Serasah merupakan salah satu penyumbang bahan organik dalam ekosistem mangrove, salah satu bahan organik yang disumbangkan yaitu Magnesium (Mg). Magnesium merupakan salah satu senyawa bahan organik yang memiliki peranan penting bagi mikroorganisme pengurai hingga menjadi rantai makanan. Tujuan penelitian ini adalah untuk mengetahui seberapa besar kontribusi bahan organik Magnesium (Mg) dari serasah daun mangrove Avicennia alba, Bruguiera parviflora, Rhizophora apiculata, dan Sonneratia alba. Setelah daun mangrove mengalami proses dekomposisi selama 56 hari yang laksanakan di Kawasan Konservasi Mangrove dan Bekantan (KKMB) Kota Tarakan dan analisis data bahan organik Magnesium (Mg) dilakukan di Laboraturium Ilmu Tanah Fakultas Pertanian Universitas Mulawarman Samarinda. Penelitian ini dilakukan dengan menggunakan metode penampungan serasah yang terbuat dari nilon dengan ukuran 2m x 2m dan kantong serasah dengan ukuran 30cm x 30cm yang diisi dengan 20 gram serasah daun mangrove yang telah di keringkan, kemudian diletakkan di lantai hutan mangrove. Hasil penelitian menunjukkan kontribusi bahan organik Magnesium (Mg) pada serasah daun mangrove Avicennia alba 0.006 (g/m2/hari), Bruguiera parviflora 0.004 (g/m2/hari), Rhizophora apiculata 0.007 (g/m2/hari), dan Sonneratia alba 0.010 (g/m2/hari).
Kata Kunci: Mangrove, Kontribusi, Magnesium, KKMB

Litter of mangrove leaf is one of organic contributor in mangrove ecosystem in which Magnesium (Mg). Magnesium (Mg) is an organic compound which has important role for decomposing microorganism to become food chain. The purpose of this research is to know how big the contribution of Magnesium (Mg) from mangrove leaf of Avicennia alba, Bruguiera parviflora, Rhizophora apiculata, dan Sonnera alba. After mangrove leaves had decomposing process in 56 days which is carried out in the Conservation Area Mangrove and Bekantan (KKMB) of Tarakan city and the data analysis of Magnesium (Mg) was done in Laboratory of Soil Science Faculty of Agriculture University of Mulawarman Samarinda. This research was conducted by using method of Mangrove leave collected in a nylon Shelters with a size of 2m x 2m and a litter bag with a size of 30cm x 30cm which was filled with 20 grams of mangrove leaf litter that has been dried, then placed on the floor of the mangrove forest. The results showed the contribution of organic materials Magnesium (Mg) in leaf litter of mangrove Avicennia alba 0.006 (g/m2 /day), Bruguiera parviflora 0.004 (g/m2 /day), Rhizophora apiculata 0.007 (g/m2 /day), and Sonneratia alba 0.010 (g/m2 /day). Keywords: Mangrove, Contribution, Magnesium, KKMB