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. 8.5. 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 



