Abstract. Salt marshes are important hotspots of long-term belowground carbon (C) storage, where plant biomass and allochthonous C can be preserved in the soil for thousands of years. However, C accumulation rates, as well as the sources of C, may differ depending on environmental conditions influencing plant productivity, allochthonous C deposition, and C preservation. For this study, we examined the relationship between belowground root growth, turnover, decay, above- and belowground biomass, and previously reported longer-term rates of total, labile, and refractory organic C accumulation and accretion in Spartina alterniflora-dominated marshes across two mid-Atlantic, US estuaries. Tidal range, long-term rates of mineral sedimentation, C accumulation, and accretion were higher and salinities were lower in marshes of the coastal plain estuary (Delaware Bay) than in the coastal lagoon (Barnegat Bay). We expected that the conditions promoting high rates of C accumulation would also promote high plant productivity and greater biomass. We further tested the influence of environmental conditions on belowground growth (roots + rhizomes), decomposition, and biomass of S. alterniflora. The relationship between plant biomass and C accumulation rate differed between estuaries. In the sediment-limited coastal lagoon, rates of total, labile, and refractory organic C accumulation were directly and positively related to above- and belowground biomass. Here, less flooding and a higher mineral sedimentation rate promoted greater above- and belowground biomass and, in turn, higher soil C accumulation and accretion rates. In the coastal plain estuary, the C accumulation rate was related only to aboveground biomass, which was positively related to the rate of labile C accumulation. Soil profiles indicated that live root and rhizome biomass was positively associated with labile C density for most marshes, yet high labile C densities below the live root zone and in marshes with high mineral sedimentation rates and low biomass signify the potential contribution of allochthonous C and the preservation of labile C. Overall, our findings illustrate the importance of sediment supply to marshes both for promoting positive plant-C accumulation-accretion feedbacks in geomorphic settings where mineral sediment is limiting and for promoting allochthonous inputs and preservation of labile C leading to high C accumulation and accretion rates in geomorphic settings where sediment supply is abundant.