• A nutrient cycle is a repeated pathway of a particular nutrient or element from the environment through one or more organisms and back to the environment.
• Energy flow is a unidirectional and noncyclic pathway, whereas the movement of mineral nutrients is cyclic
• Hence, Nutrient cycling occurs as animals and plants consume nutrients found in the soil, and these nutrients are then released back into the environment via death and decomposition
• Nutrient cycling is essential for life and is the vital function of the ecology of any region
• In any particular environment, to maintain an organism in a sustained manner, the nutrient cycle must be kept balanced and stable

Types of Nutrient Cycle

• Based on the replacement period a nutrient cycle is referred to as Perfect or Imperfect Cycle
• A perfect nutrient cycle is one in which nutrients are replaced as fast as they are utilised. Most Gaseous cycles are considered as perfect cycles
• Sedimentary cycles are considered imperfect, as some nutrients are lost from the cycle and get locked into sediments and so become unavailable for immediate cycling
• Based on nature of the reservoir, there are two types of cycles:
• Gaseous cycle – where the reservoir is the atmosphere or the hydrosphere
• Sedimentary cycle – where the reservoir is the Earth’s crust

 Gaseous Cycles

• The most important gaseous cycles are – water, carbon and nitrogen

Water Cycle ( Hydrologic)

• • • The hydrologic cycle begins with the evaporation of water from the surface of the ocean. As moist air is lifted, it cools and water vapour condenses to form clouds. Further, Moisture is transported around the globe until it returns to the surface as precipitation
    • Once the water reaches the ground, one of two processes may occur;
      • some of the water may evaporate back into the atmosphere or
      • the water may penetrate the surface and become groundwater
    • Groundwater either seeps its way to into the 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 

Carbon Cycle

• On Earth, the element carbon is a part of seawater, the atmosphere, rocks such as limestone and coal, soils, as well as all living things
• Carbon is able to move from one of these realms to another as a part of the carbon cycle
• Carbon moves from the atmosphere to plants. In the atmosphere, carbon is attached to oxygen in a gas called carbon dioxide (CO2).
• Through the process of photosynthesis, carbon dioxide is pulled from the air to produce food made from carbon for plant growth.
• Carbon moves from plants to animals.
• Through food chains, the carbon that is in plants moves to the animals that eat them. Animals that eat other animals get the carbon from their food too.
• Carbon moves from plants and animals to soils.
• When plants and animals die, their bodies, wood and leaves decays bringing the carbon into the ground. Some is buried and will become fossil fuels in millions and millions of years
• Carbon moves from living things to the atmosphere.
• Each time you exhale, you are releasing carbon dioxide gas (CO2) into the atmosphere.
• Animals and plants need to get rid of carbon dioxide gas through a process called respiration
• Carbon moves from fossil fuels to the atmosphere when fuels are burned
• Further, Carbon moves from the atmosphere to the oceans. The oceans, and other bodies of water, absorb some carbon from the atmosphere. The carbon is dissolved into the water
• Carbon moves through our planet over longer time scales as well.
• For example, over millions of years weathering of rocks on land can add carbon to surface water which eventually runs off to the ocean.
• Over long time scales, carbon is removed from seawater when the shells and bones of marine animals and plankton collect on the sea floor. These shells and bones are made of limestone, which contains carbon
• Also, The carbon can be released back to the atmosphere if the limestone melts or is metamorphosed in a subduction zone. 

Nitrogen Cycle

• Nitrogen is one of the primary nutrients critical for the survival of all living organisms. Although nitrogen is very abundant in the atmosphere, it is largely inaccessible in this form to most organisms.
• The processes in Nitrogen cycle can be explained as follows:
• Nitrogen fixation
• Nitrogen fixation is the process wherein N2 is converted to ammonium, or NH4+.
• This is the only way that organisms can attain nitrogen directly from the atmosphere; the few that can do this are called nitrogen-fixing organisms
• Certain bacteria, including those among the genus Rhizobium, are able to fix nitrogen (or convert it to ammonium) through metabolic processes
• Nitrogen-fixing bacteria often form symbiotic relationships with host plants.
• This symbiosis is well-known to occur in the legume family of plants (e.g., beans, peas, and clover).
• In this relationship, nitrogen-fixing bacteria inhabit legume root nodules and receive carbohydrates and a favourable environment from their host plant in exchange for some of the nitrogen they fix
• In addition to nitrogen-fixing bacteria, high-energy natural events such as lightning, forest fires, and even hot lava flows can cause the fixation of smaller, but significant, amounts of nitrogen
• Nitrogen uptake
• The ammonium (NH4+) produced by nitrogen-fixing bacteria is usually quickly taken up by a host plant, the bacteria itself, or another soil organism and incorporated into proteins and other organic nitrogen compounds, like DNA
• 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
• When organisms die, decomposers (such as bacteria and fungi) consume the organic matter and lead to the process of 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
• Nitrification
• Nitrification requires the presence of oxygen, so nitrification can happen only in oxygen-rich environments like circulating or flowing waters and the surface layers of soils and sediments.
• The process of nitrification has some important consequences. Ammonium ions (NH4+) are positively charged and therefore stick (are absorbed) to negatively charged clay particles and soil organic matter.
• Denitrification
• Through denitrification, oxidized forms of nitrogen such as nitrate (NO3-) and nitrite (NO2-) are converted to dinitrogen (N2) and, to a lesser extent, nitrous oxide gas (NO2)
• Denitrification is an anaerobic process that is carried out by denitrifying bacteria, which convert nitrate to dinitrogen in the following sequence:

