The Comprehensive Nitrogen Cycle

The Atmospheric Nitrogen Pool and Biological Fixation

The nitrogen cycle begins with nitrogen gas in the air, represented as $N_2$. Although the atmosphere is composed of approximately 78% nitrogen, this gaseous form is chemically inert and cannot be directly utilized by most living organisms. For nitrogen to enter the biological system, it must undergo a process of fixation. In terrestrial ecosystems, this is often facilitated by legumes, known as $\text{Balgplanter}$. These plants have a symbiotic relationship with specialized root nodule bacteria, or $\text{Knoldbakterier}$.

Additionally, general soil bacteria ($\text{Jordbakterier}$) and other specific bacteria play a role in the splitting ($\text{Spaltning}$) of the strong triple bonds in the $N_2$ molecule. This biological fixation converts atmospheric nitrogen into reactive forms that plants can eventually incorporate into their tissues.

Decomposition and the Process of Ammonification

Nitrogen is also cycled through the breakdown of organic material. Dead organic matter ($\text{Dødt organisk stof}$), which includes fallen leaves, dead animals, and waste products, contains nitrogen-rich compounds like proteins and DNA. This matter is decomposed by the collective action of fungi ($\text{svampe}$) and bacteria ($\text{bakterier}$).

During this decomposition process, a specific biochemical pathway known as ammonification ($\text{Ammonifikation}$) occurs. In this stage, the organic nitrogen is converted into ammonium ($NH_4^+$). This step is vital as it releases nitrogen stored in biomass back into the soil in an inorganic form that can be further processed by the microbial community.

Nitrification and Nitrate Production

The ammonium produced during ammonification does not usually remain in that state for long in well-aerated soils. It undergoes a process called nitrification ($\text{Nitrifikation}$). This is a biological oxidation process where specialized soil bacteria convert ammonium ($NH_4^+$) into nitrate ($NO_3^-$).

Nitrate is the primary form of nitrogen that is taken up by the vast majority of plants. Because it is highly mobile in the soil solution, it is easily accessible to plant roots. However, this mobility also introduces risks to the ecosystem balance, as nitrate is not bound to soil particles as strongly as ammonium.

Denitrifikation and the Return to the Atmosphere

To balance the cycle, nitrogen must be returned to the atmosphere from the soil. This is achieved through denitrification ($\text{Denitrifikation}$). This process is carried out by certain bacteria in anaerobic (oxygen-poor) conditions, such as in saturated or compacted soils. These bacteria use the oxygen atoms in nitrate ($NO_3^-$) for their metabolic processes, which results in the reduction of nitrate back into gaseous nitrogen ($N_2$). This release of nitrogen gas back into the air completes the loop of the nitrogen cycle.

Environmental Loss through Leaching

One significant pathway within the nitrogen cycle that has environmental implications is leaching, referred to as $\text{Udvaskning}$. Because nitrate ($NO_3^-$) is highly soluble in water, it can be washed out of the soil profile by rainwater or irrigation. This nitrogen is then carried away from the root zone of plants and can enter groundwater, streams, or other aquatic ecosystems. Excessive $\text{Udvaskning}$ can lead to a loss of soil fertility and contribute to the pollution of water bodies, highlighting the delicate balance required in managing nitrogen levels within the soil.