Low nutrient availability in salt marsh ecosystems can potentially limit primary productivity and subsequent carbon export to coastal waters. In temperate marshes with low external nitrogen inputs, nitrogen (N₂) fixation may enhance availability of usable nitrogen to marsh plant communities and increase their growth. The effects of sub-surface soil amendment of polysaccharides on rhizosphere N₂ fixation, and on plant growth and tissue characteristics of the California cordgrass, Spartina foliosa, were tested in microcosms in a tidal simulator. Polysaccharide was added as alginate extract, and rhizosphere N₂ fixation and plant characteristics were measured over 9 weeks. N₂ fixation rates increased 10-fold in the alginate treatment compared to controls after 1 week of exposure as a result of increased availability of a readily usable form of carbon. N₂ fixation rates in amended sediment were significantly higher than rates in control sediments at weeks 1 and 3. Although plant characteristics did not differ between treatments, trends of increased number of leaves and shoots in amended plants compared to controls appeared between weeks 7 and 9. The potential for increased plant growth following an increase in N₂ fixation rates warrants further investigation as it may have implications for recovery of N-limited disturbed and restored salt marshes.