Nitrogen (N) limitation of primary production is common in temperate salt marshes, even though conservative N recycling can fulfill a large proportion of plant N demand. In nutrient poor young marshes, N limitation may be more severe and new N sources, such as N fixation, more important for plant growth. We measured N fixation and the response of salt marsh primary producers (Spartina alterniflora and benthic microalgae) to N fertilization in one mature (>150 year) and two young (7 and 15 year) naturally developing marshes at the Virginia Coast Reserve LTER site. S. alterniflora aboveground biomass in the mature marsh (1,700+/-273 g m(-2)) was 1.8 and 2.8 times higher than in the 15 year and 7 year old marshes, respectively. Fertilization significantly increased S. alterniflora biomass in the two young marshes (160-175%) and areal aboveground tissue N in the youngest marsh (260%). Microalgal chlorophyll a (Chl a) in the mature marsh was nearly 2-fold lower than in the 7-year-old marsh, and there was no evidence that this was due to light limitation. However, Chl a in fertilized plots was 30% higher than control plots at the youngest site. Daily N fixation decreased with increasing marsh age in summer, when rates were highest at all sites. Autotrophic N fixation (difference between rates in the light and dark) was most important in the summer, but we saw no indication of a shift in dominance between autotrophic and heterotrophic N fixers during marsh development. Estimated annual N fixation was 2- to 3-fold higher in the young marsh (18.3+/-1.5 g N m(-2) year(-1)), than in the intermediate-aged (9.0+/-0.7) or mature marsh (6.1+/-0.5). In the young marshes, N fixation was sufficient to provide a substantial proportion of aboveground S. alterniflora N demand. Our results suggest that both benthic microalgae and S. alterniflora in young salt marshes are N limited, and that this limitation decreases as the marsh matures. The high rates of N fixation by autotrophic and heterotrophic bacteria in the sediment could provide an important source of N for primary producers during marsh development.