Proses dekomposisi serasah akan menghasilkan bahan organik yang berperan dalam menentukan kesuburan perairan salah satunya adalah karbon. Kawasan Konservasi Mangrove dan Kepiting telah mengalami penurunan luasan yang mengakibatkan berkurangnya pasokan bahan organik karbon sehingga akan mempengaruhi produktivitas perairan. Tujuan penelitian ini adalah untuk menguji kandungan bahan organik karbon yang terdekomposisi pada serasah daun mangrove jenis R. mucronata di Kawasan Konservasi Mangrove dan Kepiting Kota Tarakan. Penelitian ini dilakukan pada bulan Juni sampai dengan Agustus 2022 dengan metode deskriptif kuantitatif. Penelitian ini dilakukan di tiga stasiun, dimana stasiun 1 terletak di aliran Sungai Pamusian, stasiun 2 terletak di dekat Menara Pengawas, dan stasiun 3 terletak di dekat pemukiman masyarakat. Hasil penelitian menunjukkan laju dekomposisi serasah daun mangrove R. mucronata di Kawasan Konservasi Mangrove dan Kepiting Kota Tarakan berkisar 0,38-0,43 g/hari. Laju dekomposisi tertinggi terjadi pada 14 hari pertama dan menurun sampai hari ke 56. Kandungan karbon pada serasah daun R. mucronata yang terdekomposisi selama 56 hari berkisar 21,57-22,48 %.
Kata kunci: Karbon, Laju Dekomposisi, Mangrove, R. mucronata, Serasah

The decomposition process will produce organic matter, which plays a role in determining water fertility, one of which is carbon. The Mangrove and Crab Conservation Area has experienced a decrease in the area resulting in a reduced supply of carbon organic matter, which will affect aquatic productivity. The main purpose of this research was to examine the organic matter content of decomposed carbon (Rhizophora mucronate) mangrove leaves in the Mangrove and Crab Conservation Area of Tarakan City. This research was conducted from June to August 2022 using a quantitative descriptive method. This research was conducted in three sessions, where the first session (station) was located on the Pamusian River, the second session (station) was located near the Supervisory Tower, and the third session (station) was located near a community settlement. The research result showed that the decomposition rate of (Rhizophora mucronate) mangrove leaves in the Mangrove and Crab Conservation Area of Tarakan City ranged from 0.38-0.43 g/day. The highest decomposition rate occurred on the first 14 days and decreased until the 56th day. The carbon content in (Rhizophora mucronate) mangrove leaves, which decomposed during 56 days, ranged from 21.57-22.48%. Keywords: Carbon, decomposition rate, mangrove, Rhizophora mucronate, litter