Soil Carbon Storage in Forest and Agriculture Land Use in the Tanralili Watershed

Soil is a long-term store of carbon in terrestrial ecosystems and plays an important role in the global carbon cycle. Sequestration of soil organic carbon is considered as one of the climate change mitigation strategies and is related to carbon storage in the soil. This study aims to determine soil carbon storage based on land use in forest areas and dry land agriculture in the Tanralili watershed.Soil samples were taken at a depth of 0-10 cm, 10-20 cm, and 20-30 cm, repeated three times in succession purposive sampling on the use of forest land and dry land agriculture. Soil chemical properties observed are bulk density, soil organic carbon, nitrogen, and C:N ratio. Research results show that soil carbon storage is higher in forest land use compared to dry land farming. Forest land with mixed tree crop species had the highest carbon store, namely 96 tons/ha, while dry land with horticultural crop types rotated with various crops had the lowest carbon store, namely 43 tons/ha. Soil carbon accumulation is most abundant in the topsoil layer of 0-10 cm. The availability of soil organic carbon can be seen from the C:N ratio, increasing C:N will reduce the ability to absorb soil carbon.


Introduction
The use of land resources will increase along with the increase in human activities in sustaining life, currently most of these activities have been, are being and will continue to take place on land.Soil is part of the environment as a natural resource that plays a very important role in various human interests.The function of land eventually develops along with increasing human needs [1].
Soil is a long-term store of carbon in terrestrial ecosystems and plays an important role in the global carbon cycle because soil accumulates more carbon than is contained in plant biomass and the atmosphere.Carbon stored in the soil also contributes to preventing greenhouse gas emissions [2].
Sequestration of soil organic carbon is considered as one of the climate change mitigation strategies and is related to carbon storage in the soil [3].The more carbon stored in the soil in the form of organic carbon, it will reduce the amount of carbon in the atmosphere, thereby helping to reduce the problem of global warming and climate change [4].Soil organic carbon plays a very important role in controlling global warming because it is a carbon storage component that can reduce carbon dioxide (CO2) emissions in the atmosphere.
Currently, land use in the watershed area is increasingly massive, including land use in the form of dry land agriculture.One of the potential environmental problems that can occur on dry land is a decrease in carbon (C) stores, both on the surface and in the soil, which is not only related to global environmental problems in the form of climate change, but also local problems, in the form of soil sensitivity to erosion, high levels of nutrient leaching and low ability of the soil to store water, as well as a decrease in soil quality [5].
Decline in soil quality will be followed by a decrease in land productivity.Low soil quality is characterized by little organic matter so that the soil organic carbon content is low [6].Low soil organic 1230 (2023) 012036 IOP Publishing doi:10.1088/1755-1315/1230/1/012036 2 carbon due to changes in land use is not limited to the surface, but changes can occur in even deeper layers [7].More than 50% of global soil carbon is stored underground and used for plant growth [8].
This threat can occur in the Tanralili watershed area, where the conversion of forest land into agricultural land is increasingly widespread.Various land-based activities that exist in watersheds that bring economic benefits often result in reduced carbon stocks.If not properly planned, land use activities can contribute to increased carbon emissions or have a negative impact on economic growth and food security [9].

Study area
The research locations are in the Tanralili Sub-watershed, Maros Regency (Tompobulu District) and Gowa Regency (Tombolo Pao District), which are sub-watersheds of the Maros Watershed.Geographically, the Tanralili sub-watershed is located between 5º5' -5º13' S and 119º40' -119º55' E. Analysis of soil samples was carried out at the Laboratory of Soil Chemistry and Fertility, Department of Soil Science, Faculty of Agriculture, Hasanuddin University, Makassar.

Data collection
The data used in this study consisted of primary data and secondary data.Primary data was obtained through purposive soil sampling based on land unit maps to determine unit weight, soil organic carbon and total soil nitrogen.Secondary data in the form of spatial data consists of the Tanralili Watershed Map, the Tanralili Watershed Land Cover Map, the MapTilt Tanralili watershed slopes and Rainfall Data for the last 10 years (2011-2021).

Methodology 2.3.1. Observation location
The location of observation was determined by overlaying maps, including land cover map, slope map and rainfall map to form land unit map (as shown in figure 1).Determination of sampling location was based on the representation of land unit resulting from overlay and the accessibility of the location.

