Elements of ecology (1954) Elements of ecology elementsofecolog00clar Year: 1954 Decomposition and Transformation 299 genera Nitrosomonas and Nitrosococcus, and the oxidation of nitrite to nitrate is carried out by bacteria of the genus Nitrobacter. The supply of available nitrogen in the soil and in the water may be further augmented as the result of nitrogen fixation by specialized bacteria belonging to the genera Azotobacter (aerobic) and Clos- tridium (anaerobic), which are free living, and Rhizobium, which live symbiotically in root nodules on certain higher plants, notably: locust trees
Elements of ecology (1954) Elements of ecology elementsofecolog00clar Year: 1954 Decomposition and Transformation 299 genera Nitrosomonas and Nitrosococcus, and the oxidation of nitrite to nitrate is carried out by bacteria of the genus Nitrobacter. The supply of available nitrogen in the soil and in the water may be further augmented as the result of nitrogen fixation by specialized bacteria belonging to the genera Azotobacter (aerobic) and Clos- tridium (anaerobic), which are free living, and Rhizobium, which live symbiotically in root nodules on certain higher plants, notably: locust trees, alfalfa, clover, beans, peas, and other legumes. These bacteria take in free nitrogen, build it into their bodies and, in the • Nitrate( NO3I salts-<- in soil and water Fig. Diagram of the nitrogen cycle, showing the principal components and processes. Compounds not included in circles are free in the environment. (Modified from Principles of Modern Biology by Marsland and Plunkett. By permission of Henry Holt and Co., Copyright, 1945.) case of the symbiotic forms, pass it on to their hosts. After these bacteria or the tissues of the host plants die and decompose, the sup- ply of fixed nitrogen that they contain is made available through the activities of the nitrifying bacteria. Free nitrogen can also be fixed by certain photosynthetic bacteria, sulfate-reducing bacteria, and blue-green algae. Other types of bacteria cause denitrification, or the loss of fixed nitrogen from the environment, by reducing nitrate to nitrite and nitrite to nitrogen, or even to ammonia (Frobisher, 1944, Ch. 25; Welch, 1952, Ch. 10). Thus nitrogenous material derived from dead animals and plants
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