower than the C content in the top layer [28].Despite the fact that the soil profile's C distribution pattern largely remains the same, the Andisols and Mollisols arranged in the following order: Entisols, Inceptisols, Oxisols, and Ultisols, have quite different patterns.In Andisols soil, the C content line is to the right of the x-axis line to a depth of 1.0 m, indicating that the C content is not low.However, as the soil layer deepens, the pattern of changes in C content tends to decrease.The distribution of C is about 50 g kg -1 at the soil surface, increasing at a depth of 10 cm, then decreasing again to a depth of 100 cm.However, up to a depth of 100 cm, the C content of the soil is above the low level.The distribution pattern of C in Mollisols is almost the same as in Andisols, only that the C content of the soil is slightly lower than the C content of Andisols.At a depth of 40-100 cm, the C content of Mollisols soil has entered a low level.
Furthermore, in the orders Entisols, Inceptisols, Oxisols, and Ultisols, it turns out that the distribution pattern and changes in C content from the topsoil layer to a depth of 100 cm are in areas with low levels and all fall into the category below low category.This indicates that the storage capacity of C in these soils is very low.The low content in the soils of Aceh Besar can also be seen visually from the color of the soil.The dark color of the soil matrix is found in Andisols and Mollisols, while the soil order Entisols is slightly lighter.In the other three soil orders (Inceptisols, Oxisols, and Ultisols) the soil color is yellowish brown to reddish yellow.According to [29], the color of a soil profile is closely related to SOM and processes such as oxidation-reduction, glazing, melanization, and pedoturbation.

Contents of Soil Nitrogen
The total N content of soil at various soil depths (profiles) in Aceh Besar dryland can be seen in Table 2. Andisols is one of the soil orders containing the highest total N compared to other soil orders.In Andisols it can be seen that the soil layer with the highest N content is at a depth of 0.05-0.1 m (6.95±0.55g kg -1 ) and the lowest is at a layer of 0.7-1.0m at 3.20±0.54g kg -1 .The total N in Andisols varies from 0.32-0.69%or is in the medium to high range (Soil Research Centre, 2005) [19].The second order is followed by the soil order Mollisols with a total N content varying from 0.52±0.03g kg -1 to 2.71±0.10g kg -1 or 0.05-0.27%,which is between very low to moderate criteria.The total N content in Entisols ranged from 0.42±0.06 to 2.21±0.24g kg -1 or 0.04-0.22%which was included in the moderate to very low criteria, while the soils of the order Inceptisols, Oxisols, and Ultisols the total N content was in the category low to very low category.Based on this data, it can be said that Andisols and Mollisols belong to the two soil orders that contain relatively high total N, while the other soils include soils containing low total N.One of the most important macronutrients for plant growth is nitrogen.The primary source of nitrogen in soils is organic matter.Because N cannot be obtained from minerals, soil organic matter is crucial to plants' ability to obtain N [30].The findings of this study demonstrate that the N content of the soil in various soil orders in Aceh Besar's dryland is closely related to the soil's SOC content.The ratio of C and N content in organic matter is generally around 10:1 so the ratio of N content in soil is usually around 1/10 of the soil C content [31].
Figure 3 shows the difference in the total N content at each soil depth of each soil order.Soils containing high total N are only found in the Andisols soil profile in layers of 0.05-0.20 m or layers between 5-20 cm which are marked with a black histogram, while other soil layers are included in the medium criteria.In these Andisols, it is also observed that the total N content of the soil is all above the low-level line.This shows that the N content in this soil is considered quite good because it is at a medium to high level.Except for the order of Mollisols where N is slightly above the low (critical) level, all other soil orders are below the low level.From this study, it can be said that most of the dryland in Aceh Besar has problems with nutrient N in the soil because it is below a low threshold or critical level, so it needs to be a concern in soil management.

Nitrogen Distribution Pattern
Figure 4 shows that the distribution pattern of total soil N in the six soil orders in Aceh Besar dryland is relatively similar.Slight differences occurred in Andisols and Mollisols because the N content in these two soil orders was higher than the other soil orders.The SOC distribution pattern, which declines with deeper soil layers, and the N distribution pattern are essentially identical.In Andisols, the pattern of soil N changes tends to increase from 0-0.1 m depth and at subsequent depths up to 1.0 m.The soil N content decreases again and the decrease becomes stable at a depth of 40 cm down to 100 cm.In Mollisols and other soil orders, generally similar, where the pattern of soil N subsidence began to stabilize at a depth of 20-30 cm.The pattern of changes in soil N is closely related to the pattern of changes in soil C because N is an element that is not found in soil minerals.An important source of N in the soil is from the supply of organic matter or applied fertilizers and soil microbial activity for example through symbiotic and non-symbiotic N fixation [32].Symbiotic N activity, for example through N fixation by Rhizobium sp. which is often affiliated with the roots of legumes and certain other plants [33].Many studies have reported that the N contribution from this symbiotic fixation by Rhizobium bacteria in legumes can reach 150-300 kg per hectare of N [34].The majority of non-symbiotic N fixation is done by blue algae, Azolla pinnata, and others [32].

