Studies on the Release of Ammonium Nitrogen from the Bottom Sediments in Freshwater Regions

Abstract

In the bottom sediments of aquatic regions, the decomposition of organic nitrogenous matter and the generation of ammonium nitrogen result in the vertical distributions of their concentrations. On the assumption that the sedimentation rate has been almost constant through the last several ten years, the elapsed time of the sediments in near surface layer from their deposition on the lake floor as well as the decomposition rate of the organic matter can be estimated. The rate constant in the decomposition process in the sediments was estimated to be 10-4/day in the center of Lake Biwa, and the similar value was obtained in each 0.5 cm layer through a depth of several centimeters. Ammonium salts produced by the decomposition of organic nitrogenous matter enter into the interstitial water, then diffuse through the porous media of the sediments into the overlying water. The diffusion coefficient of ammonium nitrogen in the interstitial water was estimated to be 1 × 10-6cm2/sec and the rate of its release into the overlying water was found to be 4.2μg atoms/cm2/year in the center of Lake Biwa.