The nitrogen cycle is a biogeochemical cycle through which nitrogen is converted into different chemical forms as it circulates among the atmosphere, terrestrial and marine ecosystems. Nitrogen conversion can be conducted through biological or physical processes.
The important processes in the nitrogen cycle include nitrification, denitrification, fixation, and ammonification. The majority of the atmosphere of the earth is nitrogen. It accounts for 78% making it the leading source of nitrogen. However, atmospheric nitrogen has limited availability for biological use. This leads to a scarcity of the usable nitrogen in many types of ecosystems.
The nitrogen cycle is of great interest to ecologists because the availability of nitrogen may affect the rate of important ecosystem processes such as decomposition and primary production. Human activities like fossil fuel combustion, artificial nitrogen fertilizer use, and the release of nitrogen in wastewater have greatly changed the global nitrogen cycle. Human alteration of the global nitrogen cycle can negatively affect the system of the natural environment and also human health.
NITROGEN CYCLE PROCESSES
Nitrogen is available in the environment in different chemical forms like organic nitrogen, nitrite (NO-2), ammonium (NH4+), nitrous oxide (N2O), nitrate (NO3), inorganic nitrogen gas (N2) or nitric oxide (NO).
Organic nitrogen can be in the form of humus, a living organism, or the intermediate products of decomposition of organic matter. Nitrogen cycle processes are to transform nitrogen from one form to another. Many of these processes are conducted by microbes, in their effort to accumulate nitrogen or harvest energy. For example, nitrogenous wastes in the urine of animals are broken down by nitrifying bacteria in the soil for use by plants.
NITROGEN FIXATION
The conversion of nitrogen gas into nitrites and nitrates via industrial, biological and atmospheric processes is known as nitrogen fixation. Atmospheric nitrogen must be (fixed) or processed into usable forms in order for it to be taken up by plants. Fixation can be done by lightning strikes but most of it is done by free-living or symbiotic bacteria called diazotrophs. Most biological nitrogen fixation takes place by the activity of Mo-nitrogenase that is found in a wide variety of bacteria and in some Archaea. An example of free-living bacteria is Azotobacter. Symbiotic nitrogen-fixing bacteria like Rhizobium normally live in the root nodules of legumes like peas and alfalfa. They then form a mutualistic relationship with the plant, producing ammonia in exchange for carbohydrates.
ASSIMILATION
Plants may absorb ammonium or nitrate from the soil through their root hairs. In cases where nitrate is absorbed, it is first reduced to nitrite ions and then ammonium ions for incorporation into chlorophyll, amino acids and nucleic acids.
AMMONIFICATION
When an animal or a plant dies or an animal excretes waste, the initial form of nitrogen is organic. Fungi or bacteria convert the organic nitrogen in the remains back into ammonium (NH4+), a process known as mineralization or ammonification. Some of the enzymes involved include Gln synthetase and Glu dehydrogenase.
NITRIFICATION
The conversion of ammonium to nitrate is done by soil-living bacteria and other nitrifying bacteria. Bacteria like the Nitrosomonas species perform the primary stage of nitrification, the oxidation of ammonium. This converts ammonia to nitrites. Other bacterial species like Nitrobacter perform the oxidation of nitrites (NO2) into nitrates (NO-3).
DENITRIFICATION
Denitrification refers to the reduction of nitrates back into nitrogen gas. This completes the nitrogen cycle. This process is conducted by bacterial species like Paracoccus and Pseudomonas under anaerobic conditions.
OTHER PROCESSES
Despite nitrogen fixation being the primary source of plant-available nitrogen in most ecosystems, in areas that have bedrock rich in nitrogen, the breakdown of this rock also serves as a nitrogen source.
