Salt marshes provide a buffer between the terrestrial landscape and estuaries and may be important in preventing the movement of land-derived nutrients into coastal waters. We examined the response of S. patens in the field to additions of dissolved inorganic nitrogen (N) and phosphorus (P) and found significant (p < 0.05) positive N effects on aboveground biomass, leaf chlorophyll, tissue nutrient concentrations, and induction of fluorescence kinetics of chlorophyll a. Mean endomycorrhizal colonization among treatments was 22%, and fungal colonization ranged from 2% to 61% in the plots. We found no significant effect of N or P on endomycorrhizal colonization, but there was a significant inverse relationship (r = −0.66, p = 0.005) between the belowground biomass and fungal colonization. This study showed that S. patens could sequester 44–100% of the added N and 82–100% of the added P in its leaves, roots, and rhizomes. However, it is unclear how long-term nutrient overenrichment and the resulting changes in the S. patens–microbe–sediment system might alter the marsh buffering capacity.