Mean soil respiration rates (carbon dioxide efflux from bare soils) among salt marshes in Narragansett Bay, RI ranged from 1.7–7.8 µmol m⁻² s⁻¹ in Spartina patens in high marsh zones and 1.7–6.0 µmol m⁻² s⁻¹ in S. alterniflora in low marsh zones. The soil respiration rates significantly increased along a gradient of increasing watershed nitrogen (N) loads (S. alterniflora, R²  =  0.95, P  =  0.0008; S. patens, R²  =  0.70, P  =  0.02). As the soil respiration increased, the percent carbon (C) and N in the soil surface layer decreased in the S. alterniflora, suggesting that in part, the increased soil respiration rates are contributing to the increased turnover of labile organic matter. In contrast, there were no apparent relationships between the soil respiration rates in the high marsh and the soil C and N contents of the surface layer. However, there was a broad-scale pattern and significant inverse relationship between the high marsh soil respiration rates and the landscape belowground biomass of S. patens. As more and more salt marsh systems are subjected to increasing nutrient loads, decomposition rates of soil organic matter may increase in marsh soils leading to higher turnover rates of C and N.