Potential Carbon Accumulation Rate in the Sediment Mangroves at Ujung Kulon National Park, Banten, Indonesia

As one of the wealthiest countries in the world in terms of the mangrove, Indonesia has a significant potential contributor to blue carbon. This study determines carbon stock (Corg) and sediment accumulation rate (SAR) to find the carbon accumulation rate (CAR) in sediment mangroves at Ujung Kulon National Park. Sampling was taken in 3 locations perpendicular to the shoreline: interior, fringe, and mudflat. The result shows that the interior area has the highest CAR (31.94 ± 3.6 g C m−2 yr−1) even though the carbon stock (90.19 Mg C ha−1) is the lowest from the other two. On the contrary, mudflat, which has the highest carbon stock (169.6 Mg C ha−1), shows the lowest CAR (17.03 ± 5.21 g C m−2 yr−1). This suggests that differences in sedimentation rates contribute to the variations in CAR.


Introduction
Blue carbon is carbon that is absorbed and stored in marine and coastal ecosystems such as mangrove forests and seagrass.Mangroves are carbon-rich ecosystems capable of storing three times more carbon per hectare than terrestrial forests [1], and most of this carbon is stored in sediments.Indonesia has 2.9 million hectares of mangroves, with total carbon storage estimated at 3.14 billion tonnes, making it one of the richest countries in the world in terms of coastal blue carbon [2,3].These significant carbon reserves in Indonesia's coastal blue carbon ecosystems hold great potential for effectively mitigating climate change.Unfortunately, this blue carbon ecosystem is under tremendous pressure from unsustainable economic development.In the last three decades, 40% of Indonesia's mangroves have been degraded [4].The loss of mangroves causes carbon stocks buried in the soil to be released into the air.This makes Indonesia one of the most significant contributors to CO2 emissions from mangrove soil due to the loss of mangroves [5].
The largest carbon pool in a mangrove is in its sediment; hence it is essential to research to keep this most significant contribution.Research on blue carbon in the sediment that has been carried out generally focuses on organic carbon stocks (Corg).However, Corg measurements alone cannot fully describe the carbon capacity in the sediment or as a comparison between locations [6].Therefore, carbon 1291 (2024) 012010 IOP Publishing doi:10.1088/1755-1315/1291/1/012010 2 accumulation rate (CAR) measurement can be a solution to find out how much carbon is stored at a specific period and quantify ongoing stock sediment.In addition, in comparing between sites, it is also easier to use CAR.Unfortunately, research on the CAR still needs to be carried out in the Indo-West Pacific area [7].
On the other hand, isotope technology can be used for blue carbon research, especially in sediments.Radioisotopes such as 210 Pb, 14 C, and 7 Be can determine the age and sedimentation rate. 210Pb can determine sedimentation rate and age until one hundred years ago.This method suits modern sediments, such as mangrove sediment [7].Several studies in mangroves, such as research conducted in Papua, Indonesia [8], and Moreton Bay, Australia [9], use 210 Pb to calculate age and sedimentation rate.The age and sedimentation rate information helps quantify the CAR, which is essential in blue carbon research.
Numerous studies have been conducted on the carbon stocks in aboveground biomass and sediment mangrove in Indonesia.In contrast, research on carbon accumulation rates in mangroves in Indonesia is relatively rare compared to research on carbon stocks.One of the carbon stock studies on mangrove sediments was conducted in Totok Bay, North Sulawesi.This research found that areas with high biomass values store more carbon in mangrove sediments, and tides affect carbon stocks in sediments [10].On the other hand, research conducted in Java and Kalimantan island shows that the carbon accumulation rate in non-degraded areas is higher than in degraded areas [11].
This study aims to analyze the sedimentation and carbon accumulation rates in mangrove sediments in Ujung Kulon National Park (UKNP).The 210 Pb sediment dating approach was used to quantify the sedimentation and carbon accumulation rate.We present data on carbon accumulation rates from low intertidal zone in the mudflat to higher intertidal zones in the fringe and interior mangroves at UKNP.Carbon accumulation rate, carbon stock, and soil chemical properties across sampling locations were assessed and compared.