NO3- → NO2- → NO → N2O → N2.

• Once converted to dinitrogen, nitrogen is unlikely to be reconverted to a biologically available form because it is a gas and is rapidly lost to the atmosphere
• Denitrification is the only nitrogen transformation that removes nitrogen from ecosystems (essentially irreversibly), and it roughly balances the amount of nitrogen fixed by the nitrogen fixer
• Thus a large part of nitrogen is fixed up and stored in plants, animals, and microbes. Nitrogen leaves the living system in the same amount it is taken in from the atmosphere and the input and outflow of nitrogen are balanced in the ecosystem 

Sedimentary Cycles

• Sedimentary cycles are a type of biogeochemical cycle, in which the reservoir is Earth’s crust

Phosphorous cycle

• • • Phosphorus moves in a cycle through rocks, water, soil and sediments and organisms.
    • Over time, rain and weathering cause rocks to release phosphate ions and other minerals. This inorganic phosphate is then distributed in soils and water.
    • Plants take up inorganic phosphate from the soil. The plants may then be consumed by animals. Once in the plant or animal, the phosphate is incorporated into organic molecules such as DNA. When the plant or animal dies, it decays, and the organic phosphate is returned to the soil.
    • Within the soil, organic forms of phosphate can be made available to plants by bacteria that break down organic matter to inorganic forms of phosphorus. This process is known as mineralisation
    • Phosphorus in soil can end up in waterways and eventually oceans. Once there, it can be incorporated into sediments over time

Sulphur cycle

• Most of the earth’s sulphur is tied up in rocks and salts or buried deep in the ocean in oceanic sediments.
• Sulphur can also be found in the atmosphere. It enters the atmosphere through both natural and human sources
• Natural recourses can be for instance volcanic eruptions, bacterial processes, evaporation from water, or decaying organisms.
• When sulphur enters the atmosphere through human activity, this is mainly a consequence of industrial processes where sulphur dioxide (SO2) and hydrogen sulphide (H2S) gases are emitted on a wide scale.
• When sulphur dioxide enters the atmosphere it will 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).
• Sulphuric acid may also be produced from demethylsulphide, which is emitted to the atmosphere by plankton species.
• All these particles will settle back onto earth, or react with rain and fall back onto earth as acid deposition.
• The particles will than be absorbed by plants again and are released back into the atmosphere, so that the sulphur cycle will start over again.

Calcium Cycle

• • • Calcium is primarily present as rock, minerals or as structural calcium built into mineral crystal lattices of soil particles and is not readily available.
    • Calcium can also be added as fertilizer, lime or by-products. Water can carry calcium into the soil through weathering and natural dissolution
    • When in the soil, most of the calcium is in an insoluble form until it is ‘weathered off’ of minerals or when organic matter is broken down by microbes into soluble calcium

• However, some of the calcium are held loosely or tightly on soil’s cation exchange complex (CEC) or in the soil solution and are available to plants and microorganisms

• When animals, microorganisms, or plants decay, their bodies decompose and the calcium is mineralized and released back into the soil.
• Roots also regularly leak minerals, sugars, and other compounds back into the soil including calcium.
• Since calcium is a positively charged ion, it is adsorbed in the soil to the surface of clay and organic particles which are negatively charged. Positively charged ions (cations) adsorb to soil particles and are termed “exchangeable ions” because they can be exchanged by other ions present in the soil solution.
• When absorbed by plants or microorganisms, calcium enters an organic phase.  In this form, calcium is continually recycled between the plant roots, microorganisms, and soil.
• After a plant, animal, or soil fauna dies, decomposers break down the organism and calcium is released back to the soil in a soluble form.
• Thus, Calcium routinely moves back and forth between the soluble (and available) and the insoluble (unavailable) phases.