Mike Elkinton, age 10, of Redstone Arsenal, Alabama, for his question:

What is the nitrogen cycle?

Nitrogen is needed to make proteins and every living cell needs proteins. Since nitrogen makes up almost 80 per cent of the air, you would think that there is enough of it to supply all the people, plants and animals. So there is. But nature cycles and recycles it to make sure that the world never runs short of supplies. Ordinary nitrogen is changed to useable nitrates, which are changed to nitrates that return the original nitrogen to the air.

The water cycle is fairly easy to understand; the nitrogen cycle is more complicated. Yet both are based on the same general idea.

Water and nitrogen are two basic chemicals needed by all living things. After they have been used, nature changes them back to their original condition to be reused. This cycling and recycling of basic chemical supplies goes around and around.

Plants get their necessary nitrogen from the soil; animals get theirs by eating plants. Both use nitrogen to make proteins, but only a few simple algae and bacteria can get any use from the plentiful supplies of nitrogen in the air. Plants must have nitrogen in a certain chemical form. Animals must eat plant proteins and remodel them to make their own special proteins.

The complicated nitrogen cycle depends on a number of chemical changes. It involves the air, water and soil. Several groups of busy bacteria plus a few fungi are needed to make it work    and sometimes a flash of lightening lends a helping hand.

When a flash electrifies the air, it separates pairs of nitrogen atoms and combines them with oxygen. Nitrates are made of molecules that contain one atom of nitrogen and three atoms of oxygen. Rain dissolves these nitrate compounds and washes them into the soil. There they are absorbed by roots and plants can use this form of nitrogen to build their proteins.

Certain soil bacteria can use nitrogen directly from the air. They combine it with oxygen and carbohydrates to make nitrogen compounds that plants can use. This is called "nitrogen fixing" and the bacteria that do it form small nodules on the roots of the bean family.

Sooner or later, the proteins created by plants and remodeled by animals are returned to the earth. Various groups of bacteria break up animal wastes and decaying plants. During this process, some of the nitrogen may be reused by plants. But eventually the nitrogen is separated and returned to the air.

Decay bacteria remodel nitrogen to form ammonia, which certain plants may be able to use. Nitrifying bacteria change the nitrates in waste proteins into chemical nitrites    which plants cannot use. Finally, denitrifying bacteria break up the nitrites and the original nitrogen is free to cycle back into the air.