Understanding the Biogeochemical Cycles: The Carbon, Nitrogen, and Sulfur Cycles

Understanding biogeochemical cycles is crucial for comprehending how the Earth’s ecosystems function and sustain life. Biogeochemical cycles involve the movement and transformation of different elements and compounds, including carbon, nitrogen, sulfur, and phosphorus, among others. These cycles are crucial as they ensure that the materials required for life are continuously recycled and available to living organisms.

In this article, we will explore three essential biogeochemical cycles: the carbon cycle, nitrogen cycle, and sulfur cycle. We will discuss the processes involved, the major reservoirs, and the impact of human activities on these cycles. Understanding these cycles is critical for appreciating the Earth’s interconnectedness and the impact of human activities on the environment.

The Carbon Cycle

The carbon cycle is one of the most important biogeochemical cycles of the Earth. It allows for carbon to be recycled and reused throughout the biosphere and all of its organisms. Various materials, including different nutrients and metals, move in the ecosystem in a cyclic manner.

The major reserves or storage compartments of the materials are known as reservoirs. When the major reservoir of a nutrient is in the atmosphere, it is known as a gaseous cycle. In contrast, when the reservoir is in the earth’s crust or sediments, it is known as a sedimentary cycle. The carbon cycle is a complex series of processes through which all of the carbon atoms in existence rotate.

Reservoirs of Carbon

There are five major interconnected reservoirs of carbon, as shown in Table 1. The annual movements of carbon, the carbon exchanges between reservoirs, occur because of various chemical, physical, geological, and biological processes. The ocean contains the largest active pool of carbon near the surface of the Earth.

Major Reservoirs of Carbon

Movement of Carbon

Carbon is released into the atmosphere through various processes, including the respiration performed by plants and animals, decay of animal and plant matter, and combustion of organic material. Volcanic eruptions and metamorphism also release gases into the atmosphere. Carbon is transferred within the biosphere as heterotrophs feed on other organisms or their parts. Most carbon leaves the biosphere through respiration.

The carbon cycle starts with plants absorbing carbon dioxide from the atmosphere and using it, combined with water they get from the soil, to make the substances they need for growth.

The process of photosynthesis incorporates the carbon atoms from carbon dioxide into sugars. Animals eat the plants and use the carbon to build their own tissues. These animals return carbon dioxide into the air when they breathe, and when they die, the carbon is returned to the soil during decomposition. The carbon atoms in soil may then be used in a new plant or small microorganisms.

The Nitrogen Cycle: Understanding the Movement of Nitrogen on Earth

Nitrogen is a crucial element for the growth and development of all organisms. However, most organisms cannot use atmospheric nitrogen, which is the largest reservoir. Therefore, the nitrogen cycle is essential as it involves the transformation of nitrogen from atmospheric nitrogen into a form that can be used by living organisms.

The nitrogen cycle consists of five key processes: nitrogen fixation, nitrogen uptake, nitrogen mineralization, nitrification, and de-nitrification.

1. Nitrogen Fixation

Nitrogen fixation is the process that converts atmospheric nitrogen (N2) into ammonium (NH4+). This conversion is essential because it is the only way that organisms can attain nitrogen directly from the atmosphere. Certain bacteria, such as those in the genus Rhizobium, are the only organisms that fix nitrogen through metabolic processes.

Nitrogen fixing bacteria often form symbiotic relationships with host plants, where they receive carbohydrates and a favourable environment in exchange for some of the nitrogen they fix. In aquatic environments, blue-green algae are an important free-living nitrogen fixer.

2. Nitrogen Uptake

The ammonia produced by nitrogen fixing bacteria is usually quickly incorporated into protein and other organic nitrogen compounds, either by a host plant, the bacteria itself, or another soil organism.

3. Nitrogen Mineralization

After nitrogen is incorporated into organic matter, it is often converted back into inorganic nitrogen by a process called nitrogen mineralization, otherwise known as decay. During this process, a significant amount of the nitrogen contained within the dead organism is converted to ammonium.

4. Nitrification

Some of the ammonium produced by decomposition is converted to nitrate (NO3–) via a process called nitrification. Nitrification requires the presence of oxygen, so it can only occur in oxygen-rich environments like circulating or flowing waters and the very surface layers of soils and sediments. The process of nitrification has some important consequences, such as decreased soil fertility and nitrate enrichment of downstream surface and groundwater’s.

5. De-nitrification

Through de-nitrification, oxidized forms of nitrogen such as nitrate and nitrite (NO2) are converted to di-nitrogen (N2) and, to a lesser extent, nitrous oxide gas. De-nitrification is an anaerobic process that is carried out by denitrifying bacteria. Nitric oxide (NO) contributes to smog, and nitrous oxide (N2O) is an important greenhouse gas, thereby contributing to global climate change.

Human Influence on the Nitrogen Cycle Humans significantly influence the global nitrogen cycle through the use of nitrogen-based fertilizers. These fertilizers can result in excess nitrogen entering the environment, leading to environmental problems like eutrophication, a process that can lead to excessive algae growth and subsequent depletion of oxygen in the water.

Sulphur Cycle

Sulphur is an essential component of proteins and vitamins, playing a vital role in the functioning of proteins and enzymes in plants, and in animals that depend upon plants for sulphur. In this section, we will discuss the sulphur cycle, which explains the movement of sulphur in the environment.

Sources of Sulphur

Sulphur enters the atmosphere through both natural and human sources. Natural sources include volcanic eruptions, bacterial processes, evaporation from water, and decaying organisms. Human activity also contributes to the release of sulphur, primarily through industrial processes, where sulphur dioxide (SO2) and hydrogen sulphide (H2S) gases are emitted on a wide scale.

Transformation of Sulphur

When sulphur dioxide enters the atmosphere, it undergoes various transformations. It may react with oxygen to produce sulphur trioxide gas (SO3), or with other chemicals in the atmosphere to produce sulphur salts. Sulphur dioxide may also react with water to produce sulphuric acid (H2SO4). Demethyl-sulphide, emitted to the atmosphere by plankton species, can also contribute to the production of sulphuric acid.

Impact of Sulphur Cycle on the Environment

All these particles settle back onto earth, or react with rain and fall back onto earth as acid deposition. The particles are then absorbed by plants again and are released back into the atmosphere, so that the sulphur cycle starts over again. This process can have negative impacts on the environment, including acid rain and soil acidification, which can harm plants and animals.

Alternative Energy Sources

Fossil fuels like coal and petroleum are important energy resources that are getting exhausted. Hydrocarbon fuel-based resources create pollution levels and greenhouse gases. Finding alternative energy sources and improving technology for their management is necessary to reduce the negative impacts of energy production.

Sustainable Resource Management

Prudent and sustainable use of resources, both at an individual and collective level, can benefit a wide cross-section of society and meet the needs of future generations. Sustainable resource management practices can help reduce negative environmental impacts and ensure a healthier planet for generations to come.