『Abstract
　A large pool of nitrogen in the sediment pore fluid of a eutrophic lake in Iowa, USA, was mapped in this study. Previously, the lake had supported fishing and boating, but today it no longer supports its designated uses as a recreational water body. In the top 5 cm of the lake bottom, the pore water nitrogen ranges between 3.1 and 1,250 μg/cm³ of sediments, with an average of 160.3 μg/cm³. Vertically, nitrate concentrations were measured as 153 μg/cm³ at 0-10 cm, 162 μg/cm³ at 10-20 cm, and 32μg/cm³ at 20-30 cm. Nitrate mass distribution was quantified as 3.67×10³ kg (65％) in the bottom sediments, 172 kg (3％) in suspended particulates, and 1.83×10³ kg (32％) in the dissolved phase. soil runoff nutrients arrive at the lake from the heavily fertilized lands in the watershed. Upon sedimentation, a large mass of nitrogen desorbs from mineral particles to the relatively immobile pore fluid. Under favorable conditions, this nitrogen diffuses back into the water column, thereby dramatically limiting the lake's capability to process incoming nutrients from farmlands. Consequently, a condition of oxygen deficiency disrupts the post-season biological activities in the lake.

Keywords: Fertilizer; Iowa; Lake-sediment; Nitrogen; Nutrient; Pore water』

Introduction
Nitrogen chemistry
Study area
Objectives and hypothesis
Materials and methods
　Core sampling
　Water sampling
　Analytical procedure
　Nitrate extraction
　Analysis of dissolved nitrate
Results and discussions
　Lake geometry and sediment distribution
　Mapping of nitrogen in the lake
　Nitrogen in bottom sediments
　Mass calculations (bottom sediments)
　Nitrate in suspended particulates
　Nitrate in dissolved phase
　Conceptual nitrogen transport model
Conclusions
Acknowledgments
References