Study Area
Ujung Kulon National Park (UKNP) is located in the Pandeglang district on the southwest tip of the island of Java, Indonesia.In 1992, UNESCO designated Ujung Kulon National Park as a Natural World Heritage Site.Based on SK.775/Menhut-II/2009, UKNP has an area of 122,956 ha with details of 78,619 ha of terrestrial area and 44,337 ha of marine area.The average annual rainfall is 3250 mm, with the highest rainfall generally in December and January (400mm per month) and the lowest from May to September (100 mm per month).The temperature ranges from 25-30 o C, with a relative humidity range between 65%-100% [12].As of 2020, the conservation assessment of the UKNP is considered "good with some concerns" by the IUCN World Heritage Outlook [13].
Mangroves are found in UKNP along the northern coast of the isthmus, the Cikalong river, Handeuleum Island, and Panaitan Island.Several types of species found in UKNP include Avicennia Spp, Soneratia alba, Bruguiera Spp, Nypa fructicans, Acrostichum aureum, Rhizophora spp and Lumnitzera racemosa [14].Sampling was carried out at UKNP at 3 locations perpendicular to the shoreline shown in Figure 1, to represent intertidal gradient, namely mudflat (6°49'28.0"S,105°26'44.7"E),fringe (6°49'29.9"S,105°26'40.9"E),and interior (6°49'31.3"S,105°26'37.9"E).The mudflat area is located close to the sea and has sparse vegetation.The interior area is close to the upland and is a high intertidal zone with dense vegetation.The fringe area is the transition zone between the mudflat and the interior.Sampling was conducted during the rainy season in September 2021.

Sample Collection and Preparation
The sample was collected using sediment corer in 3 locations; interior, fringe, and mudflat.Then sediment cores were sliced every 2 cm at the top 10 centimeters and 4 cm afterward.After that, the subsample was collected in a plastic bag and then transferred to the laboratory for preparation.All collected subsamples were dried at 60 o C in the laboratory using an oven until the weight was stable.Then subsamples were grounded using a mortar and sieved through mesh 60 (250mm).

Corg and Total Nitrogen
Organic carbon was measured using the Walkey-Black method, and total nitrogen using the Kjeldahl method [15,16].Both organic carbon and total nitrogen measurements were conducted in the SUA Analysis Tanah laboratory at IPB University.
2.4. 210Pb Analysis 5 gr dried sediment added with 0.2 ml radioactive tracer 209 Po, dissolved using HCl, HNO3, H2O, and H202 and dried over a water bath.10 ml HCL and 40 ml aquadest were added to the dried sample and heated for 10 minutes.Then the sample was filtered using Whatman filter paper 42 and washed using HCl 0.3N.The filtrate was dried over a water bath.Once dried, 4 ml HCl 1:1 and HCl 0.3 N were added until the solution volume reached 50 ml.The solution was platted on the copper plate for several minutes.The results in the copper plate were analyzed using the Alpha Spectrometer at the Research Center for Radiation Process Technology, Research Organization for Nuclear Energy, National Research and Innovation Agency (BRIN), Jakarta [17].
Sample analysis using Alpha Spectrometer will produce a 210 Po activity value.Both 210 Po and 210 Pb are decay products from 238 U.In the sediment, activities of 210 Po and 210 Pb can be assumed equilibrium.Hence total 210 Pb can be measured by analyzing its daughter 210 Po activity value in the alpha spectrometer [18][19][20]. 210Pb supported calculated from the constant average concentrations of total 210 Pb at the bottom layer.Then the 210 Pb unsupported value will be determined using the formula below: To determine the age and rate of sediment accumulation, we use Constant Rate Supply (CRS) model.This model assumes that the rate 210 Pb unsupported in the atmosphere is constant and allows a nonmonotonic-decrease in 210 Pb concentration [21].The equation of the CRS model is as follows : Where A0 is the total 210 Pb from the core depth to a certain depth, A is the 210 Pb value at a specific depth layer, λ is the decay rate constant, and t is the age.The value of age and sediment accumulation rate is calculated using the equation below: Where r is the rate of mass accumulation rate (kg m -2 y -1 ), and C is the concentration of unsupported 210 Pb at a specific depth (Bq/kg)

Carbon Accumulation Rate
Once the sediment dating and carbon organic calculation finish, the carbon accumulation rate (CAR) was estimated using the below equation[6]: Where (%   ×   ) is mass Corg per unit area at layer i, mt is total mass cumulative at over period t.MAR is the mean mass accumulation rate obtained from the 210 Pb calculation.

Statistical Analysis
The significant difference of Corg and nitrogen between the interior, fringe, and mudflat were analyzed using One Way ANOVA.The normality of data distribution was tested using Shapiro-Wilk.Natural logarithmic was used if the data were not distributed normally.

