Carbon Stocks and Greenhouse Gas Emissions (CO2 and CH4) in Mangrove Forests and Aquaculture Ponds in East Kalimantan, Indonesia

Mangroves, as part of the blue carbon ecosystem, are considered a cost-effective nature-based solution pathway to help mitigate climate change and achieve the Paris Agreement’s aim to limit warming to 1.5˚C. The accurate quantification of greenhouse gas (GHG) emissions and carbon accounting has become a key challenge for policymakers and scientists addressing climate change.  Globally, Indonesia emits the highest potential CO₂ emissions from soils in the mangrove ecosystems because of its high rates of mangrove losses in recent decades. Unfortunately, there are limited studies on carbon and GHG emissions from Indonesian mangroves. This study aims to quantify carbon loss due to mangrove conversion due to aquaculture development by combining carbon stocks and GHG emissions data located in Tabalar Muara Village, Berau, East Kalimantan, Indonesia. We collected data from aboveground (vegetation, downwood) and belowground (roots and soil) carbon stocks in five and three transects of mangrove forests and aquaculture ponds, respectively. Soil bulk density and carbon concentration in various soil depth intervals were also analyzed. In addition, we conducted three consecutive days of regular monthly monitoring of CO₂ and CH₄ fluxes associated with soil physicochemical properties in mangrove forests and aquaculture ponds from January – December 2023. Total ecosystem carbon stocks in mangrove forests and aquaculture ponds were 926 ± 20 and 658 ± 45 Mg C ha⁻¹, respectively. Thus, it implies 984 Mg CO₂ ha⁻¹ of potential carbon loss during mangrove forest conversion to aquaculture ponds. Soil carbon stocks between 0 and 300 cm depth varied significantly, where carbon stock in aquaculture ponds (658 Mg C ha⁻¹) was 18% lower than in mangrove forests (777 Mg C ha⁻¹). Soil carbon dominates total ecosystem carbon stocks by up to 88% in mangrove forests.  For GHG fluxes, mangrove forests have six times higher heterotrophic CO₂ emissions (79.44 ± 4.47 Mg CO₂ ha^-1 yr^-1) compared to that from the aquaculture ponds (13.88 ± 0.88 Mg CO₂ ha^-1 yr^-1). The annual total CH₄ flux was 17 times higher in mangrove forests (7.72 ± 0.50 Mg CO₂e ha^-1 yr^-1) than in aquaculture ponds  (0.46 ± 0.04 Mg CO₂e ha^-1 yr^-1). The results of this research are useful to refine GHG emissions accounting on mangroves by providing higher Tier of emission factors to fulfill Indonesia’s Enhanced Nationally Determined Contributions.