THE NITROGEN CYCLE
& Nature's Interconnectedness
Living things absolutely need the chemical element known as nitrogen. This is easy to believe when we remember that an atom of nitrogen lies at the heart of all amino acids, which are not only the building blocks of protein, of which muscles and many other of the body's 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, plants and animals don't necessarily have an easy time getting all the nitrogen they need. The problem is that green plants can't use the nitrogen that's free in the atmosphere. Some chemistry must be done on every molecule of free nitrogen before it becomes useful to most living things.
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:
FIRST 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...
SECOND WAY: Special microorganisms living mostly in soil and water.
It's worthwhile to pause a moment here and reflect on the fact that what's being said is this:
Life on Earth depends on nitrogen-fixing microorganisms -- creatures we can't see without a microscope -- living in soil and water.
When you consider how humankind pollutes the Earth's soil and water with pesticides and other chemicals, sterilizes soil with slash-and-burn agriculture in much of the world's tropics, and allows agricultural soil to erode into our rivers much faster than it can be formed, you wonder why there's so little concern being expressed.
Nitrogen-fixing microorganisms, existing abundantly but practically invisibly nearly everywhere, include a few forms of bacteria, the blue-green algae, and some fungi. Some 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, nodules can be seen on the fine, wiry roots of clover, a member of the Bean Family, and considered a weed by those who don't know its importance.
Right now I push myself away from the keyboard, step from my front door and not a yard from my door instantly spot several little clover plants. I dig up one of them -- it's White Clover, Trifolium repens -- place it on my scanner, and the image that results is the one appearing at the top of this page.
The image at the right is a much-magnified section of the roots of the clover in the above photo. The brown, baglike things hanging on the larger roots are nitrogen-fixing nodules. Probably 99% of the readers of these words can find such nodules on plants in their own backyards! When I think what an important job the little clover with its nitrogen-fixing nodules was doing for me and the rest of the ecosystem, I feel a little guilty for having dug the plant up. However, it's important that you know about this, and know that so many things in nature that you haven't learned of yet are just as important...
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-).
Already you see that 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.
But, we don't want to get bogged down here. The main thing to keep in mind is that in average soil there are many billions of organisms of many different kinds working together to do a job that life on Earth depends on...
As you might expect, we humans are currently throwing the Earth-wide nitrogen cycle out of whack. You can read all about it on the Web, in an essay called Human Alteration of the Global Nitrogen Cycle: Causes and Consequences.
The point of all these words about nitrogen is not to convince you that nitrogen is wonderful stuff, though it is. The point is that nature is composed of a huge number of interrelated parts, and nitrogen with all of its jobs is just one tiny, usually ignored part. It's good to keep these insights in mind not only for the pleasure we feel when thinking about them, but also because of this: