FACTORIES UNDERFOOT: THE NITROGEN CYCLE

by Adelia Ritchie, Summer 2020
Photo by John F. Williams
Photo by John F. Williams

FACTORIES UNDERFOOT: THE NITROGEN CYCLE

by Adelia Ritchie, Summer 2020

 

What do we mean when we say that some plants are “nitrogen fixers”? No, it doesn’t mean that they are repairing nitrogen! Let’s take a moment to get a grip on this hugely important subject — nitrogen fixation and the nitrogen cycle. Life as we know it could not exist without this process.

A living system cycle is where a substance will change its form or its chemistry — in order to do different jobs that are essential to keeping the planet alive — and then change back again to its original form. And they do this over and over again, for eternity.

Specifically, the nitrogen cycle, mentioned elsewhere in this issue, describes how nitrogen moves between plants, animals, bacteria, the atmosphere, and underground. Nitrogen is an element that is vital to all life on earth, and in its elemental form it composes nearly 80% of the air we breathe. Nitrogen is a vitally important component of all life, as part of our amino acids, proteins and our DNA. It’s also needed to make chlorophyl in plants which is required in order for photosynthesis (the process by which plants make their food) to take place.

However, atmospheric nitrogen, which we represent as the symbol N2, is useless as a nutrient or fertilizer. N2 requires a dramatic change requiring several complex steps in order to support life. For this conversion, and ultimately back again to atmospheric N2, we can thank the millions of helpful bacteria that live in the soil working tirelessly around the clock. The diagram below shows a simplified nitrogen cycle.

Diagram by Adelia Ritchie
Let’s start at the beginning.

Atmospheric Nitrogen. How does nitrogen in the air get into the soil? There are three ways: The first, not shown on the diagram, is by lightning! The energy from a bolt of lightning causes atmospheric nitrogen (N2) and water (H2O) to combine to form ammonia (NH3) and nitrates (NO3), which dissolve in rainwater and fall to the ground. These are absorbed into the soil where they can be assimilated directly by plants. The second way is by simple diffusion of N2 into the surface of the soil. There are also industrial processes that convert nitrogen into forms useful for plants, also known as commercial fertilizers, which are typically applied directly to planting areas.

 

Nitrogen-fixing bacteria in root nodules. Because nitrogen in its gaseous form (N2) can’t be used by most living things, it has to be converted or “fixed” to a more usable form through a process called fixation. In host plants (e.g., alder trees, peas and clover), nitrogen-fixing bacteria “invade” the root hairs and form root nodules, which are enlargements of plant cells and bacteria. Inside these nodules, the bacteria convert free nitrogen (N2) to ammonia (NH3), which the host plant can use to develop and grow. But not all plants have this same association with bacteria.

 

Nitrogen-fixing bacteria, free-living in soil. Other types of free-living bacteria in the soil do the same thing without the help of a host plant, but the process requires a few more steps. There are other organisms at work here too, the “decomposers,” which also break down animal waste, dead plants and other organic matter into ammonia. Then, in a two-step process called “nitrification,” this ammonia is ultimately converted into nitrites and nitrates that can be taken up and used by plants.

Assimilation. Finally, arriving at a happy place for all plants, nitrogen compounds that have been produced from “thin air” can now be taken up by roots of all plants in the neighborhood, which will then be eaten by animals and insects. But wait! Where does the atmospheric nitrogen come from?

 

Denitrification. Yet another type of bacteria is the agent of the process of converting unused nitrates (NO3) back to atmospheric nitrogen (N2). These bacteria obtain energy by “consuming” nitrates and releasing nitrogen gas back into the atmosphere, and the cycle starts all over again.

 

The next time you take a walk outdoors, it might be fun to think about what’s going on right under your feet! So many different organisms, so many fascinating processes going on, all unnoticed by most of us, but without them there would be no life on earth. The nitrogen cycle is just one of many of these invisible processes that serve as the engine that drives all life. And as you inhale all that fresh air, consider that most of it is basic elemental nitrogen, provided for you for free by your local soil bacteria.

Adelia Ritchie grew up on a northern Virginia farm with horses, cattle, dogs, and her pet pig Porky, who ran the whole show. A long-time resident of the great Pacific Northwest, Adelia is a serial entrepreneur, scientist, educator, and artist, and currently works with educators and legislators to promote a deeper understanding of the science of climate change and its impacts on the complex ecological web of life. Adelia resides in Hansville, WA, with her garden, her dogs and a flock of very entertaining chickens.

Table of Contents, Issue #8, Summer 2020

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