ABSTRACT: Primary production is nitrogen limited in most salt marshes with the possible exception of those impacted by high anthropogenic inputs of nitrogen. It is hypothesized that mature salt marshes which receive only small inputs of 'new' nitrogen from theatmosphere, surface water runoff, groundwater, tidal creek, and nitrogen-fixation will have a conservative nitrogen cycle. We have developed a process-based N mass balance model for a short-form Spartina alterniflora marsh in Virginia, USA. Datafor the model included rates of gross mineralization, nitrification, denitrification, nitrogen fixation, above- and belowground macrophyte production, and benthic microalgal production. The annual balance between sources (mineralization, nitrogenfixation, tidal creek flux, atmospheric deposition, and sediment input) and sinks (above- and belowground macrophyte uptake, sediment microalgal uptake, sediment burial, microbial immobilization, denitrification, and nitrification) of dissolved inorganicnitrogen (DIN) was determined for both interior S. alterniflora-vegetated sites and unvegetated creek bank sites. Sediment/water exchanges of DIN species, predicted by results of the mass balance analysis, were compared to measured exchanges.Annually, sources and sinks of DIN in the vegetated marsh were in close balance. The vegetated marsh imported DIN from the adjacent creek during most of the year; the unvegetated creek bank exported NH₄⁺ to overlying tidal waterduring July and imported NH₄⁺ during other seasons. The net flux of DIN was 5.7 g N m^-2 yr^-1 from overlying water into the marsh; however, this flux was small relative to rates of internal N-cycling processes.The sediment NH₄⁺ pool turned over rapidly as a result of the high rate of gross mineralization (84 g N m^-2 yr^-1). Other microbial N-cycling rates were low (0.6 to 4 g N m^-2 yr^-1). TheNH₄⁺ supplied by mineralization was more than sufficient to support both macrophyte (33 g N m^-2 yr^-1) and benthic microalgal (5 g N m^-2 yr^-1) uptake. We propose that in order to maintainsteady state in the system approximately half of the DIN mineralized is immobilized into a readily remineralizable particulate organic N pool. Since mineralization and macrophyte uptake are temporally out of phase, the labile organic N pool may serve totemporarily sequester NH₄⁺ until it is required for plant uptake.