NITROGEN: ABSOLUTELY NECESSARY

Example of amino acid molecule, the blue N being a nitrogen atom; image courtesy of 'Techguy78' & Wikimedia Commons

Living things absolutely need the chemical element known as nitrogen. An atom of nitrogen is necessary in all amino acids, which are not only the building blocks of protein, of which muscles and many other body parts are made, but also the basic constituent of DNA, which carries the genetic code for all living things. Nitrogen atoms must also be present in molecules of ADP and ATP, which enable energy transfer during photosynthesis. Obviously, without nitrogen, life as we know it on Earth is simply impossible!

Though about 78% of the Earth's atmosphere is nitrogen, which is a gas, that nitrogen is usable to plants only when it is converted into ammonia, (NH₃). Nearly all the nitrogen in our animal bodies gets there by way of eating plants, or by eating other animals or animal products, in which originally the nitrogen came from plants.

NITROGEN FIXATION

The process of chemically altering unusable, free atmospheric nitrogen into a form usable by organisms is referred to as nitrogen fixation. In Nature, there are two main ways of "fixing" nitrogen:

• A QUICK WAY: Lightning. If you've ever been close to a lightning flash and right afterwards smelled an ammonia-like odor, that was lightning-fixed nitrogen you smelled. Only a relatively small percentage of nitrogen gets fixed in this way, however. Nature's main nitrogen fixers are...
• THE MAIN WAY: Special microorganisms living mostly in soil and water.

A POINT TO CONSIDER

Soil erosion in a wheat field in Washington state; image courtesy of Jack Dykinga

It's worthwhile to reflect on some implications of what's being said here:

Nitrogen-fixing microorganisms include a few forms of bacteria, the blue-green algae (cyanobacteria), and certain fungi. Though one handful of healthy soil contains more microorganisms than there are people on Earth, already humanity has significantly degraded about a quarter of the planet's productive soil On land, soil erosion, plowing of the soil, pesticides, heat from burning forests and fields all destroy soil microbes. In the sea, nitrogen-fixing microorganisms are killed by pesticides and other chemicals draining from land by way of polluted rivers.

NITROGEN-FIXING NODULES ON BACKYARD CLOVER

Certain nitrogen-fixing bacteria live in nodules, or small, bag-like growths on the roots of certain plants, especially members of the Bean Family. In many backyards, nitrogen-fixing nodules can be seen on the fine, wiry roots of weedy clover species. At the right, showing a clover leaf's typical 3-leaflet "clover leaf shape," is a sprig of White Clover, Trifolium repens, pulled up from beside a sidewalk in a typical small town in the US.

Clover is a member of the Bean Family, of which nearly all members are capable of fixing nitrogen, including such crop plants as soybeans, peas, alfalfa, common beans, peanuts, and lentils.

At the left you can see a much-magnified section of the roots of our pulled-up clover. The brown, baglike things hanging on the larger roots are nitrogen-fixing nodules. The nodules formed when a special soil-dwelling kind of bacteria known as rhizobia (singular rhizobium) invaded the root and multiplied within the roots' cortex cells. The plant welcomed the invasion by providing the rhizobia bacteria nutrients and energy they needed. Within a week after this friendly "infection" occurred, small nodules were visible with the naked eye.

USABLE NITROGEN, STEP BY STEP

Typically, nitrogen-fixing microorganisms do not fix free atmospheric nitrogen to a usable form in one step. Usually one set of organisms converts free nitrogen (N₂) to ammonia (NH₃). This ammonia is accompanied by its ammonium ion (NH₄⁺), which some plants can use. However, most flowering plants need nitrogen in yet another form, which microorganisms provide by converting the ammonia to usable nitrate (NO₃^-).

Therefore, various organisms must work together to accomplish this profoundly important job. However, it's even more complex than what's described above! The process of converting ammonia to nitrate, called nitrification, is usually accomplished by two different sets of bacteria working one after the other. If you want to take a real mind-trip, take a look at our generalized outline of the biogeochemical nitrogen cycle.

A UNITED NATIONS DOCUMENT ABOUT THE NITROGEN CYCLE

We humans are currently throwing the Earth-wide nitrogen cycle out of whack. You can read all about it on the Web, in a freely available UNESCO document entitled Human Alteration of the Global Nitrogen Cycle: Causes and Consequences.