Like many coastal systems, nitrogen is the critical limiting factor for mangrove net production. This study used a box model approach to assess the nitrogen budget in the Sundarban mangrove ecosystem, which acts as a sink for atmospheric nitrogen in terms of NOx, NH3, N2, and water column dissolved inorganic nitrogen. The coupling of biosphere and atmosphere in terms of atmospheric NOx and NH3 uptake showed that uptake of ammonia (130×106molyr-1) was about six fold as large as that of NOx, (22×106molyr-1). The nitrogen stored by the processes such as plant uptake of NOx, NH3 from the atmosphere, nitrogen fixation (5×109molyr-1), and sediment water exchange (8×106molyr-1) was about two times as large as that of recycled nitrogen from litter (3×109molyr-1), and could account 74% of the nitrogen required for mangrove net production. Most of the nitrogen was conserved in the living biomass (living biomass: 118×103molha-1 versus soil: 3×103molha-1). The loss of nitrogen was 23% of the total amount that was conserved from the external sources in the Sundarban mangrove system. Thus, the coastal ecosystem like Sundarban mangroves could retain only 0.2% (8×106mol) of the annual river flux of nitrogen to the coastal waters and nitrogen is generally conserved within the system. © 2014 Elsevier B.V.