Nitrification 2

Nitrogen Cycle Definition, Process and Steps/Stages

Abstract about Nitrogen Cycle

Biogenic or nutrient elements are those elements that are important for biota, life, and life processes. The nutrient cycle also termed a biogeochemical cycle in which nutrients or nutritional elements recycle in an ecosystem from living to non-living and then again to living components in a cyclic manner. Nitrogen is one of them. The major reservoir of nitrogen in the atmosphere constitutes 78% of gases.

Nitrogen is a crucial element as it is part of nucleic acid in DNA, amino acids in proteins, and also serves as a growth and nutritional element. Despite its abundance in the atmosphere, many organisms can not use this nitrogen for making amino acids or nucleic acids, or any other compound of nitrogen. So, there’s a nitrogen cycle that consists of three phases.

The first being nitrogen fixation in which nitrogen is converted to ammonia (NH3 or NH4+) or nitrate (NO3) through high-energy biological reactions. Nitrate is obtained from ammonium via an important bacterial process called nitrification. And the third is assimilation i.e., incorporation of NO3 NH3 generated through nitrogen fixation and also nitrification by animals and plants.

Nitrates are also lost as a consequence of the actions of certain soil bacteria; which in the absence of oxygen break down nitrates and release nitrogen back into the atmosphere. This process is referred to as denitrification.

Biogeochemical Cycle

The elements important for life and life processes in living organisms are called biogenic elements or nutrient elements. Macro-nutrients are elements demanded by organisms in great amounts for example carbon dioxide, hydrogen, oxygen, phosphorus, potassium, sulphur, and calcium.

Micro Nutrients are components required by organisms in small numbers or in trace amounts such as magnesium, molybdenum, iron, potassium. The nutrient cycles are also referred to as biogeochemical cycles in which the nutritional elements proceed from living to nonliving and to living components of the ecosystem in a cyclic way.

The Nitrogen Cycle

Nitrogen is an integral aspect within the nucleic acid: DNA Deoxyribonucleic acid, also a self-replicating material that is found in the majority of living organisms, the primary component of chromosomes, also a carrier of hereditary information. And RNA Ribonucleic acid, a nucleic acid found in every living cell, also acts like a messenger taking instruction from DNA for the synthesis of proteins, which is definitely the major of all biological molecules and important for living things.

DNA contains genetic information, so the directions for making a life form. When plants are not getting enough nitrogen, they are not able to create proteins (compounds which contain hydrogen and nitrogen and also create up most of the living cells, tissue, and tendons). Without amino acids, plants can not create the unique proteins which plant need to grow. Without enough nitrogen, plant growth is influenced adversely. With an excessive amount of nitrogen, plants create surplus biomass, or organic matter, like leaves and stalks, but not enough root arrangement and structure. In extreme situations, plants having quite substantial degrees of nitrogen consumed from lands may become toxic for animals and cattle that will eat them.

The main reservoir of nitrogen is the atmosphere; actually, nitrogen constitutes 78 percent of gases in the air. Since many living entities, nevertheless, can not utilize atmospheric nitrogen to generate proteins and other nitrogen containing components, they depend on nitrogen found in soil minerals. Thus, regardless of the abundance of nitrogen in the air, lack of nitrogen from the soil is frequently the major limiting factor in plant development. The procedure in which a limited amount of nitrogen is circulated and re-circulated throughout the whole world of living organisms is popularly referred to as the nitrogen cycle.

Three primary phases of the cycle include also, ammonification, nitrification, and assimilation.

Stage1: Nitrogen Fixation

Nitrogen fixation is the process in which gaseous nitrogen (N2) is converted into ammonia (NH3 or NH4+) or nitrate (NO3) through high energy biological reactions. N2 is exceptionally stable and also a high amount of energy must be required to break the bonds which combine both N atoms. N2 can be converted straight into NO3through a procedure that applies an enormous quantity of heat, pressure, and energy. Such procedures consist of combustion, volcanic activity, lightning sparks, and industrial ways.

