The Nitrogen Cycle

What are Nutrient Cycles

These are some important terms we must understand when studying nutrient cycles:

• Reservoirs - where the majority of the nutrient in question is situated; in the nitrogen cycle, this will be in the atmosphere.
• Sources - the organism or processes which return the nutrients to the reservoir; in this case, this will be the denitrifying bacteria in the soil, which convert nitrate ions back into nitrogen gas in a process called denitrification.
• Sinks - the sinks are the most prominent site of nutrient consumption, so in this instance, this will be the soil, where nitrate ions are available for plants to absorb.

These are the types of organisms which are involved in nutrient cycles:

• Producers - photosynthetic organisms harness sunlight's energy to convert inorganic molecules into organic compounds.
• Consumers - organisms that cannot create their own food must ingest producers or other consumers in the case of secondary and tertiary consumers.
• Decomposers - these organisms break down dead organic matter and release absorbable, inorganic ions in return.

This is the process by which nutrient cycles occur:

1. Producers absorb the nutrients as inorganic molecules that would otherwise be useless to other organisms and subsequently convert them into valuable organic compounds.
2. Consumers then ingest these producers and absorb these organic compounds.
3. The nutrients travel through the food chains as secondary and tertiary consumers ingest organisms containing these organic compounds.
4. Once these producers and consumers die, they are acted on by decomposers which break down the organic compounds and release them as inorganic compounds which producers can absorb.

Not all nitrogen is taken up by consumers due to a variety of reasons (not consuming the whole organism, being unable to utilise all the nutrients, and nutrient loss through urea and faeces). This does not mean that the nitrogen goes to waste, as any dead or faecal matter will be broken down by saprobionts (decomposers) and enter into the cycle.

Nitrogen Cycle Definition

The nitrogen cycle consists of 5 main processes carried out by different types of microorganisms in the soil:

• Nitrogen fixation
• Nitrification
• Assimilation
• Ammonification
• Denitrification

Fig. 1 - The nitrogen cycle consists of different processes or stages.

Here are descriptions of the processes and organisms involved.

Nitrogen Fixation

Nitrogen fixation is arguably the most important stage of the nitrogen cycle. This is because it offers a pathway for the abundance of nitrogen present in the atmosphere to become readily available for plants in the soil. Nitrogen will become present in soils through precipitation and is then acted on by saprobiontic bacteria. Atmospheric nitrogen fixation has the potential to occur when lightning strikes, and we can fixate nitrogen industrially under high pressures and temperatures. Still, it is the nitrogen-fixing microorganisms in the soil that we will be focussing on.

There are two types of nitrogen-fixing diazotrophs, namely free-living and mutualistic:

• Free-living nitrogen-fixing bacteria are abundant in the soil and convert gaseous nitrogen into ammonium by reducing it. This reduction is endothermic, resulting in energy being absorbed by the bacteria, which they can then use to manufacture various nitrogen-containing compounds such as proteins and nucleic acids. After death, these bacteria decaying releases these organic compounds into the soil.
• Mutualistic nitrogen-fixing bacteria are certain types of microorganisms that live on plants' root nodules. These bacteria will reduce gaseous nitrogen to ammonia and use this ammonia to manufacture nitrogen-containing compounds such as amino acids. The host plant will then take up these useful organic compounds and, in return, give the bacteria useful carbohydrates and sugars, which it can thrive off. This is an example of a symbiotic relationship between two organisms.

Ammonification

Ammonification is the process by which nitrogen is transferred from the living part biome to the non-living part of the biome. The saprobiontic decomposers in the soil (mostly bacteria and fungi in this case) will break down any organic material containing nitrogen and release ammonia in return. This organic material can be from protein, vitamins and nucleic acids found in dead organisms or from the urea and faeces produced by animals. The ammonia will then pick up hydrogen ions by reacting with acidic residues in the soil.

Nitrification

Nitrification is the process by which chemoautotrophic bacteria convert ammonium ions into nitrate ions which producers can then absorb. These bacteria rely on a substantial concentration of ammonium ions in the soil as they harness the energy released from the oxidation of ammonium ions for their biochemical processes. Free-living bacteria convert ammonium ions to nitrate ions in the soil; these are aerobic chemoautotrophs, so the soil must be oxygen-rich. Ammonium ions are first oxidised to nitrite ions, which are then subsequently oxidised to nitrate ions, with both reactions releasing energy.

