8 - Nitrogen Biogeochemistry and Surface-Subsurface Exchange in Streams

Abstract

This chapter reviews the predominant nitrogen forms, transformation pathways, and terrestrial sources in fluvial environments. It examines the variables related to hydrologic exchange that control the direction and rate of nitrogen transformations including hydrologic residence time, the redox environment, and ammonium sorption to sediments. Nitrogen, in conjunction with carbon, phosphorus, oxygen, and hydrogen, is one of the most important elements of living matter. Occurring primarily in proteins, nucleic acids, and other nitrogen-containing organic compounds, nitrogen constitutes about 10% dry mass in bacteria. Nitrogen-fixing bacteria form symbiotic associations with plant roots, and are free living. Symbiotic associations are the most important in terms of the amount of nitrogen fixed. Most of the fixed nitrogen is assimilated by plants, and is later released to the stream as through fall, as leachate from litterfall, or by plant decomposition. Heterotrophic bacteria liberate organically bound nitrogen as ammonia or ammonium. In addition to assimilatory nitrate reduction, obligate and facultative anaerobic bacteria dissimilatatively reduce nitrate to nitrite, nitrous oxide, and dinitrogen (denitrification) during the oxidation of organic matter in anoxic environments.