But a greater volume of available nitrogen is naturally generated via the biological conversion of N2 to NH3/ NH4+. A tiny set of bacteria and cyanobacteria are effective with the receptor nitrogenase to break the bonds among the molecular nitrogen and combine it with hydrogen.

Nitrogenase merely works in the absence of oxygen. The exclusion of oxygen is attained in many ways. Some bacteria live under layers of oxygen-excluding slime over the roots of certain plants. The main soil-dwelling bacteria, Rhizobium, live-in oxygen-free zones from nodules in the roots of legume plus other woody plants. Aquatic filamentous cyanobacteria utilize oxygen-excluding cells called heterocyst.

Kinds of Nitrogen Fixation

Atmospheric fixation: A natural process by which the energy of lightning breaks the nitrogen molecules to nitrogen oxides and is then utilized by plants.

Industrial nitrogen fixation: This is a man-made alternative process that assists in nitrogen fixation by the usage of ammonia. Ammonia is made by the proper combination of hydrogen and nitrogen by the Haber process and after that, it is changed to various fertilizers like urea.

Biological nitrogen fixation: We know that nitrogen is not usable directly from the atmosphere by plants and animals. Bacteria like Rhizobium and blue-green algae alter the unusable type of nitrogen to other substances which tend to be far more readily usable. These nitrogen compounds get fixed in the soil by these microbes.

Stage2: Nitrification

Nitrate is derived from ammonium with a crucial bacterial process called nitrification. This is a two-step procedure. The very first step in nitrification is the oxidation of ammonium to nitrite (NO3), the process performed by bacteria within the genus Nitrosomonas. After the formation of nitrite, it is rapidly oxidized further to nitrate, by bacteria within the genus Nitrobacter. The bacteria responsible for nitrification are sensitive to acidity, therefore this process does not occur at significant rates in acidic water or soil (pH level less than 7). This is the reason plants of acidic habitats are capable of utilizing ammonium as their source of nitrogen nutrition.


Stage3: Assimilation

Assimilation is the process in which plants and animals incorporate the NO3 NH3generated through nitrogen fixation and also nitrification. In comparison to the nitrification, this assimilation process requires energy. Plants use these kinds of nitrogen from their roots and incorporate them into amino acids to form special plant proteins and nucleic acids. Animals are subsequently able to utilize nitrogen from the plants by consuming them as food.


Assimilation produces large amounts of organic nitrogen, for example, amino acids, and nucleic acids. Ammonification is the conversion of nitrogen to ammonia. The ammonia produced by this process is released into the environment and is then available for either nitrification or assimilation.

Nitrogen Depletion and its Remedies


Even though the nitrogen cycle seems uninterrupted, complete, and self – replicating, nitrates are lost because of soil erosion, fire, and water percolating down the soil. Nitrates are also lost as a consequence of actions of certain soil bacteria; which in the absence of oxygen break down nitrates and release nitrogen back into the atmosphere. This process is referred to as denitrification, in badly aerated lands.

The cycle is maintained despite these losses primarily by the activities of the nitrogen-fixing bacteria, which incorporate gaseous nitrogen from the air into organic nitrogen-containing compounds. As all organisms are dependent on photosynthesis for energy, so all of them are dependent on nitrogen fixation for their nitrogen. Soil nitrogen requirements are additionally reinforced with the accession of nitrogen fertilizers by the human.

Significance of Nitrogen Cycle

Helps plants to synthesize chlorophyll out of the nitrogen compounds.

Help in converting nitrogen gas to a useable form for plants throughout the biochemical procedure.

By the process of ammonification, bacteria assist in decomposing dead plants and animals thus helping in cleaning up of the environment.

Nitrates and nitrites are discharged into the soil, which helps in improving the soil necessary nutrients required for plant growth.

Nitrogen can also be cycled by human activities including the combustion of fuels as well as the application of nitrogen fertilizers. These procedures, boost the degree of nitrogen-containing chemicals in the air.

Nitrification 2