Cycling and Global Fluxes of Nitrogen in Mangroves

Recent advances necessitate a critical reappraisal of nitrogen cycling in mangrove ecosystems. Nitrogen concentrations in soil, water and vegetation are low, but N-use and recycling efficiencies are high and N residence times are short, driven by rapid forest production and the need to conserve N. Most mangroveassociated fauna preferentially assimilate benthic algae, phytoplankton, algal detritus or complex mixtures of these forms to meet N demand. Consumer preferences, locations and types of habitat, and the relative availability of other foods determine the trophic significance of mangrove litter. A nitrogen mass balance model of the world’s mangrove forests indicates that: (1) 2687 Gg N yr -1 is required to sustain global mangrove net primary production (NPP); (2) N burial is ≈ 25% of total N input; (3) N2 fixation is ≤ 5% of total N input; (4) production in roots and litter account for 40% and 50% of mangrove NPP, respectively; (5) tidal export equates to ≈ 55% of N input; and (6) denitrification and N2O effluxes account for < 10% of total N losses. Despite proportionately large tidal losses, the global flux of nitrogen in mangrove forests is roughly in balance due to multiple adaptations and strategies to efficiently use and retain nitrogen.