Result 3.1.1. Sediment Chemical Analysis
Based on the results of the calculation of organic carbon using the Walkey-black method and total nitrogen using the Kjeldahl method, the results are shown in Figure 2 and Table 1.In the interior area, the organic carbon value varies between 1.92% to 6.62%, with an average of 4.00 ± 0.30%.The fringe area has organic carbon values between 3.87% to 6.47%, with an average of 5.03 ± 0.20%.The mudflat area has organic carbon values between 4.72% to 11.23%, averaging 7.60 ± 0.55%.The one-way ANOVA analysis showed no significant difference (p>0.05) in the organic carbon value between the interior and fringe.However, the results showed a significant difference (p<0.05) between the mudflat and interior, and also between mudflat and fringe.The total nitrogen value at each location was similar.The average value of total nitrogen in the interior area ranges from 0.05% to 0.28%, with an average of 0.14 ± 0.02.The fringe area has total nitrogen of 0.12% to 0.29% with an average of 0.18 ± 0.01%, and the mudflat area ranges from 0.11% to 0.20% with an average of 0.15 ± 0.01%.The one-way ANOVA analysis showed no significant difference (p>0.05) between interior, fringe, and mudflat.The C/N ratio in the interior area varies from 15.93 to 49.25, with an average of 31.19 ± 2.21.The fringe area has a similar value to the interior, ranging between 20.03 to 41.98, with an average of 29.63 ± 1.64.In contrast with interior and fringe, the C/N ratio in the mudflat area is higher, between 38.05-68.39,with an average of 49.59 ± 2.42.The average C/N value in the mudflat area is almost double the value in the interior and fringe.The total carbon stock up to a depth of 54 cm in the interior, fringe, and mudflat areas was 90.19, 123.31, and 169.6 Mg C ha -1 .

210 Pb Activity and Carbon Accumulation Rate
Based on the analysis using alpha spectrometer, the value of 210 Pb unsupported can be seen in Figure 3.However, for the mudflat area, there are two chart trends at a depth of 0-8 cm and 10-26 cm.The first

C/N Ratio
trend indicates the potential for sediment mixing.This mixing can occur due to several things, such as bioturbation, waves, and tides.Based on the profile of 210 Pb, the CRS method was chosen to determine the sediment accumulation rate and age.This method is suitable when the trend of the 210 Pb is not a monotonic decrease and is widely used in the vegetated coastal ecosystem [22].When the mixed sediment occurs, CRS method can be applied, but this method cannot infer dates in the mixed sections.Hence the age in the mixed section cannot be calculated.The age calculation in mudflat will start from depth 10 cm.The ages of the sediments in the interior, fringe, and mudflat areas were 80.8 ± 3.8, 100.3±7.0, and 96.9±9.8 years, respectively.The average mass accumulation rates (MAR) in the interior, fringe, and mudflat were 0.07±0.01,0.06±0.01,and 0.02±0.01g cm -2 yr -1 , respectively.The mudflat area had the lowest accumulation rate, with almost one-third of the interior and fringe.This value is converted into depth units per year (cm yr - 1 ), commonly referred to as sediment accumulation rate (SAR).The sediment accumulation rate in the interior, fringe, and mudflat areas can be seen in Figure 4.The interior has the highest average sediment accumulation rate (0.18 ± 0.02 cm yr -1 ), followed by the fringe area (0.116 ± 0.03 cm yr -1 ) and the lowest was in the mudflat area (0.073 ± 0.02 cm yr -1 ).The highest rate of carbon accumulation (CAR) over the last 50 years was in the interior (31.94± 3.6 gC m -2 y -1 ), followed by the fringe (31.89± 7.75 gC m -2 y -1 ) and mudflat (17.03± 5.21 gC m -2 y -1 ) areas.

Corg and C/N Ratio
The Corg value in this study was higher than the previous research conducted in Ujung Kulon National Park in 2018, which ranged from 0.4 -2.61% (average 1.10% ± 0.16) [23].The previous research location is about 4 km from the current research location.The high value of Corg was followed by the high carbon stock of 90.19 Mg C ha -1 to 169.6 Mg C ha -1 for a depth of 54 cm.This value is higher than the previous study, which was only around 60 Mg C ha -1 .But this value is similar to research conducted in conserved mangroves in North Sumatra, which is around 127.49 ± 33.21 Mg C ha -1 [24].However, the comparison using carbon stock needs evaluation because the different depths of the sediment core could alter carbon stock estimation.Several factors that affect the carbon stock in sediments include the type of vegetation, NPP (net primary production), forest structure, and the age of the mangrove forest [8, [25][26][27] The mean value of the C/N ratio in this study is similar to research conducted on Togian Island, Sulawesi, where for marine mangrove sediments, the C/N ratio value ranges from 29-64 [28].However, there are C/N values below 29 in the interior and fringe areas.Many factors, including the potential for allochthonous input from the upland forest, can cause low C/N values.The interior and fringe are located near the river and the forest.This location allows for input from the mainland [29].This is reinforced by the lower carbon values in both areas compared to the mudflat.The value of organic carbon in sediments that get input from the upland is generally lower because the organic carbon content is mostly diluted by mineral sediments overflowing from the upland forest [11].On the other hand, the mudflat area has the highest Corg and C/N values.The potential reason is likely due to its location close to the sea, where it has the potential for disturbances such as tides, currents, and waves that can carry organic material from mangroves [30].The vegetation in the mudflat also increases the potential for organic material to get stuck in the roots and sediment [26].This area also has a low potential to get input from the upland forest because of its location closer to the sea, thus minimizing dilution by mineral sediments and causing higher carbon values than the interior and fringe [31].

