How Do Plants And Animals Get Nitrogen? | The Nitrogen Cycle Explained

Understanding how life accesses nitrogen is fundamental to appreciating biological processes on Earth. This essential element forms the backbone of proteins and nucleic acids, making it indispensable for all living organisms. We’ll explore the intricate pathways that bring nitrogen into the biological world.

The Abundance and Unavailability of Atmospheric Nitrogen

Nitrogen gas (N2) constitutes approximately 78% of Earth’s atmosphere, making it the most abundant gas we breathe. Despite this vast quantity, most living organisms cannot directly use atmospheric N2. The reason lies in the strong triple covalent bond holding the two nitrogen atoms together.

This bond requires a significant amount of energy to break, rendering N2 largely inert and biologically inaccessible in its gaseous form. Think of it like a vast treasure chest of an essential nutrient that is securely locked, with only a few specialized organisms holding the key to open it.

Nitrogen Fixation: The Essential Conversion

The process of converting atmospheric N2 into forms usable by life is called nitrogen fixation. This vital transformation makes nitrogen available to the broader biological community.

Biological Nitrogen Fixation

The most significant amount of nitrogen fixation occurs biologically, carried out by specific microorganisms. These prokaryotes possess the enzyme nitrogenase, which can break the N2 triple bond.

• Symbiotic Bacteria: A well-known example involves bacteria of the genus Rhizobium, which form a symbiotic relationship with leguminous plants like peas, beans, and clover. These bacteria reside in specialized root nodules, where they convert N2 into ammonia (NH3) for the plant, receiving carbohydrates in return.
• Free-Living Bacteria: Other bacteria, such as Azotobacter and Clostridium, live freely in the soil and can also fix nitrogen.
• Cyanobacteria: Certain cyanobacteria, also known as blue-green algae, are significant nitrogen fixers in aquatic environments and some terrestrial settings.

The nitrogenase enzyme complex is highly sensitive to oxygen, necessitating anaerobic or microaerobic conditions for its function. Plants hosting symbiotic nitrogen-fixing bacteria often produce leghemoglobin, a protein that binds oxygen within the nodules to protect the enzyme.

For a deeper understanding of this fundamental process, you can refer to educational resources such as those provided by Khan Academy.

Abiotic Nitrogen Fixation

Nitrogen fixation can also occur through non-biological means:

• Lightning: The immense energy of lightning strikes can break the N2 triple bond, allowing nitrogen to react with oxygen to form nitrogen oxides. These compounds dissolve in rainwater and fall to Earth as nitrates (NO3-), adding small amounts of fixed nitrogen to soils.
• Industrial Processes: The Haber-Bosch process, developed in the early 20th century, is an industrial method for synthesizing ammonia (NH3) from atmospheric N2 and hydrogen gas (H2) under high temperature and pressure. This process has revolutionized agriculture by enabling the mass production of synthetic nitrogen fertilizers.

How Plants Acquire Nitrogen

Once nitrogen is fixed into usable forms, plants become the primary entry point for this element into the food web. Plants absorb nitrogen predominantly from the soil solution.

• Nitrate (NO3-): This is the most common form of nitrogen absorbed by plants. Nitrates are highly soluble in water and readily transported through the soil to plant roots. Once inside the plant, nitrates are reduced to ammonium before being incorporated into organic molecules.
• Ammonium (NH4+): Plants can also absorb ammonium ions directly. Ammonium is less mobile in soil than nitrate because it is positively charged and can bind to negatively charged clay particles. Direct ammonium uptake is often metabolically less energy-intensive for the plant than nitrate uptake and reduction.

Mycorrhizal fungi, which form symbiotic relationships with plant roots, can also enhance nitrogen uptake. These fungi extend the root system’s reach, improving the plant’s access to soil nutrients, including nitrogen. In nitrogen-poor environments, some carnivorous plants, such as Venus flytraps or pitcher plants, supplement their nitrogen intake by trapping and digesting insects.

Table 1: Forms of Nitrogen in Soil and Plant Availability

Nitrogen Form	Chemical Formula	Plant Availability
Atmospheric Nitrogen	N₂	Not directly usable by most plants
Ammonium	NH₄⁺	Directly usable by plants
Nitrate	NO₃⁻	Readily usable by plants; highly mobile
Nitrite	NO₂⁻	Generally toxic to plants; intermediate form
Organic Nitrogen	Various (e.g., amino acids)	Limited direct uptake; requires mineralization

The Role of Nitrification and Ammonification in Soil

The nitrogen cycle involves several transformation steps within the soil, mediated by various microorganisms. These processes convert nitrogen between different inorganic and organic forms.

Ammonification

Ammonification is the process where decomposer organisms, primarily bacteria and fungi, break down dead organic matter (such as dead plants, animals, and animal waste). During decomposition, the organic nitrogen compounds (proteins, nucleic acids) are converted into inorganic ammonium (NH4+).

This process is crucial for recycling nitrogen from decaying biomass back into the soil, making it available for subsequent transformations or direct plant uptake. Without ammonification, nitrogen would remain locked in organic forms, unavailable to new plant growth.

