The root:shoot ratio of grassland plants may be lower in high fertility sites than in low fertility sites as plants modify their root, rhizome and shoot morphologies (and masses) to suit prevailing nutrient availability. We conducted geographically diverse and regionally specific field sampling and measured aboveand belowground plant biomasses in western Atlantic and Gulf of Mexico salt marshes to determine whether there is a similar morphological response in Spartina alterniflora, the dominant salt marsh plant. Coastal nutrient addition/enrichment, which is widespread and ongoing, may lower root and rhizome biomass, belowground production and organic accumulation in this species. Higher soil respiration and a lower Eh (redox potential) are expected additional soil property changes. The addition of P, more than of N, seems to reduce root and rhizome biomass accumulation. The cumulative effects of increased nutrient loadings on salt marshes may be to decrease soil elevation and accelerate the conversion of emergent plant habitat to open water, particularly on the lower marsh. The recommendations for management practices intended to conserve coastal marshes include reducing nutrient loading to coastal zones, solving water quality problems with a multiple nutrient approach, and choosing monitoring metrics that are based on both aboveand belowground plant biomass.