210 Pb activity and Carbon Accumulation Rate
The value of the sediment accumulation rate at the study site is far below the global average for mangroves, which is 0.5 cm yr -1 [32], and for conserved mangroves, 0.25 cm yr -1 [30].In this study, mudflats had the lowest sedimentation rate compared to fringe and interior.This pattern is similar to research conducted on conserved mangroves in Jaring Halus, North Sumatra, where the highest sedimentation rate is in the interior area, followed by the fringe and the lowest in the mudflat.However, the sedimentation rate in this study is lower than the sedimentation rate in mangroves in Jaring Halus, which is 0.77 ± 1.01 cm year -1 [24].One contributing factor is that mudflats are disturbed by waves, currents, and tides due to their location close to the sea.This is reinforced by the 210 Pb results, which show mixing sediment in the upper layers.Vegetation can also affect sedimentation rates.The results of research conducted in Papua in 2016 showed that the value of sedimentation rates in mudflat areas could not be detected because there was no vegetation.Hence, the potential for high-mixing sediment resulted in 210 Pb fluctuations [8].In contrast to this research location, the mudflat area still has vegetation, although it is less dense than the fringe and interior areas.The root system of mangrove vegetation provides stability to sediments and reduces some hydrological impacts, such as tidal velocity and river flow [30,33,34].Such processes facilitate higher sediment and organic carbon deposition compared to unvegetated areas.The fringe area receives sediment input from the sea, but the waves are low, so the potential for sediment to deposit is higher than in the mudflat area.Vegetation in the fringe area is also more abundant than in the mudflat, so the potential for organic material and sediment to be trapped is higher in this area.Although the potential for disturbance from the sea is low in the interior area, there is the potential for input sediment from the upland and rivers so that the sedimentation rate is higher than the fringe area [29].The low C/N value adds to the suspicion of potential input from the upland.
Although the mudflat area has the highest carbon stock, the carbon accumulation rate is the lowest.Conversely, the interior area has the highest carbon accumulation rate despite the lowest carbon stock.In the mudflat area, mainly inundated by tides, disturbances such as waves can bring organic material from mangroves and the sea so that the Corg value and carbon stock are higher than the interior and fringe.On the other hand, waves can stir up sediments, resulting in less sediment deposited on the surface and lower sedimentation rates.The flat slope also makes it less likely for the mudflat to receive sediment input from the upland.Although the Corg and carbon stock values are low in the interior area, this area has minimal disturbance from waves and tides.Hence, the potential for sediment to deposit is higher than mudflat.In addition, the interior also has the potential to get sediment input from the upland so that the sedimentation rate is higher than the fringe and mudflat.On the other hand, tides bring in abundant sediments, promoting the trapping of organic carbon-rich sediments due to reduced water velocity within the mangrove [35].This study's carbon accumulation rate is relatively low compared to the global average carbon accumulation rate of 174 gC m -2 y -1 [32].One of the reasons is the low sedimentation rate.On the other side, the low sedimentation rate indicates that the level of erosion in the upland forest is low [8,36].This condition is likely to happen because the research location is a pristine and protected area, so there is minimal interference from human and other anthropogenic activities.Factors affecting sedimentation and carbon accumulation rates include geomorphology, vegetation, and hydrological [30].

Conclusion
The study results show that the interior has the lowest carbon stock value, but the SAR is twice that of the mudflat, which has the highest carbon stock.This causes the CAR value in the interior to be twice that of the mudflat.This result also shows that sediment accumulation and carbon stock affect CAR's value.On the other side, based on the %Corg profile and C/N ratio, there is also the possibility that mangrove UKNP received allochthonous input from the upland.We suggest that stable isotope 13 C combined with C/N analysis would help to identify the source of organic matter in mangrove sediment.

Acknowledgement
The first author expresses their deepest gratitude to the Lembaga Pengelola Dana Pendidikan (LPDP) for sponsoring master degree studies and supporting the publication of this research.We acknowledge the support from Ujung Kulon National Park (UKNP) for granting permission to conduct the research as well as the help during fieldwork

Figure 1 .
Figure 1.Map area of study site in Ujung Kulon National Park

Figure 2 .
Figure 2. (a) %C organic, (b)%nitrogen total, and (c)C/N ratio value in the interior, fringe, and mudflat (the boxplot shows first quartile, median, third quartile and range data)

Table 1 .
Value of %C organic, %nitrogen total, C/N ratio, carbon stock, SAR, MAR and CAR in sediment mangrove UKNP