The Nitrogen Cycle and How Your Plants Benefit From It

Understanding the nitrogen cycle as a gardener or landscaper isn't a necessity, but it's wonderfully interesting information to know. While it can be somewhat confusing, the information below is designed to explain the process thoroughly.

Nitrogen is a chemical element, known by the symbol of N, and is colorless, odorless, tasteless and constitutes approximately 78% of Earth's atmosphere by volume. Nitrogen is required for all living things on Earth to exist and grow due to it being an essential component of DNA, RNA and amino acids for protein.

Nitrogen in its gaseous form is almost entirely unusable to anything living, but during the nitrogen cycle, it's converted into a usable form.

Organic nitrogen exists in materials which are formed from manures, sewage waste, compost and decomposing roots or leaves caused by the activities of animal, humans and plants. Eventually, these organic products transform into humus, an organic soil material.

Inorganic nitrogen comes from minerals and is delivered to the soil via precipitation or fertilizers. The addition of nitrogen helps plants to grow and stay healthy, but plants are not able to use organic forms of nitrogen, so microbes in the soil convert it into inorganic forms that plants can use.

The nitrogen cycle defines the process of nitrogen undergoing chemical reactions, or when atoms and molecules recombine by forming or breaking bonds. This causes nitrogen to change form and move between the atmosphere, biosphere and geosphere in varying forms.

The basic nitrogen cycle goes like this:

Fixing > To Plants > To Animal > To Atmosphere > Fixing

There are five processes to the nitrogen cycle which are all driven by micro-organisms.

1. Fixation - This is the process of converting nitrogen in the atmosphere into ammonia. Nitrogen fixation uses energy to combine with other elements, such as when caused by lightning. Biological fixation happens when bacterial microbes such as from plants in the legume family and animals such as termites create nitrogen compounds that move up through the food chain.

2. Assimilation - The assimilation process is the conversion of inorganic nitrogen (nitrate) into an organic form, such as an amino acid. The nitrate is reduced first to nitrite by enzymes, then to ammonia. The ammonia produced during fixation is incorporated into protein and other compounds such as the host plant, another soil organism or the bacteria itself. Organism near the top of the food chain then eat and use the nitrogen.

3. Mineralization - This is a process of conversion of organic matter into an inorganic form, otherwise known as decay. When organisms die or excrete organic material, bacteria and fungi chemically alter the ammonia and consume the matter which starts the decomposing process. This then turns the nitrogen contained within to ammonium, making it available for plants to use.

4. Nitrification - This process describes the conversion of ammonium to nitrate during the decomposition phase, which is what happens to most of the ammonia produced by decay. This process can only occur in oxygen-rich environments such as surface layers of soil and flowing water. More bacteria convert ammonium into nitrates. Plants take up nitrates through roots to stem, then to the leaves where nitrate and glucose are combined to make proteins.

5. Denitrification - While the previous processes remove nitrogen from the atmosphere and cycle it through various ecosystems, denitrification returns nitrogen to the atmosphere. Microorganisms and denitrifying bacteria use nitrates in place of oxygen during respiration and reduce it to nitrogen gas which replenishes the atmosphere, ending the cycle.