Dynamic nature of the turnover of organic carbon, nitrogen and sulphur in the sediments of a Jamaican mangrove forest

Rates of oxygen uptake, sulphate reduction, carbon dioxide production, ammonium turnover, nitrogen fixation and denitrification were measured in sediments in a transect across a mangrove swamp in Jamaica. Carbon and nitrogen mineralisation increased as the transect entered the mangrove, indicating a greater availability of organic matter, probably by root excretion. Time course measurements of sulphate reduction indicated a subsurface pool of labile carbon. Carbon dioxide production within the mangrove exceeded oxygen flux across the sediment surface, indicating a subsurface source of oxygen of 28 to 179 mm01 m-' d' The turnover time for the sedimentary sulphide pools decreased as the transect entered the mangrove. The production of ammonium from organic nitrogen, measured by I5NH,+ turnover, also indicated active subsurface organic matter mineralisation. The source of this organic nitrogen is unknown. Measured rates of nitrogen fixation were too low to supply the required organic nitrogen. The estimated net availability of ammonium to the plant roots was 10 mm01 m" d-' This would give a productivity of 2000 g C m-2 yr-'.