Nitrogen Cycle

Enhanced Flash Version

Nitrogen is one of the most essential elements on earth; all living things are composed of nitrogen containing compounds, in the form of amino acids, which compose an organism's protein. Although nitrogen gas composes about 78 percent of the gasses in the atmosphere, most living things cannot use it in is free form. For this reason most plants require ammonium and nitrate which are present in the soil, but not in as abundant of quantities as the nitrogen gas in the atmosphere, this often creates an abundance of available nitrogen gas but not enough ammonium or nitrates which plants require, limiting plant growth in nitrogen deficient areas.

The cycling of the earth's limited amount of nitrogen is know as the nitrogen cycle and has three principal stages; these are ammonification, nitrification and assimilation.

1. Ammonification also known as nitrogen mineralization occurs when soil dwelling saprophytic bacteria decompose dead organic matter, which are composed of complex nitrogen containing compounds such as proteins and amino acids, and nucleic acids these bacteria use the nitrogen they obtain to create their own amino acids and proteins and release the excess nitrogen as ammonium which can then be used by plants.
2. Nitrification occurs when bacteria oxidize ammonia or ammonium ions, this chemical reaction produces energy, which the bacteria use to convert carbon dioxide into nitrite, hydrogen, and water. Since the nitrite produced by the bacteria is toxic to plants it must be converted to nitrate by another species of bacteria, once converted to nitrate, the nitrogen is available for absorption by plants
3. The assimilation of inorganic nitrogen in the form of ammonium or nitrate into organic compounds such as proteins, amino acids and nucleic acids is one of the most important processes on earth and is almost equal to photosynthesis and respiration. The greatest source of nitrogen to crop plants is in the form of nitrate, which is broken-down and reduced to ammonia, which can be quickly incorporated into organic compounds like amino acids. Although most plants receive their nitrogen in an inorganic form like nitrate some plants in the artic, where nitrogen availability is limited, are able to utilize organic nitrogen from dead organisms, these specialized plants are able to do so without going through the process of Ammonification.

The cycling of nitrogen does not occur with out some nitrogen being lost from the soil-plant system. Most of the nitrogen in the system is lost, during the process of denitrification where microorganisms, convert nitrate into nitrogen gas and nitrous oxide, which returns to the atmosphere. The process of denitrification requires there to be no oxygen present so it is called anaerobic it was long though that anaerobic conditions could only be found in water logged soils such as those of marshes and bogs, but scientists are now finding that denitrification is a universal soil process, since conditions suitable for denitrifying bacteria are present in almost all soils.

Nitrogen is not only lost due to the conversion of nitrate into nitrogen gas or nitrous oxide through the process of denitrification, it can also leave the system when plants are harvested or burnt along with soil erosion and leaching by rain water which is able to easily wash nitrogen away from a plants root zone into streams or the water table where it is lost in the deep sediments of lakes or oceans.

Fortunately nitrogen is also added back to the plant-soil system by a process called nitrogen fixation. Nitrogen fixation is the primary way in which the nitrogen in an ecosystem is replenished. This process occurs when nitrogen gas in the atmosphere is reduced to ammonium and is then made available, for the production of organic nitrogen containing compounds through assimilation by plant cells. Nitrogen fixation can only be accomplished by very specialized bacteria, upon which all organisms are dependant, just as all life is dependant upon photosynthesis to harness and convert energy from the sun.

Two types of nitrogen fixing bacteria exist, the free living and the symbiotic. The most effective bacteria are the symbiotic, which are those, which form a relationship with a plant host, most likely a legume such as alfalfa, clover, peas, and beans. In return for providing the plant with a source of nitrogen the plant provided the bacteria with energy and carbon containing molecules, which are essential for the synthesis of nitrogen containing compounds. These nitrogen-containing plants add nitrogen back into the soil when they die, enriching the soil for non-nitrogen fixing plants. Nitrogen is also fixed by free-living soil bacteria, which rely on the oxidation of organic matter to provide the needed energy for fixation rather than a plant host.

Although natural nitrogen fixation is one of the most effective ways in which nitrogen can be made available to plants it is very slow and requires an entire growing season to accomplish. For this reason, industrialized nations such as the United States nitrogen is fixed by an industrial process where nitrogen gas is reacted with hydrogen gas under extreme pressure and temperature to create ammonium. This process is extremely costly and requires large amounts of fossil fuels to provide the hydrogen gas required in the reaction. Despite this high cost industrial nitrogen fixation can account for up to one-third of the worlds newly fixed nitrogen each year.