The Nitrogen inputs into a Natural Forest or Prairie Ecosystem

How does Nitrogen enter a natural system?

Nitrogen enters natural ecosystems from two primary sources.

Biological Nitrogen Fixation – Certain microorganisms can capture nitrogen gas from the atmosphere and convert it into forms that plants can use. This process, known as nitrogen fixation, is carried out by Rhizobium bacteria that live in association with legumes such as clover, peas, and beans; by free-living soil bacteria such as Azotobacter; and by cyanobacteria that inhabit wet soils and aquatic environments.

Atmospheric Deposition – Smaller amounts of nitrogen enter the ecosystem through rainfall, dust, and lightning. These natural processes continuously deliver modest amounts of nitrogen to the soil throughout the year.

However, these new sources of nitrogen alone are not sufficient to support the productivity of a prairie, forest, or other mature ecosystem. The true strength of these systems lies in their ability to recycle and retain nitrogen. Once nitrogen enters the ecosystem, it is continually cycled through plants, roots, microbes, fungi, earthworms, and soil organic matter. As plants and organisms die and decompose, their nitrogen is released and reused by future generations of plants. This efficient recycling system allows natural ecosystems to remain productive for centuries while requiring relatively small annual additions of new nitrogen.

The Largest Source of Nitrogen Is Recycling

Scientists estimate that in mature ecosystems, 80 to 95 percent of the nitrogen used by plants comes from the recycling of organic matter rather than from new nitrogen entering the system.

In other words, nature's fertility system depends on recycling existing nitrogen and minimizing nitrogen losses. Once a plant dies, soil organisms decompose the organic matter, incorporating some of the nitrogen into their own bodies while gradually releasing the remainder back into the soil in plant-available forms. The nitrogen is then taken up by new plants, transferred through the soil food web, and incorporated into roots, leaves, microbes, fungi, and soil organic matter. This cycle repeats continuously. Much of the nitrogen remains stored within living organisms and organic matter rather than existing as free nitrate in the soil, where it is vulnerable to leaching or runoff. Because natural ecosystems maintain continuous plant cover, living roots, and active biological communities, they are remarkably effective at capturing, recycling, and retaining nitrogen. As a result, very little nitrogen leaves the system, allowing the same nitrogen atoms to support plant growth repeatedly over many years.