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Potential carbon storage of Indonesian mangroves

Even though it only covers 0.1% of the surface of the continent, mangrove forests recorded as carbon-rich forests on earth. Forest loss and degradation together, especially in the tropics contributes about 12% of the total annual anthropogenic CO2 emissions. Mangrove forests ecosystem have high carbon storage potential. The study results show that the ratio of carbon to area in mangrove ecosystems is almost 13 times greater than that of tropical rainforest ecosystems in Indonesia. This study has used published data recorded in Google Scholar and Science Direct. Indonesia’s mangroves, with a total area of 3.1 million ha, store a total carbon of 5.2 Gt. The average stand biomass or aboveground biomass is around 407.15 tons ha−1, storing carbon as much as 191.36 tons ha−1 which is equivalent to the potential for CO2 absorption (CO2e) by mangrove stands of 702.29 tons ha−1. Papua has the largest mangrove forest in Indonesia that has the potential for CO2e. Assessment of carbon storage in soil needs to be done in estimating the total carbon stored in mangrove ecosystems, given the large potential for stored carbon that plays a role in climate change mitigation.


Introduction
Mangrove forests have a higher capacity than terrestrial forests to store large amounts of carbon in the soil over thousands of years. Mangroves with waterlogged soils, high sedimentation rates, and complex root structures that are free of fire risk, and anoxic soils result in greater levels of carbon storage and a thousand times slower rate of soil carbon turnover than in terrestrial forests. Mangrove ecosystems ability to reserve major amounts of soil carbon (5-10.4 Gt globally) over thousands of years makes this ecosystem important for carbon sequestration, and reduces or prevents greenhouse gas (GHG) emissions from losing soil carbon stocks [1]. This is an important option for mitigating climate change at a low cost.
Mangrove forests are well known for a series of ecosystem services, including fiber and fisheries production, protection from hurricane or tsunami, and sediment regulation, but its potential is decreasing rapidly as a consequent of aquaculture expansion, land clearing, excessive utilization, and development. The decline of 30-50% of the area during the last half century has prompted estimates that mangrove forests can be functionally disappearing only in 100 years. The 21 st century's rapid sea level rise has also been cited as a major threat to mangrove forests, which have responded to past sea level changes by migrating to land or upward [2,3].
Mangroves cover only 0.1% of the continental surface of Earth (∼81,485 km2) [4], but have been confirmed as some of the highly carbon-rich (C) forests on earth. Indonesia itself is a country with the  [4]. However, Indonesia was also recorded as the country with the largest loss of mangroves [4][5][6]. Indonesia's annual mangrove deforestation rate is about double the global average or nearly half of all global mangrove loss [4,6].
Forest loss and degradation result in carbon dioxide (CO2) emissions that contributes to carbon sequestration capacity loss, climate change, and invaluable ecosystem services loss as well as biodiversity loss. Forest loss and degradation occurs mainly in the tropics; which accounts for approximately 12% of the total annual anthropogenic emissions of CO2 [7,8]. Therefore, this paper analyzes CO2 absorption based on carbon stocks of mangrove forests in several regions of Indonesia.

Materials and methods
This paper is the result of a review of various literature studies published in peer reviewed journals. Articles obtained from Google Scholar and Science Direct databases. CO2 sequestration analysis is calculated by converting the carbon content into CO2 using the following equation [9][10][11].
Where CO2e is the carbon dioxide equivalent (tons ha -1 ), Bm.CO2 is the relative molecular weight of the CO2 (44), Ba.C is the relative atomic weight of C (12) and C is the carbon content (ton ha -1 ) or following the equation of [12]:

Results and discussion
Net primary productivity and biomass of mangrove forests have been studied in a variety of mangrove forests around the world. The purpose of estimating the productivity and biomass of mangrove forests is important in ecosystem management and evaluating carbon stocks. However, production of biomass depends on the interaction between various factors, such as climate and topography and there are also some methodological differences in the estimation of net primary productivity [13]. Mangrove forests have an important role in managing the global climate and providing ecosystem products and other services [2,8,[14][15][16]. Mangrove forests are recorded as the highly carbon-rich forests in the tropical areas and their carbon sequestration potential is counted to be up to 5 times that of tropical rain forests [2]. A recent valuation of carbon stored in various mangrove areas found that the Indo-Pacific is one of the highly carbon-rich forests in the tropics comprising, on average, 1,023 tC ha -1 , most of which is deposit in soils with depth > 30 cm [2].
The total carbon potential of mangroves can be quantified by adding up the potential above-ground carbon (AGC), subsurface carbon or root system (BGC), and soil carbon (SOC). According to Rahadian [17], it is shown that in Indonesia, the ratio of carbon stock to mangrove forest area is almost 13 times greater than that of rainforest. He compared the total value of mangrove carbon storage with rainforest through reference to carbon stock by MoEF [18] of 125.02 t C ha -1 , and then multiplied it by the area of remaining rainforest in Indonesia (93.95 million ha) [18], hence the total rainforest carbon storage is approximately 11.74 Gt C. Meanwhile, mangroves, which only cover a total area of 3.1 million ha, store carbon reaching 5.2 Gt C (Table 1). Thus, if the mangrove ecosystem is disturbed then the resulting emissions would also be higher than the terrestrial ecosystems in the same area. Table 2 shows that mangroves in Indonesia have an average aboveground biomass (AGB) of around 407.15 tons ha -1 , or carbon of 191.36 tons ha -1 , so the potential for CO2 absorption by mangrove stands is 702.29 tons ha -1 . Papua has a major contribution in carbon sequestration because it has the largest mangroves compared to other regions in Indonesia.  Due to the absorption of CO2 and carbon storage of mangroves is much greater than rainforests, mangroves are very important in mitigating climate change and becoming an asset in reducing GHG emissions. When mangroves are part of Nationally Determined Contributions (NDC), it will provide an opportunity to fill the gap in emission reduction programs. The mangrove area, which is still considered very small compared to the rainforest, has neglected the mangrove ecosystem role in climate change mitigation. This is because the value of mangrove soil carbon storage is not included, causing an underestimate of the total carbon value in the mangrove ecosystem [17]. Most of the carbon storage of mangrove ecosystems is stored in the soil [2,[19][20][21].

Conclusions and recommendations
Indonesia has a ratio of carbon stock to area in mangrove forests nearly 13 times greater than rainforests. Indonesia's mangroves with a total area of 3.1 million ha store a total carbon of up to 5.2 Gt C with an average AGB of around 407.15 tons ha -1 , storing carbon of 191.36 tons ha -1 , and the potential for CO2 absorption by mangrove stands amounting to 702.29 tons ha -1 . Papua has a biggest contribution with the largest mangroves and has the potential for high CO2 absorption. Soil carbon storage needs to be included in estimating the total carbon value in mangrove ecosystems, given the large potential for stored carbon and CO2 absorption which plays an important role in mitigating climate change.