Nitrogen Cycle

Essential Nutrients for Plant Growth

Plants, like all organisms, require essential nutrients to survive. These include carbon, nitrogen, sulfur, and phosphorus. While trace minerals are also necessary, they are generally abundant in the soil and rarely a limiting factor.

Carbon Acquisition Through Photosynthesis

Plants obtain carbon from atmospheric CO2. During photosynthesis, the carbon in CO2 is converted into sugars such as glucose. This glucose is either metabolized during respiration or transformed into other substances, including sucrose, cellulose, starch, vitamins, and various carbon-based molecules.

Nitrogen as a Growth-Limiting Factor

Because phosphorus and sulfur are not typically found in most carbon-based molecules, nitrogen availability often becomes the primary limiting factor in the growth of plants and microorganisms.

Sources and Fixation of Nitrogen

Natural Sources of Nitrogen

Nitrogen is derived from several sources: atmospheric nitrogen (N2), mineral nitrogen from volcanic activity, and nitrogen released from decomposing organic matter.

Nitrogen Fixation by Soil Bacteria

Certain bacteria possess the ability to fix atmospheric nitrogen by converting N2 into NH2, a form usable by plants. Rhizobium bacteria perform this function and are found in nodules associated with the roots of leguminous plants such as beans, peas, alfalfa, vetch, and soybeans. These plants do not require nitrate or ammonium fertilizers due to their symbiotic relationship with nitrogen-fixing bacteria.

Genetic Research on Nitrogen Fixation

For decades, efforts have been made to clone nitrogen-fixing genes from Rhizobium into plants to eliminate the need for chemical fertilizers. No successful outcomes have been documented to date.

The Nitrogen Cycle and Ammonification

Decomposition and Nutrient Recycling

When living organisms—plants, animals, fish, birds, and microorganisms—die, their bodies are decomposed by other microorganisms in the soil or oceans. A critical part of this decomposition is the conversion of nitrogen from cell proteins into reusable forms, a process known as ammonification.

Conversion Processes in the Soil

During ammonification, NH2 from proteins is converted into NH3 (ammonia), which has a characteristic odor often associated with decay. Ammonia, a gas at room temperature, dissolves in soil moisture and forms NH4+ (ammonium ions), which plants absorb and convert into NH2 to form new molecules.

Nitrification and Environmental Impact

Soil Bacteria and Nitrate Formation

Ammonia can also be converted into nitrates by soil bacteria. Initially, NH3 is transformed into NO2 (nitrite) by Nitrosomonas. Since nitrites are not highly soluble in water, they are further converted into nitrates by Nitrobacter. Nitrates, being highly water-soluble, contribute significantly to fertilizer contamination in the water table.

Uptake and Utilization by Plants and Microorganisms

Plants absorb nitrates and convert them into NH2 for use in growth and development. Microorganisms also utilize both nitrates and ammonium ions in their metabolic processes.

Continuity of the Nitrogen Cycle

When animals, birds, or humans consume plants or other animals, nitrogen is transferred and converted into forms usable for their own growth. Upon death, their remains contribute to restarting the nitrogen cycle through the activity of bacteria and fungi. The continuous activity of microorganisms in the soil and ocean is essential to sustaining life on Earth.