Nitrogen Cycle
The nitrogen cycle is a vital ecological process that describes the transformation of nitrogen across various chemical forms through biological, physical, and geologic actions on Earth. Nitrogen is crucial for all living organisms as it forms a key component of proteins and nucleic acids. Most nitrogen exists as molecular nitrogen (N2) in the atmosphere, but it must be converted into biologically usable forms by certain bacteria through a process known as nitrogen fixation. This process allows nitrogen to enter food webs, beginning with plants that absorb it from the soil.
The cycle includes several key processes: ammonification, nitrification, and denitrification. Ammonification involves the conversion of organic nitrogen into ammonia, nitrification transforms ammonia into nitrates, and denitrification returns nitrogen to the atmosphere by converting nitrates back into nitrogen gas. Human activities, such as the use of nitrogen fertilizers and fossil fuel combustion, significantly disrupt this natural cycle, leading to issues like water pollution and eutrophication, where excess nitrogen causes harmful algal blooms. As climate change impacts the environment, the dynamics of the nitrogen cycle are increasingly important to understand for maintaining ecosystem health and mitigating environmental challenges.
Nitrogen Cycle
The nitrogen cycle is the shift between different chemical forms of nitrogen through Earth's biological, physical, and geologic processes. Nitrogen is an essential element for all living things. It is a building block of biological molecules such as proteins and nucleic acids. Most nitrogen on the planet is in the form of molecular nitrogen in the air. Only certain bacteria can convert nitrogen into biological molecules that occur mainly inside living cells. Humans are interfering with the nitrogen cycle by making nitrogen fertilizers and oxidizing atmospheric molecular nitrogen by extensively burning fossil fuels.
Cycling of Nitrogen on Earth
Most of Earth’s chemical elements circulate through biological, physical, chemical, and geologic processes. These processes operate in circles and are called biogeochemical cycles.
Nitrogen is a key element in human activities and biological, physical, chemical, and geologic processes. Estimates show that more than 20 million tons of nitrogen exist on every square mile of the planet. The atmosphere contains up to 78 percent molecular nitrogen (N2), which is mainly cycling through biologic processes.
Four major nitrogen-transformation (biologic) processes exist in nature: nitrogen fixation, ammonification, nitrification, and denitrification. Mineralization is the only geologic process that is involved in the circulation of nitrogen. Earth's primary mineral nitrogen sources are Bengal saltpeter (KNO3) in India and other Asian countries and Chile saltpeter (NaNO3) in South America. Natural gas also contains nitrogen.
Vast amounts of nitrogen are circulated by physical and chemical processes. Nitric oxide (NO) is formed in the air from N2 and O2 (molecular oxygen) during thunderstorms by lightning. Nitric oxide oxidizes further to nitrogen dioxide (NO2) and later reacts with water to form nitric (or nitrous (HNO3) acids. Acids fall to the ground during rain and form nitrates (NO-3) and nitrites (NO-2) in the soil (acid rain).
Living things require nitrogen as a component for proteins, nucleic acids (deoxyribonucleic acid, or DNA, and ribonucleic acid), and other organic compounds. Nitrogen is often a limiting plant nutrient. Plants take up nitrogen from soil mainly as ammonium ions, nitrate, or nitrogenous organic compounds and incorporate nitrogen into organic molecules such as proteins or nucleic acids. The nitrogen then follows food webs from plant eaters (herbivores) to decomposers (mainly microbes). Animals use only organic forms of nitrogen.
Nitrogen Fixation
The utilization of molecular nitrogen (N2) by particular bacteria is called nitrogen fixation. Some of these bacteria (Rhizobium) live in symbiosis with certain legume plants; others are free-living bacteria such as cyanobacteria or Azotobacter. Legume plants include soybeans, clover, alfalfa, beans, and pears. Symbiotic nitrogen-fixing cyanobacteria provide nitrogen to other plant species, such as the water-fern Azolla, liverworts, and cycads.