Field survey activities
Field survey activities include preparation of observation plots carried out using Practical Instructions for Carbon Measurement from Land Level to Landscape by [10].Soil sampling was taken by making observation plots with a plot size of 25m x 25m on each representative land unit.Soil samples were taken using a soil sampler box measuring 10x10x10 cm.The soil sampling method was carried out on a plot by selecting 3 points by purposive sampling .Soil sampling was carried out at depth intervals of 0-10 cm, 10-20 cm, and 20-30 cm.From 3 points soil samples were taken and then composited, namely mixing the soil samples according to their depth.Then analyzed in the laboratory.

Calculation of soil carbon in land use
Identification of soil carbon in land use is estimated based on the weight of soil mass and organic carbon content of each soil layer is calculated using the formula [10]: The weight of the soil is calculated with the equation: Where W is total dry weight of the soil sample (g), W1 is total wet weight of soil (g), W2 is wet weight of soil sample (g), W3 is dry weight of soil sample (g) and V is soil volume in ring soil sampler (cm 3 ).
Soil mass is calculated by the equation: Where Mtn is mass of soil in soil layer n, for example 0-10 cm layer, Vtn is soil volume in layer n and BIn is weight of the content on layer n.

Soil carbon content:
After obtaining the results of organic carbon laboratory analysis (%), organic carbon content per ton of soil is calculated, assuming for example in a layer of 0-10 cm, results of organic carbon laboratory analysis are obtained with a value of 1%, meaning that in a mass of 100 g of soil there is 1 g C. Thus 1% C = 10 g C/kg soil = 0.01-ton C/ton soil [10].This applies to the next layer of soil.
The carbon content of a 1 hectare soil is calculated by the equation: Where MCn is mass of soil carbon (tons) at layer n, Mtn is mass of soil (tons) at layer n, MCn is mass of soil carbon (tons) at layer n.
To find out carbon content in a 0-30 cm layer of soil covering an area of 1 ha, add up the final results of soil carbon calculation in each layer.

Soil Carbon Storage in each Land Use
In this study, soil carbon storage was divided into two categories of land use, namely forestry and agriculture.In detail, soil carbon storage in forest land use can be seen in table 1 and agricultural land use is seen in table 2. Soil carbon storage in each research location is closely related to land use.The existence of soil carbon is very dependent on the organic matter in the soil, if the soil organic matter is high, then the soil organic carbon content will also be high.Organic carbon in the soil can be stored if there is vegetation on the soil surface as a source of organic matter, decomposition processes and the environment.[11] state that Soil Organic Matter and Soil Organic Carbon determine the availability of carbon in the soil.
The results showed that (table 1 and table 2), across all types of land use soil carbon concentrations decreased with increasing soil depth due to higher accumulation of litter in the surface layer of the soil than in the deeper layers of the soil.[12] explained that organic matter tends to be enriched in the topsoil because most of the supply of soil organic carbon comes from litter in the surface layer, which decreases with increasing soil depth, so the soil organic carbon content gets lower.Soil with a fairly high organic matter content can affect the weight or density of the soil.This condition is influenced by the presence of organic matter on the soil surface.Adding organic matter to the soil increases the pore space of the soil, creating a loose soil structure that reduces soil weight.Increasing soil organic carbon can increase the stability of soil aggregates, reduce density, and reduce soil density [13].
Results The study found that soil carbon stores differed between unmanaged (forest) and managed (agricultural) land.Of all the research locations, both forest land use and agricultural land use categories (table 1 and table 2), which have high soil carbon in the forest land use category (table 1), at land location 5 is 98 tons/ha, and the lowest on land 4 is 80 tons/ha.In the category of agricultural land use (table 2) the highest soil carbon is on land 6 is 78 tons/ha, and the lowest is on land 7 is 43 tons/ha.
Forests containing old vegetation components will continue to provide biomass and litter which will then undergo weathering and become available carbon in the soil.Compared to active agricultural land with annual crops, the biomass is rather low, so the carbon content is also low.[14] in a forest litter biomass will continue to be contributed by fallen leaves and branches.This litter enriches the soil with organic matter which will break down into humus, so it is not surprising that the soil carbon stock is highest in forests.A study by [15] on the Gacheb watershed in Ethiopia showed that forest soils had the highest total carbon stock at 412 mg/ha compared to agricultural land at 357 mg/ha.Research by [16] revealed that in less than 10 years, the conversion of natural forest to corn fields in Central Sulawesi's Lore Lindu National Park resulted in a 29% decrease in soil organic carbon at a depth of 0-10 cm and decreased at a depth of 20 cm by 7%.
Comparison of soil carbon in forest land use categories (table 1) at each observation location on different carbon stores.The density of trees affects the amount of litter produced, the higher the density of trees, the more litter is produced.Variations in litter productivity can be caused by differences in tree age, crown or stand density [17].
The location of land 5 has a higher soil organic carbon content than land 4 and land 8 which is thought to be affected by differences in litter content.From these locations, land location 5 with high vegetation diversity and density can increase soil organic carbon through carbon supply, thus providing a rich distribution of organic matter compared to land 4 and land 8 locations which are vegetated with pine trees, even though at the time of observation in the second field this location has a thick forest floor due to accumulation of litter, this indicates that the process of litter decomposition is very slow compared to land 5.According to [18] although there is a lot of pine litter on the forest floor, this is not related to C content -high organic.Pinus merkusii leaf litter has a very high lignin component so it is recalcitrant.Litter with these properties is difficult to degrade by decomposer organisms.Some studies also show that natural forests contain more carbon than pine forests.Research by [11] in Mount Merbabu National Park shows that forests with natural vegetation have the highest organic carbon content (4.89-7.62%),while pine forests contain organic carbon (2.45-2.85%).Thereby also research done [19]in IR Forest Park.H.Juanda, the mixed forest has a soil carbon stock of 21 Mg/Ha while the pine forest has the lowest soil carbon stock of 14.9 Mg/Ha.
In the location of land 4 and land 8, even though they have the same vegetation they have different soil carbon stores.Land location 8 had higher soil carbon stores, this was due to the growing ground cover canopy at location and intercropped with coffee plants which also contribute litter.The existence of a ground cover canopy will protect the soil from leaching and erosion [15].
In the category of agricultural land use (table 2) as wellown differences in soil carbon stores.These differences can result from the management or treatment of each land.Land 3 and land 6 are included in the Agroforestry category.Agroforestry and forestry have similarities in that the organic carbon content comes from litter, but the density of vegetation in agroforestry is not as high as in forest land, so that the soil carbon stores in this location are higher than those in land 1, land 2, and land 7.These three locations are intensively managed land.The low carbon content of agricultural soil is thought to be because farmers do not return crop residue and tend to use more inorganic fertilizers to increase crop yields.Anthropogenic activities, such as the use of chemical fertilizers and herbicides, can reduce the soil organic carbon content in the topsoil [20].[21] on agricultural land, litter biomass from plant residues is one of the soil organic carbon inputs, so returning plant residues to the soil is the key to preventing loss of soil organic carbon on agricultural land [22].
Land 7 has the lowest carbon storage of all land uses due to intensive tillage by farmers.Tillage is a very important factor in soil carbon storage.Intensive land cultivation will break down soil aggregates which will release carbon stored in the soil.Intensive tillage can disrupt macro-aggregates (250-2000 m) that control soil aggregates and soil organic carbon stability, making soil organic carbon protected in macro-aggregates more susceptible to decomposition or mineralization [23].Increasing active organic carbon and restoring soil fertility will take longer for soil structure and function to recover gradually [24].

