300 Nutrients 



is decomposed and transformed by different microorganisms into in- 

 organic compounds that can be utilized once more in the growth of 

 phototrophic plants. The various steps— some of them reversible- 

 involved in this vital nitrogen cycle are indicated in Fig. 8.5. An 

 example of the quantitative changes in successive nitrogenous prod- 

 ucts is shown in Fig. 8.6 depicting the course of an experiment in 



5 10 15 20 25 30 35 40 45 50 55 



Days 



Fig. 8.6. Decomposition of nitrogenous organic matter in mixed plankton under 

 laboratory conditions, showing the successive appearance of soluble nitrogen com- 

 pounds. One portion inoculated with diatoms after 45 days; dotted lines represent 

 uninoculated portion. (Von Brand, Rakestraw, and Renn, 1937, Biological Bull.) 



which marine plankton decomposed and a new growth of the plank- 

 ton took place after regeneration of the nutrients. Phosphorus, car- 

 bon, sulphur, and other elements entering the living complex take 

 part in similar cyclic movements between the organism and the en- 

 vironment. The carbon cycle is represented diagrammatically in 

 Fig. 8.7. 



Place of Decomposition 



In the terrestrial environment the decomposition of organic matter 

 takes place on and in the soil. Material in particulate form is carried 

 beneath the surface by organisms and by rain to depths varying ac- 

 cording to the ecological conditions as discussed in Chapter 3. Mate- 

 rial in solution may be transported still further by percolating water. 

 The decomposition of this organic matter thus may take place at a 

 variety of levels within the soil profile, although most of it goes for- 

 ward in the A horizon. As we have mentioned earlier, a good por- 

 tion of the products of decomposition may be retained in the soil 

 when the vegetation is suitable and the rainfall moderate, but some 



4 