The nitrogen fixation or reduction of N2 to NH3 (ammonia) is a complicated, multistep process (N2 + 8e- + 8H+ + 16ATP 2NH3 + H2 + 16ADP + 16P). Ammonia produced by this process is further converted to proteins, nucleic acids (DNA), and other nitrogen-containing organic molecules (NH3 nitrogenous organic molecules: proteins, nucleic acids, and so forth).
The nitrogen fixation is catalyzed by the enzyme nitrogenase. Nitrogenase is sensitive to molecular oxygen (O2). Nitrogen-fixing organisms possess several morphological and biochemical modifications designed to protect enzymes from oxygen inactivation. For example, the bacterium Rhizobium controls the oxygen level in cells by the protein leghemoglobin, which catches oxygen. In the case of cyanobacteria, there are specialized cells (called heterocysts) for nitrogen fixation. Heterocysts show high rates of respiration, which ultimately reduces oxygen levels in these cells.
Nitrogen fixation is an energy-consuming process, which explains why cyanobacteria normally have only 5 to 10 percent of heterocysts among their cells. To maintain nitrogen fixation, other cyanobacterial cells (vegetative cells) work to generate enough energy for heterocysts.
All life on Earth depends on nitrogen fixation because the main reservoir of nitrogen on Earth is in the air as molecular nitrogen (N2). The main path of nitrogen from the air into biological nitrogen-containing molecules of different organisms is through nitrogen fixation. Nitrogen fixation also is of enormous importance to agriculture because it supports the nitrogen needs of many crops. This process was discovered by Russian microbiologist Sergei Winogradsky.
Besides natural nitrogen fixation, the industrial Haber-Bosch process converts molecular nitrogen to ammonia. In this process, nitrogen fertilizers are made for agriculture. Haber-Bosch is an energy-consuming route, and the process of manufacturing nitrogenous fertilizers consumes up to 50 percent of the energy input in modern agriculture.
Ammonification
Ammonification is the process of making ammonia or ammonium ions (NH4+) by living things. Ammonium ions are produced as waste from animals such as fish and during the decomposition of organic nitrogen wastes by bacteria and by the metabolism of some bacteria. Bacteria, for example, can convert nitrate into ammonia in soils or the human gut.
Globally, only a small amount (15 percent) of nitrogen reaches the atmosphere as ammonia, compared with N2 and N2O. Most ammonium ions are quickly consumed in soil and water by microorganisms and plants. Ammonium ions are returned to the environment at different points in the food web.
Nitrification
Nitrification is caused by the sequential action of two separate groups of soil bacteria: the ammonia-oxidizing bacteria (the nitrosifyers) and the nitrite-oxidizing bacteria (nitrifying bacteria). These bacteria obtain energy by consuming nitrogen compounds and can feed only on inorganic compounds. The end product of nitrification is nitrate, a valuable nitrogen source for plants.
Nitrification is a two-step process. Nitrosifyers, such as the bacterium Nitrosomonas, convert ammonium ion into nitrite first (NH4+ + O2 NO-2 + H2O + H+). Later, nitrifying bacteria, such as the bacterium Nitrobacter, oxidize nitrite into nitrate (NO-2 + O2 NO-3 ).
Nitrosifyers and nitrifying bacteria are common in soil and water. They live especially in areas where ammonia is present in high amounts, such as sites of ammonification and wastewater and manure. Nitrification does not contribute significantly to agriculture. Although liked by plants, nitrate is not always available for plants in soils. Microorganisms quickly consume nitrate during denitrification. Additionally, one species of Archaea (microorganisms similar to bacteria) undergoes nitrification by oxidizing ammonia in the oceans.
Denitrification
The conversion of nitrate into gaseous nitrogen compounds such as N2O, NO, and N2 by different bacteria in soils is called denitrification, or nitrate reduction. Bacteria use nitrate as a substitute for oxygen during respiration and convert it to different nitrogenous compounds according to the following chain of reactions: NO-3 NO-2 NO N2O N2.
