EFFECTS OF FLOOD PULSES ON NITRIFICATION RATES IN UPPER MISSISSIPPI RIVER FORESTED FLOODPLAINS T.M.Jicha1, 2, B.H. Hill1, L. Johnson3, C.M. Elonen1, and M.S. Pearson1 1 US Environmental Protection Agency, Mid-Continent Ecology Division, Duluth, MN 2 University of Minnesota-Duluth 3 Natural Resource Research Institute, UMN- Duluth, MN Flood frequency and season are the major drivers of floodplain nitrification. The movement of nutrients between terrestrial and aquatic ecosystems during flooding of large floodplain rivers plays an important role in nutrient fluxes downstream. Human control over the natural flood regime is thought to be directly related to downstream phenomena, e.g.; Gulf of Mexico hypoxia. Understanding how flooding affects nitrogen processes is important for ecosystem management of our large floodplain rivers. In this study, the effect of flood pulses on nitrification was examined at 30 sites in the forested floodplain of the Upper Mississippi River Pool 8 in April, August, and November, 2006. Sites were equally divided among three flood pulse categories: rare, moderate, and frequent, based on elevation and the HEC-RAS flood probability model developed by the Corp of Engineers. ANOVA showed a significant difference in nitrification rates among flood pulse categories: rare > moderate > frequent, and over season: summer> spring> autumn. Linear Regression (r2=0.35, p=0.0049) showed that total carbon, pH, moisture, and percent sand explained a significant amount of variation in nitrification rates. Well oxygenated conditions of dry, sandy soils in rarely flooded areas allow for high nitrification and presumed low denitrification rates. This nitrification/denitrification combination results in a surplus of nitrate available for return to the river and downstream with the next flood. This abstract does not necessarily reflect US EPA policy. Keywords: ecosystem services and functional roles, wetlands of the Upper Mississippi River, nutrient trading, nitrification, Effects of flooding.