THE CARBON AND NITROGEN CYCLES loi 



Ammonta 



Nifrit 



An/ma/s 



es 



N/ira'te^ 



remain constant. But it is not — there are many leaks in the 

 cycle. Some of the plants and animals may be burned with the 

 liberation of free nitrogen. Millions of pounds of it reach sew- 

 ers, and from here rivers, lakes, and oceans. In time this is 

 broken down and the nitrates so formed are reduced by denitri- 

 fying bacteria 

 with the liberation 

 of gaseous nitro- 

 gen. The pro- 

 cesses of decay 

 continually going 

 on may also liber- 

 ate free nitrogen. 

 Furthermore, mil- 

 lions of pounds of 

 nitrogen are re- 

 turned to the air 

 by explosives. In 

 some of the battles 

 of the World 



War enough nitrogen was liberated to produce the food of the 

 civilized world for a year. Hence the combined nitrogen 

 would continue to grow smaller were it not that other factors 

 are at work in nature causing them to combine. Every flash of 

 lightning causes some nitrogen to combine as oxides, but the 

 quantity of combined nitrogen thus produced is relatively insig- 

 nificant. The major factors are biological. There are within 

 the soil two great groups of bacteria which possess the power of 

 fixing nitrogen. The first — ^the non-symbiotic nitrogen-fixing 

 organisms living free in the soil — ^are able, with the energy they 

 obtain from the oxidation of organic carbon, to build up complex 

 organic nitrogen compounds. There are two groups of these 

 organisms — the aerobic and the anaerobic, the first being the more 

 important. The other class of nitrogen fixers is the symbiotic. 

 These live in conjunction with legumes and obtain from them 

 carbonaceous material, and in return give combined nitrogen. In 



Ph/fi- Profein 

 Fig. 25. — Illustrating nitrog-en cycle in nature. 



