|Floodplains are important landscapes that act as boundaries between terrestrial and aquatic ecosystems. Past research efforts have illustrated that floodplains perform many beneficial functions, such as sediment retention and nutrient transformations, but little is known about old-growth riparian forests due to historical wetland losses in the United States. Thus, the goal of this study was to examine the effect of floodplain landscape variability on nutrient storage and transformations within an old-growth floodplain forest at Congaree National Park (CONG), South Carolina. Our specific objectives were to quantify: (i) soil organic carbon (SOC) storage, (ii) key biogeochemical indices such as microbial biomass, net nitrogen (N) mineralization, and soil phosphorus (P) saturation, and (iii) woody debris decomposition rates and nutrient dynamics. We measured SOC pools in four distinct floodplain landscapes (natural levee, flats, hydric mineral soil wetlands, hydric organic soil wetlands) within CONG. Mean SOC stocks (0-200 cm) were significantly greater in hydric mineral and organic soils when compared to non-wetland soil landscapes because of deep carbon (C) stocks associated with buried surfaces. We also measured biogeochemical indices and woody debris decomposition over a 2 year period on a soil toposequence ranging from the well drained river levee to a poorly drained backswamp. Soil microbial biomass was relatively consistent throughout the study, unlike net N mineralization, which showed significant seasonal variations. Nitrate (NO3-N) was the predominant product of net N mineralization and soil P saturation was low across all toposequence positions. Prolonged drought conditions during the study allowed red imported fire ants to infest and damage the outer bark of downed woody debris. Invertebrate activity and microbial mineralization contributed to extremely fast wood decay rates in all landscapes. Woody debris C content decreased throughout the study, while N and P displayed periods of net immobilization. Results from this research indicate that the soils in the old-growth floodplain forests of CONG function as a sink for C, N, and P. However, rapid woody debris turnover rates and net NO3-N production suggest that the Park may mineralize nutrients quickly from forest litter and be relatively leaky with regard to ecosystem N retention.