Environment Biology | Calicut University Second Semester BSc. Zoology NOtes FRee PDF | Module 1 Unit 3

 Biogeochemical Cycles

Hey everyone! Biogeochemical cycles involve biological, geological, and chemical processes working together to circulate elements essential for life. These cycles maintain nutrient balance, regulate ecosystem productivity, and influence climate and soil fertility.

🌍 Carbon Cycle

The carbon cycle is one of the most important biogeochemical cycles because carbon forms the structural backbone of all organic molecules. It involves the movement of carbon through the atmosphere, land, oceans, and living organisms.

Carbon exists in multiple forms, including atmospheric carbon dioxide, organic carbon in biomass, dissolved inorganic carbon in oceans, and fossil carbon stored underground.

🌿 Terrestrial Carbon Cycle

Carbon enters the atmosphere primarily through combustion, both natural and anthropogenic. Natural wildfires release carbon stored in vegetation, while human activities such as burning coal, petroleum, and natural gas release fossil carbon that had been locked away for millions of years. This rapid release significantly alters atmospheric carbon balance.

Photosynthesis is the major biological process that removes carbon dioxide from the atmosphere. Green plants, algae, and some bacteria use solar energy to convert atmospheric CO₂ and water into glucose. In this process, carbon is stored in plant biomass, while oxygen is released into the atmosphere.

Respiration occurs in all living organisms and represents the reverse process. During respiration, organic molecules are broken down to release energy, and carbon dioxide is returned to the atmosphere.

When organisms die, decomposition becomes the dominant process. Decomposers such as bacteria and fungi break down complex organic matter. Under aerobic conditions, carbon is released as CO₂, while anaerobic conditions may lead to methane production, a potent greenhouse gas.

🌊 Oceanic Carbon Cycle

Atmospheric CO₂ dissolves in surface seawater, a process influenced by temperature and pressure. Once dissolved, carbon dioxide participates in carbonate chemistry, forming carbonic acid, bicarbonate ions, and carbonate ions. This carbonate system buffers ocean pH and maintains chemical stability.

Marine photosynthetic organisms such as phytoplankton use dissolved carbon dioxide to produce organic matter. These organisms form the base of marine food webs and account for a significant fraction of global carbon fixation.

Carbon is transported within the ocean through a combination of biological and physical processes. Organic particles sink to deeper layers when organisms die or are consumed, effectively transferring carbon away from the surface. Calcium carbonate shells formed by marine organisms also sink and accumulate as sediments, storing carbon over long geological periods.

Ocean currents and vertical movements such as upwelling and downwelling redistribute carbon. Upwelling brings deep, carbon-rich water to the surface, potentially releasing CO₂ back to the atmosphere, while downwelling transports surface carbon to deeper layers for long-term storage.

🌾 Nitrogen Cycle

Nitrogen is an essential element required for the synthesis of amino acids, proteins, nucleic acids, and chlorophyll. Although nitrogen makes up nearly 78% of the atmosphere, it exists primarily as molecular nitrogen (N₂), which is biologically unusable.

🔄 Steps of the Nitrogen Cycle

Nitrogen fixation is the first critical step. Atmospheric nitrogen is converted into ammonia by nitrogen-fixing bacteria and cyanobacteria using the enzyme nitrogenase. Fixation can also occur through lightning or industrial processes used in fertilizer production.

Nitrification follows fixation and involves the biological oxidation of ammonia into nitrite and then nitrate by nitrifying bacteria. Nitrate is the most readily absorbed form of nitrogen for plants.

Assimilation occurs when plants absorb nitrate or ammonia and incorporate it into organic compounds such as amino acids and proteins. Animals obtain nitrogen by feeding on plants or other animals.

Ammonification takes place during decomposition. Organic nitrogen from dead organisms and wastes is converted back into ammonia by decomposers, returning nitrogen to the soil or water.

Denitrification completes the cycle by converting nitrates back into gaseous nitrogen under anaerobic conditions. This process releases nitrogen back into the atmosphere and maintains nitrogen balance in ecosystems.

🪨 Phosphorus Cycle

The phosphorus cycle differs from the carbon and nitrogen cycles because it lacks a significant gaseous phase. Phosphorus circulates primarily through rocks, soil, water, and living organisms.

🔄 Steps of the Phosphorus Cycle

Phosphorus enters ecosystems through the weathering of phosphate rocks. Physical and chemical weathering releases phosphate ions into soil and water bodies.

Plants absorb inorganic phosphates through their roots, while phytoplankton absorb dissolved phosphates in aquatic ecosystems. Assimilated phosphorus becomes part of organic molecules.

Through food chains, phosphorus moves from producers to consumers. Herbivores obtain phosphorus from plants, and carnivores obtain it from other animals.

When organisms die, decomposers mineralize organic phosphorus, converting it back into inorganic phosphate. These phosphates re-enter soil or aquatic systems, completing the cycle.

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