Carbon Sequestration and Nutrient (Nitrogen, Phosphorus) Accumulation in River‐Dominated Tidal Marshes, Georgia, USA

Abstract

Soil organic C, N, and P were measured in salt, brackish, and tidal freshwater marshes in river‐dominated estuaries (Ogeechee, Altamaha, and Satilla) of the Georgia coast to evaluate the effects of salinity on C, N, and P storage and accumulation. Tidal freshwater marshes had greater concentrations of organic C (10.81% w/w) and N (0.71% w/w) than brackish (7.71% C, 0.50% N) or salt (5.95% C, 0.35% N) marshes. Soil accretion rates of 137Cs were greater in tidal freshwater (4.78 mm yr−1) and brackish marshes (4.41 mm yr−1) than in salt marshes (1.91 mm yr−1). Consequently, organic C and N accumulation was greater in tidal freshwater (124 and 8.2 g m−2 yr−1) and brackish (93 and 6.5 g m−2 yr−1) marshes than salt marshes (40 and 2.4 g m−2 yr−1). Phosphorus accumulation was greater in the brackish marshes. Lower salinity tidal freshwater and brackish marshes remove more C, N, and P; however, salt marshes dominate the spatial extent of the study area (60%) vs. brackish (33%) and tidal freshwater marshes (7%). Combining measurements of C, N, and P accumulation with tidal marsh area, we estimated that tidal freshwater, brackish, and salt marshes stored or removed the equivalent of 2 to 20% of watershed N inputs entering the estuaries from the terrestrial landscape. After accounting for N2 fixation and denitrification, tidal marshes collectively removed the equivalent of 13 to 32% of the N entering estuaries. Tidal marshes, especially tidal freshwater and brackish marshes, are important for improving water quality and decreasing the impacts of N eutrophication of estuarine ecosystems.