Plant Physiology: A Treatise, Volume VIII: Nitrogen Metabolism focuses on the physiological aspects of nitrogen metabolism in plants. This book considers the descriptive biochemistry by which nitrogen compounds and their reactions are recognized; the way the feasible reactions are assembled into metabolic pathways and cycles that give meaningful purpose and direction to the course of metabolism; and the ongoing fate of nitrogen compounds in relation to ontogeny, growth, and development. This volume is organized into four chapters and begins with an introduction to developments in basic and applied biological nitrogen fixation, focusing on the agriculturally important mechanisms by which atmospheric nitrogen is converted to plant protein. The biology and biochemistry of nitrogen-fixing associations and their practical application in agriculture are discussed, along with nitrogen metabolism in the context of various actual situations in which cells divide and grow and specific plants develop, mature, and reproduce. The next chapter deals with the metabolism and turnover of proteins, citing specific proteins (the enzymes ribulosebisphosphate carboxylase and nitrate reductase and seed protein) as the type cases to illustrate the fate of special proteins through their formation and turnover. The mechanisms and biological settings of protein degradation, as distinct from breakdown during protein turnover, are also covered in detail. This book is intended for teachers, research workers, and students with specific interest in plant physiology.