Carbon stock estimation in post-mining reclamation area of open coal mining in PT Berau Coal, Berau, East Kalimantan

Postmining land reclamation and revegetation activities play an important role in restoring vegetation, degraded land and controlling climate change because CO2 is sinking from the atmosphere due to the impact of mining activities. This study aims to estimate the potential for above-ground and below-ground carbon and CO2 absorption capacity at the Binungan Mine Operations Site (BMO), PT Berau Coal, Berau, East Kalimantan. Vegetation analysis was used to estimate the potential carbon stock and CO2 absorption capacity at plant ages 1-10 years using the purposive sampling method of 5 plots (20 x 20 m) each in the post-mining area studied. Biomass and carbon stocks were estimated using existing allometric models. The results showed 14 tree species from 10 families dominated by Fabaceae. Biomass was estimated at 1.13 - 128.31 Mg ha-1, carbon stocks were 0.53-60.3 Mg ha-1, and CO2 absorption was 1.95 - 221.32 Mg ha-1. Reclamation and revegetation in 10 year-old plants on reclamaied land can restore biomass, carbon stock, and CO2 absorption capacity of 70% from the initial baseline.


Introduction
Changes in the landscape from open pit mining activities impact the loss of vegetated land resulting in loss of carbon stocks, decreased soil productivity, and heavy metal pollution in the soil [1, 2, 3].The loss of carbon stocks in vegetated lands causes a decrease in the ability of a tree to absorb carbon dioxide (CO2) in the atmosphere.Based on Minerba One Data Indonesia (MODI) (2023), coal production in Indonesia has experienced an increasing trend in 2020 -2022 by 17.4%, which indicates an increase in land clearing for mining activities which disrupts the balance of the carbon cycle in ecosystems.
Carbon stocks and CO2 sinks are important in the issue of climate change and global warming [4, 5,6,7].The impact of the loss of the ability of vegetation to absorb CO2 can be minimized by implementing proper reclamation and revegetation of postmining land [8].Revegetation of postmining can then be converted into biomass and carbon, reducing greenhouse gas effects and restoring forest ecological functions [9].Calculation of forest carbon is needed to identify its potential and influence on the carbon cycle [10].
The CO2 sequestration approach can be calculated using the approach to the amount of forest carbon stocks stored in biomass.Biomass is an organic material contained in the tree structure [11].Carbon composes about half of the total biomass [12].Postmining reclamation and revegetation activities aim to increase biodiversity, canopy cover, and stratification [13].Data estimation in tropical forest is generally indispensable, because of their ability to store carbon, and subsequently reduce atmospheric CO2 levels [14], making it an important step in planning for the sustainable postmining revegetation.
Indonesia's commitment to climate mitigation and reducing greenhouse gas emissions is stated in the 2022 Enhanced Nationally Determined Contribution (ENDC) of 31.89%independently and 43.2% with International support in 2030.Data regarding the stock of carbon stocks in post-mining revegetated land ecosystems is still minimal.Biomass content and carbon stocks can indicate the success of postmining revegetation activities [15].Furthermore, this research aims to estimate the potential of biomass and carbon stock in postmining revegetation land of PT Berau Coal, Berau, East Kalimantan.

Necromass type
Correction factor Dead tree without leaves 0.9 Dead tree without leaves and branches 0.8 Dead tree, without leaves, twigs and branches 0.7 Nekromassa is a dead tree trunk either still upright or fallen.Calculation of necromass biomass considers the correction factor for calculating necromass (Table 2).The calculation of total biomass uses the equation: Furthermore, the carbon stock and CO2 absorption capacity of postmining land revegetation plants are determined based on the formula [12]:

Result and discussion
Vegetation analysis results (Table 3) show that Binungan Mine Operation postmining revegetation area consists of 14 plant species, consisting of 10 families and dominated by the Fabaceae family.The stem diameters range from 1 -43.6 cm, with a canopy height of 1 -43.5 m.P. falcataria species dominate the total number of species.The dominance of P. falcataria in postmining revegetation stands because this species is a pioneer plant species.This type of pioneer plant generally comes from the Fabaceae family because it is fast growing and its ability to symbiosis with Rhizobium bacteria [23].
The stand density observed in the revegetated plots aged 1 -10 years varied.Revegetated land has a different succession process and does not depend on the age of the stand [24].Enrichment planting aims to increase species diversity on revegetated land.Furthermore, this accelerates carbon absorption and storage, which aims to restore forest function in post-mining reclamation areas [25].

Figure 1. Stand density on stands of post-mining revegetated land Binungan Mine Operation PT Berau Coal
The highest stand density (Figure 1) was observed on post-mining revegetation land aged 8 years with a value of 1370 individual ha -1 , and the lowest density was on post-mining revegetation land aged 1 year with a value of 575 species ha -1 .According to the Minister of Forestry Regulation No. 60 of 2009, the minimum value of the stand density considered successful in post-mining land revegetation activities is 625 individual ha -1 .The difference in stand density is due to competition between individuals.Carbon stocks stored in individual plants in biomass can be the basis for estimating individual plants carbon dioxide (CO2) uptake [26].One of the carbon stocks is influenced by individual density.High individual density indicates more trees and higher carbon stocks in stands.Biomass potential in post-mining revegetated land aged 1 -10 years (Table 4) is 1.51 -132.44 Mg ha -1 .The biomass estimation results have a positive correlation with the potential for carbon storage and CO2 absorption.The potential for carbon storage and CO2 absorption in this study was observed in the range of 0.71 -62.25 MgC ha -1 and 2.60 -228.45MgCO2 ha -1 .Differences in potential biomass, carbon storage, and CO2 absorption at each stand age of post-mining land revegetation are indirectly influenced by diameter, height, density, soil conditions, and plant age [26].
DF, Dipterocarpa forest (concession area) [24]; Bag, above-ground biomass; C, Carbon stock; CO2, Carbon dioxide absoprtion The success of revegetation activities in post-mining land reclamation areas has positive causality with the biomass content and carbon sequestration ability [14].The estimated carbon stock in the Dipterocarpa forest area around the revegetated land (Figure 2) is 74 MgC ha -1 [25].Therefore, postmining land revegetation stands at the age of 10 years and can recover 84% of the carbon stored lost due to mining activities.This proves that this study's post-mining land revegetation activities are in good condition and can restore carbon stores and CO2 absorption.

Conclusion
The postmining revegetation area in this study had 14 species from 10 families and was dominated by P. falcataria and the Fabaceae family.The highest density of postmining revegetation stands was observed in postmining revegetated land aged 8 years with a value of 1370 species ha -1 .The potential of postmining revegetated standing biomass is 1.51 -132.44 Mg ha -1 , with carbon stocks of 0.71 -62.25 MgC ha -1 , and CO2 absorption 2.60 -228.45MgCO2 ha -1 .Post-mining revegetation with a 10-year age can restore biomass, carbon stock, and CO2 absorption by 84% from the initial baseline.Mg ha -1 Age Biomass Carbon stock Carbondioxide B ag (Mg ha -1 ) C (Mg ha -1 ) CO 2 eq (Mg ha -1 )

Table 1 .
The allometric equation used to estimate the biomass of each tree species at Binungan Mine Operation PT Berau Coal

Table 3 .
Vegetation analysis results on postmining revegetated land Binungan Mine Operation PT

Table 4 .
Biomass potential, carbon stocks, and CO2 absorption in postmining revegetated land Binungan Mine Operation PT Berau Coal