Eventually, nitrogen is released into the atmosphere as N2O and NO or as N2. Simultaneously, bacteria decompose significant amounts of organic matter within the soil.
Denitrification negatively affects on agriculture, as it removes nitrogen from soils. In contrast, denitrification can be useful in wastewater treatment.
Human Interference in Nitrogen Cycle
Human interference in the nitrogen cycle can be significant. Human activities are generally responsible for adding excessive amounts of inorganic nitrogen to water or soil, which causes rapid cultural eutrophication of water bodies.
Most inorganic nitrogen in water comes from soil fertilization, industrial and domestic wastes, septic tanks, feed-lot discharges, domestic animal wastes (including birds and fish), and discharges from car exhausts. Eutrophication occurs regularly in nature but does so slowly, often through hundreds or thousands of years.
During human eutrophication of water bodies, algae grow fast, choke waterways, and consume large amounts of dissolved oxygen. Some algae also produce toxins. This rapid and uncontrollable growth of algae—a process that produces algal blooms or red tides—causes decay and, eventually, the destruction of aquatic ecosystems. Fish and other aquatic organisms die by the thousands, suffocated by oxygen depletion or killed by the action of toxins. Algae need inorganic nitrogen ions as a nitrogen source for their growth (for making proteins).
Ammonium and nitrate ions are the main human nitrogen pollutants in water. Inorganic nitrogen ions must be removed from industrial and domestic wastewater using physicochemical and biological methods to protect water quality.
Human contamination of soil by inorganic nitrogen is another substantial problem caused by nitrogen fertilization. In such cases, nitrogen derives from industrial nitrogen fixation or from using leguminous plants (soy, beans, peas, and alfalfa).
In addition, the extensive burning of fossil fuels by humans converts atmospheric nitrogen (N2) into nitrogen oxides. Nitrogen oxides react in the atmosphere with the ozone (O3) to make nitric acid. Nitric acid is one of the components of acid rain, which damages soils and forest richness by destroying communities of organisms. Thus, humans' burning of fossil fuels contributes to acid rain and the destruction of the ozone layer in the atmosphere.
As concerns over global climate change dominate discourse in the twenty-first century, the nitrogen cycle has become an even more pertinent phenomenon. As climate change causes increased precipitation and severe weather, an increased amount of nitrogen will run off from agricultural areas and into bodies of water. Further, temperature and precipitation changes like the ones seen due to climate change can influence the rates of nitrogen fixation, nitrification, and denitrification, altering the mechanics of the nitrogen cycle. Anthropogenic climate change and the nitrogen cycle are intrinsically linked, and mitigation efforts to reduce nitrogen emissions are essential to halt the adverse effects of global climate change.
Principal Terms
ammonia: a colorless and highly toxic gas with a strong odor; the odor of ammonia is frequently detected in stables or sewage; salts of ammonia are used as fertilizer for plants
ammonium ion: produced as a waste of such animals as fish, during decomposition of organic nitrogen wastes by bacteria, and by metabolism of some bacteria; forms after dilution of ammonia in water; acidic and toxic to humans because it interferes with respiration
biogeochemical cycle: cycling of chemical elements such as nitrogen, carbon, and phosphorus
enzyme: biological catalyst made of proteins
eutrophication: process in which water bodies (rivers, ponds, lakes, and oceans) receive excess nutrients (mainly nitrogen and phosphorus) that stimulate abundant growth of algae and plants
food web: the complex web of feeding relationships in nature
nitrate: the ion of nitric acid; an essential nutrient for plants
nitric acid: nitrogen-containing strong acid used by medieval alchemists to separate gold from silver; now used in the manufacturing of dyes, plastics, and drugs and laboratories
nitrite: the ion of nitrous acid; source for some microorganisms; extremely hazardous to humans, especially babies
nitrogen: a key chemical element on Earth; a colorless and odorless component of air
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