Soil nitrogen pools were lower in a 4-yr-old constructed salt marsh than in an adjacent natural marsh of San Diego Bay. Aboveground biomass and foliar nitrogen content of Spartina foliosa were both lower for the constructed marsh. Soil organic carbon, which was highly positively correlated with total nitrogen, was also lower in the constructed marsh. Rates of nitrogen fixation were higher for the natural marsh in surface soils (1 cm depth) but not in the rhizosphere (10 cm depth). Experimental additions of organic matter increased rates of nitrogen fixation substantially for both the constructed and natural marsh soils, with glucose stimulating greater increases than Spartina foliosa detritus (roots and rhizomes). In comparison with natural marshes studied elsewhere, the San Diego Bay sites have low nitrogen pools and little soil organic carbon. Nitrogen mineralization rates (in situ incubations) were high in both marshes studied. The low nitrogen pools reflect low tidal import and infrequent streamflow influxes and, possibly, high nitrogen demands of vegetation stressed by hypersaline soils.