• Nutrient cycling is the repeated movement of nutrients or elements from the environment through organisms and back to the environment. Unlike energy flow, nutrient cycling is a cyclic pathway.
• Plants and animals consume nutrients from the soil, and these nutrients are returned to the environment through death and decomposition.
• Nutrient cycling is a vital process for life and essential to the ecology of any region. A balanced and stable nutrient cycle is necessary to sustain organisms in a particular environment.
• Nutrient cycles can be classified into two types based on the replacement period: perfect or imperfect cycles. A perfect nutrient cycle is one in which nutrients are replaced as quickly as they are utilized. Most gaseous cycles, such as the carbon and nitrogen cycles, are considered perfect cycles. On the other hand, sedimentary cycles, such as the phosphorus cycle, are considered imperfect because some nutrients are lost from the cycle and become locked into sediments, making them unavailable for immediate cycling.
• Another way to categorize nutrient cycles is based on the nature of the reservoir. There are two types of cycles: gaseous and sedimentary cycles. Gaseous cycles have the atmosphere or the hydrosphere as the reservoir, such as the carbon and nitrogen cycles. Sedimentary cycles have the Earth’s crust as the reservoir, such as the phosphorus cycle.

Gaseous Cycle:

Gaseous cycles are essential for the functioning of ecosystems, with the three most important ones being the water, carbon, and nitrogen cycles.

1. The water cycle, also known as the hydrologic cycle, begins with the evaporation of water from the ocean’s surface. As moist air rises and cools, water vapor condenses to form clouds. Moisture is then transported around the globe until it falls back to the surface as precipitation. Once the water reaches the ground, two processes may occur: it may either evaporate back into the atmosphere or penetrate the surface and become groundwater. Groundwater either seeps into oceans, rivers, and streams or is released back into the atmosphere through transpiration. The balance of water that remains on the Earth’s surface is runoff, which empties into lakes, rivers, and streams and is carried back to the oceans, where the cycle begins again.

2. The carbon cycle involves the movement of carbon between various realms on Earth, including seawater, the atmosphere, rocks like limestone and coal, soils, and all living things.

• Carbon dioxide (CO2) is attached to oxygen in the atmosphere, and through photosynthesis, carbon is pulled from the air to produce food for plant growth. Carbon then moves from plants to animals through food chains. As animals eat plants or other animals, they obtain the carbon from their food. Carbon is constantly moving from one realm to another as part of this cycle.
• Carbon from plants and animals is brought into the ground when their bodies, wood, and leaves decay. Some of this carbon is buried and can become fossil fuels in millions of years.
• Living things release carbon dioxide gas (CO2) into the atmosphere through respiration, and when fossil fuels are burned, carbon is released into the atmosphere as well.
• The oceans and other bodies of water absorb some carbon from the atmosphere, which is dissolved into the water. Carbon also moves through the planet over long time scales. For example, over millions of years, weathering of rocks on land can add carbon to surface water, which eventually runs off into the ocean.

3. The nitrogen cycle is crucial for the survival of living organisms, as nitrogen is a primary nutrient. Despite being abundant in the atmosphere, nitrogen is mostly inaccessible in this form for organisms. The cycle includes the following processes:

