Nitrogen Cycling - April 16, 2003
Jeff Schalau, County Director, Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County

Nitrogen is an essential nutrient and is most likely limiting growth in horticultural and garden plants. In other words, by adding some plant available nitrogen to the growing environment, you should see a corresponding increase in plant growth. This is especially true for crop plants and fruits, nuts, and vines. Conversely, native plants seem to have adaptations that allow them to succeed with naturally available nitrogen.

All living cells need nitrogen to make nucleic acids, proteins, and other cellular constituents. Plants absorb nitrogen, then other organisms eat the plants. Other animals eat the animals and this is how nitrogen becomes available to other organisms in this dog-eat-dog world.

Although the total amount of nitrogen on Earth is fixed, there is an abundant supply in the earth's atmosphere - nearly 79% in the form of N2 gas. However, N2 is unavailable for use by most organisms because the two nitrogen atoms are held together by very tight chemical bonds, making the molecule almost inert. In order for nitrogen to be used for plant growth it must be "fixed" in the form of ammonium (NH4) or nitrate (NO3) ions.

Microorganisms play an important role in converting the inert atmospheric nitrogen into plant available nitrogen. This process is called nitrogen fixation. Biological nitrogen fixation occurs primarily in agricultural fields (35%), forested lands and wildlands (20%), and the oceans (14%). Non-living factors also convert N2 into plant available nitrogen through lightning (4%), combustion (8%), and industrial fertilizer production (20%).

The biological nitrogen fixation process is driven primarily by bacteria which are either free-living or symbiotically associated with plants. The most familiar of these symbiotic associations is between Rhizobium bacteria and plants in the legume family. Whatever pathway is taken, the nitrogen must be converted from N2 to a plant available form.

After plant available forms of nitrogen are in the soil, there are three things that can happen: (1) plants may take it up, (2) it may be leached below the root zone by water, or (3) denitrifying bacteria can use it for an energy source and release it back to the atmosphere as N2. The nitrogen released to the atmosphere may start the cycle over again at some point. The nitrogen in groundwater will remain there as a contaminant or can be pumped to the surface where it could be taken up by a plant.

If nitrogen is taken up by a plant and converted to protein or nucleic acids, then other organisms (insects and animals) can get nitrogen by eating that plant (herbivory). Otherwise, the plant simply dies and become soil litter. Carnivores may also eat the grazing animal as a source of nitrogen. Whether herbivore or carnivore, waste products result. The manure, dead plant litter, and dead animal/insect litter all contain nitrogen that is useable by soil microorganisms. This pathway requires warm soil temperatures. It also bypasses the atmosphere and results in the release of organic matter and mineral nutrients (including nitrogen) directly to the soil and root zone. These decomposition processes are going on in a compost pile.

The relationship between nitrogen and living organisms is critical to life as we know it. In wildland settings, nitrogen is often in balance with the ecosystems and local climate. In crop production, nitrogen is managed to increase crop yields. Respect nitrogen and manage it wisely.

The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site:

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: April 10, 2003
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