Congaree National Park contains one of the only remaining old-growth bottomland hardwood forests in the United States. Assessment of biogeochemical processes in riverine forests is important because these properties can influence net primary productivity by controlling the rate of transformation and subsequent availability of inorganic (plant-available) nutrients. In addition, there are also numerous water quality improvement functions associated with bottomland forests, such as sediment trapping and phosphorus (P) retention from flood waters. Thus, the primary objectives of our study are to quantify i) in situ net nitrogen (N) mineralization rates; ii) microbial biomass carbon (C) and N, and iii) soil P saturation. The Congaree floodplain contains a range of heterogeneous ecosystems composed of unique fluvial landscapes with distinct sedimentation patterns, hydroperiods, soil properties, and vegetation communities. Therefore, biogeochemical indices will be quantified from 20 research plots situated along transects perpendicular to the Congaree River. These research plots represent a toposequence of common floodplain microsites ranging from the well drained natural river levee to a poorly drained backswamp. Net N mineralization and microbial biomass are being assessed seasonally along these transects (January, April, July, October) for 2 years to evaluate temporal trends in biogeochemical properties. Initial net N mineralization data collected from October 2010 through May 2011 ranged from 55-286 g/ha/day, with a mean rate of 116 g/ha/day. Initial mean microbial biomass C (range 162-2325 μg/g) and N (range 15-253 μg/g) data indicated significantly greater (α = 0.05) microbial biomass on the higher elevation plots (close to the natural levee) compared to low-lying backswamp landscapes. Further analyses of soil P saturation will be quantified using composite samples from each of the 20 research plots. Additional site-specific microsite factors such as sedimentation rates, relative elevation, and soil drainage class will be measured to see if they affect biogeochemical processes. Results from our study can be transferred to similar old-growth bottomland ecosystems and allow us to better understand the fundamental processes that drive forest productivity in riverine forests of the southeastern United States.