The relationship between soil chemical properties and soil carbon availability
The relationship between soil properties and soil carbon availability at forest locations can be seen in table 3 and at agricultural locations in table 4.
From tables 3 and 4 it can be seen that the soil at the study site has medium-high soil organic carbon, while the total nitrogen content is low.At all sites, soil organic carbon and total nitrogen decreased with increasing soil depth.The higher organic carbon content in the topsoil is due to the decomposition of plant residues which are mostly spread over the topsoil as well as Total Nitrogen which has a similar distribution pattern [20].
Availability of soil organic carbon and total nitrogen in forests and agroforestry through the continuous defoliation of litter from trees and shrubs at the site.On agricultural land, removal of crop residues, land management, washing and absorption Plants impact the availability of organic carbon and soil nitrogen [15].
During seasonal defoliation and senescence, carbon and nitrogen in plant tissues contribute significantly to the soil surface and underground in the form of detritus.Microorganisms need carbon as an energy source and nitrogen to form proteins.Microorganisms will fix nitrogen based on carbon availability.When carbon availability is limited (the C:N ratio is too low), not enough compounds are available as an energy source for microorganisms to fix all the free nitrogen.In this case, a certain amount of free nitrogen is released in the form of NH3 gas.If carbon is available in excess (high C:N ratio) and nitrogen is limited, then this becomes a limiting factor for microbial growth [25].The ratio of carbon to nitrogen (C:N) is very important in providing nutrients to the soil.Soil organic carbon decomposition is influenced by the balance of carbon, nitrogen, and soil microorganisms, so soil C:N greatly affects the rate of soil C-organic decomposition [26].
Residues with a high C:N ratio generally decompose more slowly than those with a low C:N ratio.Sources of organic matter that decompose quickly will be able to increase soil nutrient content, while sources of organic matter that are difficult to decompose will increase the soil C:N ratio, which means the soil is low in nutrients [27].
In the research results (table 3 and table 4), C:N ranges from high to moderate.In all land uses, the C:N ratio at the soil surface is higher because the weathering is not perfect in the litter that still resembles its original shape, increasing the C:N soil depth will decrease indicating the level of weathering is complete, the litter is already integrated with the soil and its original shape is no longer visible.Components of organic matter that are easy to decompose are easier for microorganisms to use, thereby increasing the carbon content of soil which is difficult to oxidize [28].
In the forest land use category (table 3) there are differences in organic carbon and total soil nitrogen in each land.The availability of organic carbon and total soil nitrogen at each location produces a different C:N.Higher soil C:N was found in land 4 and 8 compared to land 5.The C:N ratio in this forest type is closely related to the dominant vegetation type in those locations.[27] explained that the high lignin content in pine litter resulted in slow decomposition because lignin was resistant to biological, enzymatic and chemical degradation, whereas broadleaf tree litter was easily broken down by soil decomposers, especially earthworms [29].These results corroborate that higher soil C:N ratios were found in coniferous forests compared to Danish broadleaf forests [30].[31] C:N ratio the lowest on mineral soils found in broadleaf forests in the species Robinia pseudoacacia L. with a C:N ratio of 12, and Alnus glutinosa L. with a C:N ratio of 13.5 while the highest C:N ratio was found in pine stands with spiky leaves of the species Scots pine with a C:N ratio of 22 and Lodgepole pine with a C:N ratio of 25.5.In the category of agricultural land use (table 4).Each location has a different content of organic carbon and total nitrogen.Land 3 and land 6 are included in the Agroforestry category.Agroforestry is similar to forest where C-organic and nitrogen comes from litter.The C:N ratio at these two locations is lower than that of land 1, land 2 and land 7 which are areas with intensive agriculture.
Compared to agroforestry lands, the c-organic content in intensive agricultural lands is lower due to less organic matter being returned to the soil.Organic carbon only comes from the gradual decomposition of the remaining stems and roots of corn plants which were pruned after harvesting in field 1 and field 2. In field 7 there was no return residue from plants.Fertilization carried out by land owners, namely Urea and NPK fertilizers, contributes to the presence of total nitrogen in the soil.Nitrogen in inorganic fertilizers (Urea and NPK) is mobile in the soil, easily soluble, easily lost or evaporated to atmosphere, and quickly absorbed by plants because it is available in the form of NH3 and NH4 resulting in competition between plants and microbes for nitrogen.The imbalance of organic carbon and nitrogen at these three locations results in C:N in the high category.
Research by [32] revealed that a longer rate of decomposition by soil microarthropods showed a higher C:N ratio in inorganic agricultural land.The increase in the decomposition rate is also due to the instability of aggregate macro soils, which are disturbed by intensive tillage and crop rotation [33].

