NITRATE REDUCTION IN SEDIMENTS OF LOWLAND TROPICAL STREAMS DRAINING SWAMP FOREST IN COSTA-RICA - AN ECOSYSTEM PERSPECTIVE

Nitrate reduction and denitrification were measured in swamp forest st reams draining lowland rain forest on Costa Rica's Atlantic slope foot hills using the C2H2-block assay and sediment-water nutrient fluxes. D enitrification assays using the C2H2-block technique indicated that th e full suite of denitrifying enzymes were present in the sediment but that only a small fraction of the functional activity could be express ed without adding NO;. Under optimal conditions, denitrification enzym e activity averaged 15 nmoles cm(-3) sediment h(-1). Areal NO, reducti on rates measured from NO3- loss in the overlying water of sediment-wa ter flux chambers ranged from 65 to 470 umoles m(-2) h(-1). Oxygen los s rates accompanying NO3- depletion averaged 750 umoles m(-2) h(-1). C orrected for denitrification of NO3- oxidized from NH4+ in the sedimen t, gross NO3- reduction rates increase by 130 umoles m(-2) h(-1), indi cating nitrification may be the predominant source of NO; for NO, redu ction in swamp forest stream sediments. Under field conditions approxi mately 80% of the increase in inorganic N mass along a 1250-m reach of the Salto River was in the form of NO3- with the balance NH4+. Scruti ny of potential inorganic N sources suggested that mineralized N relea sed from the streambed was a major source of the inorganic N increase. Despite significant NO, reduction potential, swamp forest stream sedi ments appear to be a source of inorganic N to downstream communities.