Nitrification

Nitrification is a two-step microbial process that converts ammonium (NH4+) into nitrate (NO3-), the most common form of nitrogen absorbed by plants. This process is carried out by distinct groups of chemosynthetic bacteria.

1. First Step (Nitritation): Ammonia-oxidizing bacteria, such as those in the genus Nitrosomonas, convert ammonium (NH4+) into nitrite (NO2-). This reaction releases energy that these bacteria use for their metabolic processes.
2. Second Step (Nitratation): Nitrite-oxidizing bacteria, primarily from the genus Nitrobacter, then convert the nitrite (NO2-) into nitrate (NO3-). Nitrate is highly soluble and easily leached from soils, particularly in agricultural systems with heavy rainfall or irrigation.

Nitrification is important because it produces the form of nitrogen most readily taken up by many plants. It also contributes to nitrogen loss from terrestrial ecosystems through leaching.

How Animals Obtain Nitrogen

Animals, being heterotrophs, cannot fix atmospheric nitrogen or absorb inorganic nitrogen from the soil like plants. Their sole method of acquiring nitrogen is by consuming other organisms.

• Dietary Consumption: Herbivores obtain nitrogen by eating plants, which contain nitrogen in the form of proteins, nucleic acids, and other organic molecules. Carnivores acquire nitrogen by consuming other animals, which have already assimilated nitrogen from their diet. Omnivores obtain nitrogen from both plant and animal sources.
• Digestion and Assimilation: Within an animal’s digestive system, complex nitrogenous compounds from food are broken down into simpler units, primarily amino acids and nucleotides. These smaller molecules are then absorbed and transported to cells throughout the body.
• Building Blocks: Animals use these absorbed amino acids to synthesize their own proteins, enzymes, hormones, and structural components. Nucleotides are used to build DNA and RNA. Excess nitrogen, not incorporated into new molecules, is typically excreted as waste products like urea (in mammals), uric acid (in birds and reptiles), or ammonia (in aquatic animals).

Table 2: Nitrogen Acquisition Pathways for Organisms

Organism Type	Primary Acquisition Method	Key Nitrogen Forms Utilized
Nitrogen-Fixing Bacteria	Biological Nitrogen Fixation	Atmospheric N₂
Plants	Root Absorption from Soil	Nitrate (NO₃⁻), Ammonium (NH₄⁺)
Herbivores	Consumption of Plants	Organic Nitrogen (proteins, nucleic acids)
Carnivores	Consumption of Animals	Organic Nitrogen (proteins, nucleic acids)
Decomposers (Bacteria, Fungi)	Breakdown of Organic Matter	Organic Nitrogen (from dead biomass)

Denitrification: Returning Nitrogen to the Atmosphere

Denitrification is the process that completes the nitrogen cycle by returning nitrogen gas to the atmosphere. This process is carried out by specific anaerobic bacteria, such as those in the genera Pseudomonas and Bacillus.

These denitrifying bacteria use nitrates (NO3-) as an electron acceptor in their metabolism when oxygen is scarce. Through a series of enzymatic reactions, nitrates are sequentially reduced to nitrite (NO2-), nitric oxide (NO), nitrous oxide (N2O), and finally back to atmospheric nitrogen gas (N2).

Denitrification commonly occurs in waterlogged soils, sediments, and anaerobic zones in aquatic systems where oxygen is depleted. While it removes fixed nitrogen from ecosystems, it is a natural and essential process for maintaining the balance of nitrogen in the atmosphere and preventing excessive accumulation of fixed nitrogen in terrestrial and aquatic environments.

The Human Impact on the Nitrogen Cycle

Human activities have profoundly altered the global nitrogen cycle, significantly increasing the amount of fixed nitrogen entering ecosystems. The development and widespread use of the Haber-Bosch process for synthesizing ammonia have been a primary driver of this change.

• Synthetic Fertilizers: The massive application of synthetic nitrogen fertilizers in agriculture has dramatically boosted crop yields. However, excess fertilizer can leach into waterways, causing eutrophication. This leads to algal blooms, which deplete oxygen when they decompose, creating “dead zones” harmful to aquatic life.
• Fossil Fuel Combustion: Burning fossil fuels in vehicles and industries releases nitrogen oxides (NOx) into the atmosphere. These compounds contribute to air pollution, including smog formation and acid rain. Nitrogen oxides also play a role in stratospheric ozone depletion.
• Greenhouse Gas Emissions: Agricultural practices, particularly the use of nitrogen fertilizers and animal waste management, contribute to the release of nitrous oxide (N2O). Nitrous oxide is a potent greenhouse gas, significantly more effective at trapping heat than carbon dioxide over a 100-year period.
• Land Use Changes: Deforestation and conversion of natural ecosystems to agricultural land can alter natural nitrogen cycling rates, often leading to increased nitrogen losses through runoff and emissions.

Understanding these impacts is essential for developing sustainable agricultural practices and mitigating environmental consequences. More information on these impacts can be found through resources like the Environmental Protection Agency.