Conclusion
Soil carbon storage in each land use at the research location is different and is closely related to land use.The forest land use category has the highest soil carbon storage compared to the agricultural land use category.
The high soil carbon storage in forest land use is due to the high litter biomass on the forest floor.The highest soil carbon content was in the secondary dry forest of mixed tree species with a value of 96 tonnes/ha and the lowest in industrial pine plantations with a value of 80 tonnes/ha.This difference is influenced by the chemical nature of the litter on pine needles which is difficult to decompose because it contains lignin.
In the use of agricultural land, the highest soil carbon storage values were in agroforestry land for mango and teak plantations and clove plantations with values of 77 tons/ha and 78 tons/ha.The lowest soil carbon storage is found in horticultural land with a value of 43 tons/ha.The difference in soil carbon storage is caused by differences in vegetation and land management that exist in agroforestry and horticultural farming systems, thereby affecting supply of organic litter as a source of carbon reserves.Agricultural practices carried out on horticultural land rotated produce low soil carbon due to the use of inorganic fertilizers, intensive tillage, and no return of plant residues in the form of litterin land.Soil carbon storage can be seen from the C:N ratio, an increase in C:N will reduce the ability of the soil to absorb carbon.

Table 1 .
Soil carbon storage in forest land use Notes: Criteria based on Balittan in 2009.h is high, m is medium, l is low.

Table 2 .
Soil carbon storage in agricultural land use Notes: Criteria based on Balittan in 2009.h is high, m is medium, l is low.

Table 3 .
Chemical properties of soil on forest landNotes: Criteria based on Balittan in 2009.h is high, m is medium, l is low.

Table 4 .
Chemical properties of soil on agriculture land Notes: Criteria based on Balittan in 2009.h is high, m is medium, l is low.