Nitrogen cycle – Processes

The atomic number 7 is assigned to the chemical element nitrogen, which has the symbol N. Nitrogen is a nonmetal and the lightest member of Group 15, often known as the pnictogens, on the periodic table. It is a common element in the cosmos, with the Milky Way and the Solar System having the fifth highest total abundance. 

Nitrogen cycle: 

The nitrogen cycle is a biogeochemical cycle in which nitrogen is transformed into various chemical forms as it moves through ecosystems such the atmosphere, land, and sea. Both biological and physical methods can be used to convert nitrogen. The nitrogen cycle includes processes such as fixation, ammonification, nitrification, and denitrification. Atmospheric nitrogen makes up the majority of the Earth’s atmosphere (78%), making it the most abundant source of nitrogen. 

Stages of nitrogen cycle: 

The nitrogen cycle includes the phases of nitrogen fixation, nitrification, assimilation, ammonification, and denitrification. The following are the steps that these processes are broken down into: 

Nitrogen fixation process: 

The nitrogen cycle begins with this phase. Atmospheric nitrogen (N₂), which is predominantly available in an inert form, is transformed to the useful form -ammonia- in this process (NH₃). 

The inert form of nitrogen gas is deposited mostly through precipitation into soils from the atmosphere and surface waters during the nitrogen fixation process. Later, the nitrogen proceeds through a sequence of changes that result in the separation of two nitrogen atoms, which combine with hydrogen to form ammonia (NH₄⁺). 

Types of nitrogen fixation:

• Atmospheric fixation: A natural occurrence in which lightning energy splits nitrogen into nitrogen oxides, which are then utilised by plants. 

• Industrial nitrogen fixation: Is a man-made alternative that uses ammonia to help in nitrogen fixation. The direct interaction of nitrogen and hydrogen produces ammonia, which is then transformed into different fertilisers such as urea. 

• Biological nitrogen fixation: Bacteria such as Rhizobium and blue-green algae convert the useless form of nitrogen into more readily used molecules. The nitrogen molecules in the soil are repaired by these microbes. 

Nitrification: 

The presence of microorganisms in the soil converts ammonia to nitrate in this process. The oxidation of ammonia with the help of Nitrosomonas bacterium species produces nitrates. 

The following is the reaction that occurs during the nitrification process: 

2NH₄⁺ + 3O₂ → 2NO₂⁻ + 4H⁺ + 2H₂O

2NO₂⁻ + O₂ → 2NO₃⁻

Assimilation: 

Plants receive nitrogen molecules from the soil via their roots, which are available in the form of ammonia, nitrite ions, nitrate ions, or ammonium ions and are used to make plant and animal proteins. 

Ammonification: 

When plants or animals die, the nitrogen in organic matter is released back into the soil. Decomposers, which are bacteria or fungi present in the soil, turn organic waste back into ammonium. During the decomposition process, ammonia is created, which is then used in other biological activities. 

Denitrification: 

Denitrification is the process of turning nitrate (NO₃⁻) into gaseous nitrogen and releasing nitrogen compounds back into the atmosphere (N). In the absence of oxygen, this is the penultimate stage of the nitrogen cycle. 

Nitrogen cycle in marine ecosystem: 

In both the marine and terrestrial ecosystems, the nitrogen cycle operates in the same way. The main difference is that it is carried out by bacteria that live in the sea. 

As sediments are compacted over time and form sedimentary rock, nitrogen-containing chemicals fall into the ocean. These sedimentary rocks are moving to land due to geological uplift. Until recently, no one realised that these nitrogen-rich sedimentary rocks were an important source of nitrogen. However, recent studies have shown that the weathering of rocks causes nitrogen to be released into the plants. 

Human impact on nitrogen cycle: 

The nitrogen cycle is influenced by a variety of human activities. The amount of biologically accessible nitrogen in an ecosystem can be greatly increased by burning fossil fuels, applying nitrogen-based fertilisers, and other activities. Large variations in nitrogen availability can induce severe adjustments in the nitrogen cycle in both aquatic and terrestrial ecosystems, because nitrogen availability typically limits the primary productivity of many ecosystems. Since the 1940s, industrial nitrogen fixation has expanded at an exponential rate, and human activity has doubled global nitrogen fixation. 

Consequence of human modification of the nitrogen cycle: 

Impacts on natural systems: 

Nitrogen deposition has been found to have a variety of deleterious consequences on both terrestrial and aquatic ecosystems. Certain plant species can be directly hazardous to nitrogen gases and aerosols, affecting the aboveground physiology and growth of plants near significant point sources of nitrogen pollution. Plant species may change when the accumulation of nitrogen compounds increases their availability in a given ecosystem, affecting species composition, plant diversity, and nitrogen cycle. 

Impacts on human health – air quality: 

Human activities have also radically affected the global nitrogen cycle by producing nitrogenous gases, which is linked to global nitrogen pollution in the atmosphere. Fluxes of reactive nitrogen (Nr) in the atmosphere come from a variety of places. Agricultural sources of reactive nitrogen can release ammonia (NH₃), nitrogen oxides (NOₓ), and nitrous oxide into the atmosphere (N₂O). Combustion operations in energy production, transportation, and manufacturing can also result in the formation of additional reactive nitrogen via the accidental waste product NOₓ. 

Conclusion: 

The nitrogen cycle aids plants in the production of chlorophyll from nitrogen compounds. Through the metabolic process, it also aids in the conversion of inert nitrogen gas into a useful form for plants. Nitrates and nitrites are released into the soil, assisting in the enrichment of the soil with the nutrients essential for agriculture. Nitrogen is an essential component of the cell, and it is used to make a variety of critical chemicals and proteins.