• Nitrogen fixation: N2 is converted to ammonium, or NH4+. Only a few organisms can directly obtain nitrogen from the atmosphere, known as nitrogen-fixing organisms. Bacteria such as Rhizobium can fix nitrogen through metabolic processes.
• Nitrogen-fixing bacteria have a symbiotic relationship with host plants, which is commonly observed in the legume family of plants like beans, peas, and clover. These bacteria reside in nodules present in the roots of legume plants, receiving carbohydrates and a suitable environment from their host plant. In return, they provide some of the nitrogen they fix to the plant.
• Apart from nitrogen-fixing bacteria, high-energy natural events like lightning, forest fires, and hot lava flows can also cause the fixation of smaller but notable amounts of nitrogen.
• Nitrogen Uptake:
• The nitrogen produced by nitrogen-fixing bacteria is quickly taken up by the host plant, the bacteria itself, or another soil organism and incorporated into proteins and other organic nitrogen compounds.
• Nitrogen Mineralization:
• After nitrogen is incorporated into organic matter, it can be converted back into inorganic nitrogen by a process called nitrogen mineralization, also known as decay. When organisms die, decomposers such as bacteria and fungi consume the organic matter and lead to decomposition. During this process, a significant amount of the nitrogen contained within the dead organism is converted to ammonium. Once in the form of ammonium, nitrogen is available for use by plants or for further transformation into nitrate (NO3-) through the process called nitrification.
• The nitrogen cycle involves several processes, including nitrification and denitrification. Nitrification is the conversion of ammonium (NH4+) to nitrate (NO3-) by bacteria, which occurs in oxygen-rich environments such as the surface layers of soils and sediments, as well as circulating or flowing waters. During this process, ammonium ions can be absorbed by negatively charged clay particles and soil organic matter.
• Denitrification, on the other hand, is an anaerobic process carried out by denitrifying bacteria that convert nitrate and nitrite into dinitrogen (N2) and, to a lesser extent, nitrous oxide gas (NO2). Once converted to dinitrogen, nitrogen is lost to the atmosphere, and it is unlikely to be reconverted to a biologically available form. Denitrification balances the amount of nitrogen fixed by nitrogen-fixing organisms, and it is the only nitrogen transformation that removes nitrogen from ecosystems irreversibly.
• Ultimately, the input and outflow of nitrogen are balanced in the ecosystem, with nitrogen leaving the living system in the same amount it is taken in from the atmosphere. Nitrogen is fixed and stored in plants, animals, and microbes, and it moves through the nitrogen cycle via various processes.

Sedimentary Cycle:

• Sedimentary cycles involve a reservoir in the Earth’s crust and are a type of biogeochemical cycle.

1. The phosphorus cycle is a cycle that involves the movement of phosphorus through rocks, water, soil, sediments, and organisms. Over time, rocks release phosphate ions and other minerals due to rain and weathering, which are distributed in soils and water. Plants absorb inorganic phosphate from the soil, and animals may consume the plants, leading to the incorporation of phosphate into organic molecules like DNA. When plants or animals die, the organic phosphate is returned to the soil through decay.

Organic forms of phosphate in the soil can become available to plants through mineralization, which is the process of bacteria breaking down organic matter into inorganic forms of phosphorus. Phosphorus in soil can eventually end up in waterways and oceans, where it can be incorporated into sediments over time.

2. The Sulphur cycle: Most of the earth’s sulphur is present in rocks, salts, or buried deep in the ocean’s sediments. However, sulphur can also be found in the atmosphere, entering it through natural and human sources. Natural sources include volcanic eruptions, bacterial processes, evaporation from water, or decaying organisms. Human activities that contribute to the release of sulphur dioxide (SO2) and hydrogen sulphide (H2S) gases into the atmosphere include industrial processes.

When sulphur dioxide enters the atmosphere, it reacts with oxygen to produce sulphur trioxide gas (SO3) or with other chemicals in the atmosphere to produce sulphur salts. It can also react with water to produce sulfuric acid (H2SO4). Additionally, dimethyl sulphide, which is emitted into the atmosphere by plankton species, can also contribute to the production of sulfuric acid.

After being released into the atmosphere through natural and human sources, sulphur can settle back onto Earth through various processes. This can occur through the settling of particles, or through reactions with rain which then falls onto Earth as acid deposition. Once absorbed by plants, sulphur can be released back into the atmosphere, starting the sulphur cycle anew.

3. The calcium cycle involves the movement of calcium between rocks, minerals, and soil particles, making it less readily available. Calcium can also be introduced to soil through fertilizers, lime, or natural dissolution via water. Once in the soil, most of the calcium remains in an insoluble form, unless it is weathered off minerals or released via decomposition of organic matter by microbes, making it available for uptake by plants and microorganisms. Calcium can also be held in soil’s cation exchange complex or the soil solution, making it loosely or tightly available for uptake. Additionally, roots regularly release compounds, including calcium, into the soil. This cycle allows for the movement of calcium throughout the ecosystem.

Calcium in soil adheres to negatively charged clay and organic particles, which are exchangeable ions. It enters an organic phase when absorbed by plants or microorganisms, and is recycled between them and the soil. Upon decomposition of animals, plants, or soil fauna, calcium is released back into the soil in a soluble form. Calcium regularly transitions between the soluble and insoluble phases.