## 1 Introduction  
Mangrove forests are important sinks of carbon (C) throughout the tropics and subtropics [1; 2; 3] due to their high C burial rates within soils [3; 4; 5]. These observations have led to a heightened level of interest in conserving and restoring mangroves for their C values [6; 7]. Additionally, mangrove forest primary production is responsible for approximately 10% of the global export of both particulate and dissolved organic C to the oceans, which supports production in near shore waters [8; 9]. However, there are very few detailed C budgets available for mangrove forests [3; 10; 11]. Here, we describe a carbon budget for mangrove forests in Belize. Our aims are to increase knowledge of the global variation in C budgets of mangroves and the understanding of the factors giving rise to variation in C stocks, rates of C burial and the role of mangrove forests in supporting productivity of the coasts on which they occur. Gross primary productivity (GPP) and the size of the detrital pool in ecosystems are strongly influenced by variation in vegetation structure and rates of turnover [12]. Although little is known of variation in mangrove tree turnover rates [13], mangrove forests vary widely in structure and productivity across large (e.g., latitude [13]) and small spatial scales (e.g., ecotones [14]). Some of the major features of the few existing C budgets of mangrove forests indicate that despite being rooted in saline soils, they are highly productive forests, with similar productivity to terrestrial forests, and that a large proportion of fixed C is respired by the canopy [3; 13]. Carbon burial rates of detritus have been estimated to be approximately 2%-4% of GPP, but this could vary with age of the forest and productivity [3; 11]. At our study site, C burial is particularly important because of the dependence on the burial of autochthonous organic matter for maintaining the soil elevation relative to sea level [15; 16]. Finally, approximately 40% of the fixed C has not yet been accounted for in the existing budgets and is proposed to be exported from the ecosystem [10; 13; 17], although how the size of the putatively exported fraction varies among different types of mangrove forests is also not known. Mangrove forests are threatened by agriculture, industry and urbanization [18] which can lead to losses in ecosystem C stocks and alterations to C budgets. Not only are mangrove forests cleared, which results in losses of aboveground and belowground C [2; 19; 20], but anthropogenic nutrient enrichment alters a range of processes that could affect C budgets [21]. In terrestrial ecosystems, increases in the availability of nutrients have been shown to enhance primary productivity, reduce allocation to roots and increase rates of decomposition of soil organic matter [22; 23]. Mangroves also show strong sensitivity to nutrient additions for a range of processes, including above and belowground growth and decomposition [14; 24; 25]. These studies have demonstrated that scrub mangrove growth can be severely limited by phosphorus (P) availability. However, little is known of how nutrient enrichment influences the efficiency of C burial within soils through reductions in allocation to roots or enhanced rates of decomposition of soil organic matter. We used a mass balance approach of C inputs (autotrophic components) and outputs (the fate of autotrophic production) similar to those presented in Alongi [11] to identify the major C sources and sinks of mangrove ro forests of Twin Cays, Belize. Twin Cays is a small archipelago comprised of 92 ha of mangrove forest that lies within the lagoon of the Meso-American Barrier Reef [26]. In the Caribbean region, mangrove forests often grow on peat soils that overlie limestone [27; 28; 29]. Unlike mangrove forests in deltaic systems, where forests often grow on mineral soils, mangrove forest soils in oceanic settings in the Caribbean are often comprised of mangrove roots that have accumulated during the rise in sea level over the Holocene [15].