DYNAMIC NATURE OF THE TURNOVER OF ORGANIC-CARBON, NITROGEN AND SULFURIN THE SEDIMENTS OF A JAMAICAN MANGROVE FOREST

Rates of oxygen uptake, sulphate reduction, carbon dioxide production, ammonium turnover, nitrogen fixation and denitrification were measure d in sediments in a transect across a mangrove swamp in Jamaica. Carbo n and nitrogen mineralisation increased as the transect entered the ma ngrove, indicating a greater availability of organic matter, probably by root excretion. Time course measurements of sulphate reduction indi cated a subsurface pool of labile carbon. Carbon dioxide production wi thin the mangrove exceeded oxygen flux across the sediment surface, in dicating a subsurface source of oxygen of 28 to 179 mmol m-2 d-1. The turnover time for the sedimentary sulphide pools decreased as the tran sect entered the mangrove. The production of ammonium from organic nit rogen, measured by (NH4+)-N-15 turnover, also indicated active subsurf ace organic matter mineralisation. The source of this organic nitrogen is unknown. Measured rates of nitrogen fixation were too low to suppl y the required organic nitrogen. The estimated net availability of amm onium to the plant roots was 10 mmol m-2 d-1. This would give a produc tivity of 2000 g C m-2 yr-1.