Here are the chemical equations for the reactions:

Ammonium ions oxidised to nitrite ions:

2NH₄⁺ + 3O₂ → 2NO₂⁻ + 4H⁺ + 2H₂O

Nitrite ions oxidised to nitrate ions:

2NO₂⁻ + O₂ → 2NO₃⁻

Assimilation

Producers which live in the soil, or autotrophs, take up the nitrogen by actively transporting the nitrate and ammonium ions through their roots. The ions must be actively transported most of the time because there is always a high concentration of ions in the plant roots, but the soil can sometimes be nutrient deficient. Nitrate ions are transported throughout the plant and converted to ammonia, which is converted to amine groups that can easily be used to manufacture useful nitrogen-containing compounds.

Denitrification

Denitrification is the process by which nitrogen returns to the atmosphere completing the cycle. Anaerobic saprobionts called denitrifying bacteria reduce nitrate ions to oxygen and release nitrogen gas in the process. These denitrifying bacteria are abundant in waterlogged soils because the water often pushes out any oxygen available to the microorganisms in the soil. This reduction of nitrate ions can be detrimental to the plant as they require a constant supply for manufacturing nitrogen-containing compounds.

Importance of the Nitrogen Cycle

The nitrogen cycle is important for many different reasons:

• It provides producers with a sustainable source of nitrate ions that they can absorb and use to build up essential nitrogen-containing compounds like proteins, nucleic acids and chlorophyll.
• Allows important organic compounds which have nitrogen as a major component to pass through the ecosystem. Herbivorous consumers ingest the producers, and carnivorous consumers will ingest their prey.
• Saprobiontic decomposers break down dead matter and faeces; in doing so, they make nutrients readily available for producers while simultaneously cleaning up the environment.
• Allows the inert gas nitrogen to become available for terrestrial organisms.

Why do Plants Require Nitrogen

Plants require nitrogen for the manufacturing of many different structures and molecules throughout the plant:

• Chlorophyll - nitrogen is a major component of chlorophyll, so lots of nitrogen is needed to produce the maximum amount of chlorophyll pigment so the plant can photosynthesise as much as possible.
• DNA - nitrogen is present in the nucleotides, which are the building blocks of DNA. DNA is the genetic material that provides the instructions for all biochemical processes inside a plant and the hereditary information for the plant.
• ATP - the monomers which make up the energy transfer molecule adenosine triphosphate contain nitrogen. ATP is pivotal in controlling energy transfer in metabolic processes.
• Amino acids - nitrogen makes up a substantial proportion of amino acid residues, which are the building blocks when manufacturing proteins. These proteins are essential in manufacturing protective structures and useful enzymes in the plant.

Nitrogen Deficiency in Plants

Nitrate ion availability in the soil can be caused by heavy rains, which leads to waterlogged land. This wet soil leads to their being a dominating anaerobic saprobiont population, meaning that denitrifying bacteria prevail and the nitrate ion concentration of the soil decreases. Dry soils with little to no moisture can also hamper plants' development as they will struggle to uptake water-soluble ions. Plants that have a nitrogen deficiency may have the following characteristics:

• The plant's leaves will grow much slower; this is especially noticeable in younger leaves.
• Shoot development is much slower.
• Leaves may exhibit a yellow discolouration.
• Reduced flowering and fruit production.

The Nitrogen Cycle in Marine Ecosystems

Nitrogen is transferred through marine ecosystems in the same way as terrestrial ecosystems. Aquatic saprobiontic microorganisms break down dead organic matter on the sea bed and release nitrate ions which aquatic plants can absorb. Recent research has found that sedimentary rocks are a major source of nitrogen as they release gaseous nitrogen when they erode.

Use of Nitrogen Fertilisers

Nitrogen is one of the three main components of NPK fertilisers, popular with farmers worldwide as nitrogen, phosphorus, and potassium are essential for plant development. Nitrogen fertilisers may be needed when the soil is waterlogged, so it contains mostly denitrifying saprobionts, when the soil is too dry, meaning soluble nitrate ions cannot move around, or even when crops are being grown on infertile soil. Farmers almost always use fertilisers to increase the yield and size of their crops. Despite their apparent benefits, fertiliser runoff can be detrimental to nearby rivers and estuaries.