Salt marsh ecosystems rely on carbon accumulation to increaseelevation and survive sea level rise. The amount of carbon accumulated in a marsh is the net result ofcarbon dioxide emissions to the atmosphere,fixation of carbon by photosynthesis, export of dissolved carbonto the creek, and accumulation of organic carbon in sediments deposited on the surface. Because eachcomponent varies between edge and interior marsh, it is challenging to estimate carbon accumulation for awhole marsh system. It is not currently known how increasing nitrogen pollution impacts carbonaccumulation for a whole marsh. Tofind out, we compared measurements of carbon accumulation infertilized and unfertilized plots in the edge and interior of a salt marsh at Freeman Creek, North Carolina,USA. Overall, the marsh gained carbon in the interior but lost carbon on the edge, leading to a loss of about50,000 kg C year−1across the 0.5 km2marsh area. However, under fertilized conditions, Freeman Creekmarsh carbon loss increased nearlyfivefold overall as a result of the large increase in carbon lossfrom the edge marsh. This study shows that increasing nitrogen pollution in coastal waters will causeincreasing net carbon loss in marshes.