William Booth, age 10, of Charlotte, North Carolina, for his question:
How does the air keep its oxygen nitrogen balance?
We know that enormous quantities of oxygen are consumed every day by plants and animals. We know that living cells must have nitrogen, all of which comes indirectly from the air. Yet the basic ingredients in the atmosphere do not change its ratio of nitrogen and oxygen remains more or less the same. This impossible situation is maintained by recycling, by using and reusing the same basic ingredients in different forms.
The earth has a fixed quota of about 92 chemical elements. But these assorted atoms are kept extremely busy. For they are the basic building blocks available to construct all the earth's countless solids, liquids and gases. The system must work without wasting these basic materials. Nature does this by using chemical reactions. Atoms are united in molecule packages to form compounds of different substances. When their work is done, molecules are broken apart and the original atoms reused to construct other useful compounds.
This system is used to maintain the balance of nitrogen and oxygen in the air. It operates constantly on a global scale. The fine details are very complicated and it may take a long time to complete them. The raw materials are taken from the countless tons of nitrogen that make up three fourths of the air and the oxygen that makes up about one fourth. Each day, a certain quota starts a cycle of duties on the earth. Each day the same quota completes a cycle and returns to the air.
The nitrogen and oxygen cycles are necessary to support all the life processes of plants and animals. However, 'aoth operations begin with the plant world. When plants use carbon dioxide to make their food, its molecules are broken apart and some of its oxygen atoms return to the air. Other complex stages may be involved. But as a rule, the oxygen cycle is less complex than the nitrogen cycle. This is because plants cannot use nitrogen in its basic form, and its compounds are built into materials that take a long time to break apart.
Various operations can fix the air's nitrogen into useable compounds. One starts when searing, lightning forms molecules of nitrite in the air. This washes into the soil where it can be changed to nitrates, which plants can use to build and grow. Animals eat plant food to build and grow, too and finally all these materials decay. This is when the original atoms separate and the nitrogen cycles back to the air.
Naturally the nitrogen and oxygen cycles are enormous operations. Each involves a very complex series of chemical changes. What's more, the basic gases may be cycled around by different chemical routes, or run through the same route several times before returning to the air. But on the average, the returning quotas equal the quotas setting forth on the next cycling operations.