Nitrogen makes up approximately 78% of Earth’s atmosphere, but this abundance does not make it readily usable by most life forms. The element is fundamental to all organisms, serving as a constituent of essential biomolecules, including amino acids, proteins, and nucleic acids like DNA. The distinction between organic and inorganic nitrogen compounds is based on chemical structure; organic forms contain carbon-hydrogen bonds, while inorganic forms lack them. The continual conversion of nitrogen between these states sustains global ecosystems.
Defining Inorganic Nitrogen and Its Key Forms
Inorganic nitrogen refers to compounds of nitrogen that do not contain carbon-hydrogen bonds. The three primary forms of dissolved inorganic nitrogen (DIN) relevant to biology and environmental science are ammonium (\(\text{NH}_4^+\)), nitrite (\(\text{NO}_2^-\)), and nitrate (\(\text{NO}_3^-\)).
Ammonium (\(\text{NH}_4^+\)) is a positively charged ion (cation) that forms when ammonia (\(\text{NH}_3\)) accepts a hydrogen ion. Because of its positive charge, ammonium is attracted to and retained by the negatively charged surfaces of clay particles and organic matter in soil. This retention limits its mobility, meaning it does not easily wash away with water.
In contrast, nitrate (\(\text{NO}_3^-\)) is a negatively charged ion (anion) and is highly soluble in water. Since soil particles are also negatively charged, nitrate is repelled and highly mobile, making it prone to leaching into groundwater or waterways. Nitrite (\(\text{NO}_2^-\)) is an intermediate, water-soluble form positioned between ammonium and nitrate in the nitrogen cycle. These different properties dictate how each form is utilized by organisms and how it moves through systems.
The Nitrogen Cycle: Transformation and Movement
The conversion and movement of inorganic nitrogen through the atmosphere, soil, and water is governed by the nitrogen cycle, a biogeochemical process driven largely by microorganisms. The cycle begins with nitrogen fixation, which converts atmospheric dinitrogen gas (\(\text{N}_2\)) into biologically available ammonia (\(\text{NH}_3\)). This conversion is carried out primarily by specialized bacteria known as diazotrophs, which possess the nitrogenase enzyme complex.
Ammonia rapidly converts to ammonium (\(\text{NH}_4^+\)) in the soil, initiating nitrification, a two-step oxidation reaction. The first step involves ammonia-oxidizing bacteria, such as Nitrosomonas, which convert ammonium into nitrite (\(\text{NO}_2^-\)). The second step sees nitrite-oxidizing bacteria, such as Nitrobacter, further oxidize the nitrite into the plant-preferred form, nitrate (\(\text{NO}_3^-\)).
The final major stage that returns nitrogen to the atmosphere is denitrification, which occurs in environments with little or no oxygen, such as waterlogged soils. Denitrifying bacteria, including species of Pseudomonas, use nitrate (\(\text{NO}_3^-\)) as an electron acceptor during respiration in place of oxygen. This anaerobic process reduces the nitrate sequentially into gaseous forms like nitrous oxide (\(\text{N}_2\text{O}\)) and ultimately back into dinitrogen gas (\(\text{N}_2\)), completing the loop.
Essential Roles in Biology and Ecosystems
Inorganic nitrogen compounds are necessary for the synthesis of all nitrogen-containing biological molecules. Plants and other primary producers absorb the readily available forms, nitrate and ammonium, from the soil or water. Inside plant cells, these inorganic forms are reduced to ammonia and incorporated into amino acids like glutamine and glutamate.
These amino acids serve as the building blocks for proteins, which are necessary for cell structure, enzyme activity, and growth. Nitrogen is also a component of chlorophyll, the molecule that captures light energy during photosynthesis, and of nucleic acids like DNA and RNA.
The availability of inorganic nitrogen often dictates the overall productivity of an ecosystem, making it a commonly limiting nutrient for plant growth. When nitrogen is scarce, plant growth is restricted, which limits the energy available to the rest of the food web.
Human Impact: Agriculture and Environmental Concentration
Human activity has drastically altered the global nitrogen cycle, primarily through the industrial production of synthetic inorganic fertilizers. The Haber-Bosch process, developed in the early 20th century, synthesizes ammonia (\(\text{NH}_3\)) from atmospheric nitrogen (\(\text{N}_2\)) and hydrogen gas. This process revolutionized agriculture by providing a consistent and abundant source of nitrogen fertilizer, which has supported the exponential growth of the global population.
The widespread application of synthetic fertilizers leads to environmental issues when crops do not absorb all of the applied nitrogen. Since the produced ammonium is quickly converted to highly mobile nitrate (\(\text{NO}_3^-\)) in the soil, excess fertilizer easily leaches into groundwater, rivers, and coastal waters. This excess inorganic nitrogen fuels the process of eutrophication in aquatic environments.
Eutrophication begins with a rapid growth of algae, known as an algal bloom, driven by the nutrient surge. When these masses of algae die and decompose, the process consumes dissolved oxygen, creating hypoxic zones, or “dead zones,” where aquatic life cannot survive. Furthermore, nitrate leaching into drinking water sources can pose a risk to human health, notably causing methemoglobinemia, or “Blue Baby Syndrome,” a condition that reduces the blood’s ability to carry oxygen in infants.