19.1 Carbon, Nitrogen, and Phosphorus Cycles

Carbon, nitrogen, and phosphorus are vital elements in ecosystems. These nutrients cycle through living organisms, the atmosphere, and Earth's crust. Understanding their movement helps us grasp how ecosystems function and how human activities impact them.

These cycles are interconnected and influence each other. Carbon drives energy flow, nitrogen is crucial for proteins, and phosphorus is key for DNA and energy transfer. Together, they shape the dynamics of life on Earth.

Biogeochemical Cycles

Essential Nutrient Cycles

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• Carbon cycle involves the exchange of carbon between the atmosphere, terrestrial ecosystems, and aquatic systems (oceans, lakes, rivers)
• Nitrogen cycle encompasses the transformation and movement of nitrogen compounds in the environment, including the atmosphere, soil, and living organisms
• Phosphorus cycle describes the movement of phosphorus through the lithosphere, hydrosphere, and biosphere, playing a crucial role in biological systems as a component of DNA, RNA, and ATP

Carbon Cycle Processes

Photosynthesis and Cellular Respiration

• Photosynthesis is the process by which plants and other autotrophs convert sunlight, carbon dioxide, and water into glucose and oxygen, incorporating carbon into organic compounds
• Cellular respiration is the process by which organisms break down organic compounds (glucose) to release energy, producing carbon dioxide and water as byproducts, returning carbon to the atmosphere

Decomposition and Carbon Storage

• Decomposition is the breakdown of dead organic matter by decomposers (bacteria and fungi), releasing carbon back into the atmosphere as carbon dioxide or storing it in the soil as organic carbon
• Carbon can be stored in various reservoirs, including the atmosphere (as carbon dioxide), oceans (dissolved carbon and calcium carbonate in shells), soil (organic carbon), and fossil fuels (coal, oil, natural gas)

Nitrogen Cycle Processes

Nitrogen Fixation

• Nitrogen fixation is the process of converting atmospheric nitrogen (N2) into biologically available forms (ammonia, NH3) by nitrogen-fixing bacteria (Rhizobium) or through lightning
• Nitrogen-fixing bacteria can be free-living in the soil or form symbiotic relationships with plants, such as legumes (soybeans, alfalfa), in root nodules

Nitrification and Denitrification

• Nitrification is the process by which ammonia (NH3) is oxidized into nitrite (NO2-) and then nitrate (NO3-) by nitrifying bacteria (Nitrosomonas and Nitrobacter) in the soil, making nitrogen available for plant uptake
• Denitrification is the process of reducing nitrate (NO3-) back into atmospheric nitrogen (N2) by denitrifying bacteria in anaerobic conditions, such as waterlogged soils or aquatic sediments, removing nitrogen from the ecosystem

Phosphorus Cycle Processes

Weathering and Erosion

• Weathering is the physical and chemical breakdown of rocks and minerals, releasing phosphorus in the form of phosphate ions (PO4-) into the soil and water
• Erosion is the process by which weathered materials, including phosphorus-containing particles, are transported by wind, water, or ice to other locations, redistributing phosphorus in the environment

Biological Uptake and Sedimentation

• Biological uptake occurs when plants and microorganisms absorb phosphate ions (PO4-) from the soil or water, incorporating phosphorus into their biomass (DNA, RNA, ATP)
• Sedimentation is the process by which phosphorus-containing particles settle to the bottom of aquatic systems (lakes, oceans), forming sedimentary layers that can store phosphorus for long periods or be uplifted and weathered, reintroducing phosphorus to the cycle