C/N ratio
The C/N ratio is an indicator of the activity of soil microorganisms and a good predictor of the amount of N available to plants.In cases where C/N is high (>25), soil N will be immobilized by microbes so that it is not available for plants, whereas if the C/N ratio is low (10~15), N will be available for plants [31].To remain active, microorganisms require a 25-35 molar carbon-nitrogen ratio.A high C/N ratio can cause the decomposition period to be long and if the C/N is low, it will cause easy nitrogen loss [31].Typically, the SOC content of dryland farming is between 0.7% and 4% (or ranges from very low to moderate depending on the soil type.Table 3 shows that the soil C/N varied from 8.9±1.43 to 14.3±1.13 or was between low to moderate criteria [19].However, the soil C/N values were relatively not significantly different between soil orders.These data indicate that the N and C composition of the soil and the characteristics of the organic matter in the dryland of Aceh Besar have decomposed so that it is close to the C/N of microorganisms, which is around 10:1 [27].According to [27], nitrogen is more important to microorganisms than carbohydrates and protein is mostly essential for the construction of tissues, while carbohydrates provide carbon for energy.Figure 5 from the study's findings shows that, even though the C/N ratio of various soil orders observed in the drylands of Aceh Besar ranges from 8.0 to 14.3, the pattern of C and N distribution in Ultisols each soil order is comparatively similar.What is different is the content of C and N only, while the distribution pattern in the soil profile is the same.Furthermore, when compared between soil orders, it is seen that only the soil of the Andisols order contains C and N which is different from other soil orders.The Andisols soil order differs from other soil orders in terms of the composition of soil organic matter (humus), which causes this distinction.Lignin and other compounds with a high carbon content make up most of the humus.Humus contains more carbohydrates than microbial bodies which are very high in protein [35].Protein contains about 16 percent nitrogen, but soil humus is resistant to rapid decomposition so it stays in the soil more.Fresh organic matter with a low C/N ratio swiftly decomposes, causing soil C loss to happen quickly [36].Therefore, to increase soil C storage, it is necessary to amend materials that contain a lot of carbon such as compost and biochar.The soil humus carbon-nitrogen ratio is more stable and almost unaffected by the addition of nitrogen fertilizers.For sustainable humus buildup, nitrogen-rich organic matter must be applied.There are few studies that compare the carbonnitrogen ratios of soils that have been chemical vs organically treated, but there is still room for further study on this topic.

Sustainable Soil Management
A significant portion of soil's organic matter is carbon, which contributes to the soil's structure, fertility, and ability to retain water.The carbon buildup in soil aggregate is highly stable and can remain over thousands of years [37].Generally, soil organic carbon can be increased by increasing organic carbon input and/or reducing losses.Avoid practices that can accelerate soil C and N loss such as (a) excessive tillage because it can accelerate SOC decomposition and promote loss through erosion, (b) burning land due to burning can remove C, N, and other plant nutrients contained in the soil, (c) livestock grazing (overgrazing), and (d) leaving the land in an open state without cover vegetation.
Another attempt to increase inorganic C in the soil is the addition of lime carbonate because carbonates are inorganic and have the ability to store carbon for many years, whereas soil organic matter usually stores carbon for decades.The majority of SOM of about 58 percent is mainly composed of carbon and in the laboratory, that is what is measured.Little fragments of roots, stems, and leaves (less than 2 mm), organic matter that has only partially decomposed, microorganisms, and charcoal are all examples of the organic matter that makes up soil.SOC is often divided into an active component which is usually 20-40 percent C soil and a passive or stable component which is usually 60-80 percent C soil 1230 (2023) 012062 IOP Publishing doi:10.1088/1755-1315/1230/1/01206210 [37].Active SOC is often referred to as unstable SOM and this component is relatively fast in the release of nutrients so it is closely related to biological fertility and soil nutrient availability.
Related to the above concept, for soil management on dryland in Aceh Besar, efforts are needed to increase the C and N content of the soil, such as by carrying out reforestation efforts (reforestation) on abandoned lands, applying an alley cropping system to restore organic mass, agroforestry, planting cover crops, and adding organic amendments such as compost, biochar, drum fertilizer, and bio compost, as well as biological fertilizers.In addition, the dryland farming system applied in the study area needs to be directed to become a sustainable agricultural system while maintaining and improving soil quality and environmental sustainability.This strategy can be carried out by choosing models of integrated farming, organic farming, and conservation agriculture [37], to prevent and guard against accelerated soil degradation.The involvement of the government to conduct guidance and monitoring of agricultural business actors or the community (farmers) needs to be optimized.

Conclusions
Depending on the soil order, the total C and N content of the soil of Aceh Besar's dryland varied.At a depth of 0.10-0.30m, the soil profile has the largest distribution of C and N. Soil C and N content at a depth of 0-1.0 m in the Andisols are higher than other soil orders.The C content of the topsoil (0-30 cm) ranged from 0.7-6.9%while the N content varied from 0.07-0.56%.The order of N and C content of soil according to soil order is Andisols > Mollisols > Entisols > Inceptisols > Oxisols > Ultisols.To improve and maintain the soil C and N balance in the dryland farming system of Aceh Besar, it is necessary to input organic matter by returning plant biomass or recycling organic matter through the provision of organic amendments in the form of biochar, compost, manures, and by applying organic fertilizers.Conservation farming patterns such as agroforestry, alley cropping, cover crops, and mixed agriculture are also advisable.

Figure 1 .
Figure 1.Soil SOC content at each soil depth for each soil order in Aceh Besar dryland

Figure 2 .
Figure 2. Pattern of soil SOC distribution in Aceh Besar's dryland according to soil depth in each soil order

Figure 3 .
Figure 3.The nitrogen content in each soil order in Aceh Besar dryland

Figure 4 .
Figure 4. Distribution pattern of total soil N according to soil depth and soil order in Aceh Besar dryland

Figure 5 .
Figure 5.Comparison of SOC and N distribution patterns in the dryland soils of Aceh Besar Regency

Table 1 .
The

Table 2 .
Total soil N content at each soil depth for each soil order in Aceh Besar dryland

Table 3 .
Soil C and N ratio values at each soil depth for each soil order in Aceh Besar dryland