Nitrate reduction in sediments of lowland tropical streams draining swamp forest in Costa Rica: An ecosystem perspective

Abstract

Nitrate reduction and denitrification were measured in swamp forest streams draining lowland rain forest on Costa Rica's Atlantic slope foothills using the C₂H₂-block assay and sediment-water nutrient fluxes. Denitrification assays using the C₂H₂-block technique indicated that the full suite of denitrifying enzymes were present in the sediment but that only a small fraction of the functional activity could be expressed without adding NO₃ ⁻. Under optimal conditions, denitrification enzyme activity averaged 15 nmoles cm⁻³ sediment h⁻¹. Areal NO₃ ⁻ reduction rates measured from NO₃ ⁻ loss in the overlying water of sediment-water flux chambers ranged from 65 to 470 umoles m⁻² h⁻¹. Oxygen loss rates accompanying NO₃ ⁻ depletion averaged 750 umoles m⁻² h⁻¹. Corrected for denitrification of NO₃ ⁻ oxidized from NH₄ ⁺ in the sediment, gross NO₃ ⁻ reduction rates increase by 130 umoles m⁻² h⁻¹, indicating nitrification may be the predominant source of NO₃ ⁻ for NO₃ ⁻ reduction in swamp forest stream sediments. Under field conditions approximately 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 ⁺ . Scrutiny of potential inorganic N sources suggested that mineralized N released from the streambed was a major source of the inorganic N increase. Despite significant NO₃ ⁻ reduction potential, swamp forest stream sediments appear to be a source of inorganic N to downstream communities.