18_Biogeochemical Cycles L R200305101003031515

Biogeochemical Cycles

  • Definition: A biogeochemical cycle is the pathway by which chemical elements required by life move through both biotic (biosphere) and abiotic (lithosphere, atmosphere, hydrosphere) compartments of Earth.

  • Key Elements Involved:

    • Biological: Organisms that live and die.

    • Geological: Earth and its component parts.

    • Chemical: Processes returning organic matter to elemental form.

  • Key Nutrients: Carbon, Oxygen, Nitrogen, Phosphorous, Sulphur, Water.

  • Cycle Types:

    • Gas Cycle: Nutrients cycle through the atmosphere (e.g., Carbon, Nitrogen, Oxygen cycles).

    • Sedimentary Cycle: Nutrients move from land to water to sediment (e.g., Phosphorous, Sulphur cycles).

Nitrogen Cycle

  • Importance of Nitrogen: Vital for amino acids, proteins, vitamins, DNA, and RNA. Earth’s atmosphere contains about 78% nitrogen, but it's unusable in its elemental form.

  • Conversion Process: Nitrogen must be converted into inorganic nitrogen compounds (e.g., nitrites, nitrates, ammonia) through the action of bacteria.

  • Steps in the Nitrogen Cycle:

    • Nitrogen Fixation: Conversion of atmospheric nitrogen into a biologically available form.

      • Physiochemical Fixation: Combines nitrogen with oxygen during lightning; produces various nitrogen oxides that dissolve into nitrates.

      • Biological Nitrogen Fixation: Done by free-living bacteria (e.g., Azobacter) and symbiotic bacteria (e.g., Rhizobium in legumes).

      • Industrial Fixation: Ammonia produced under high pressure and temperature using an iron catalyst.

      • Combustion of Fossil Fuels: Releases nitrogen oxides into the atmosphere.

    • Nitrogen Assimilation: Plants uptake inorganic nitrogen to produce organic compounds like amino acids and proteins.

    • Ammonification: Decomposition of organic matter returns nitrogen to the soil as ammonia through microbial activity (e.g., Bacillus species).

    • Nitrification: Conversion of ammonia to nitrites, then to nitrates via Nitromonas and Nitrobacter bacteria.

    • Denitrification: Conversion of nitrates back to nitrogen gas by denitrifying bacteria (e.g., Pseudomonas).

    • Sedimentation: Nitrates may become locked in rocks during weathering process and are eventually released.

Phosphorous Cycle

  • Significance of Phosphorous: Essential for DNA, ATP, cellular structure, and is vital for many organisms.

  • Sources: Phosphates from rocks leach into soil and water through weathering.

  • Fate of Phosphorous:

    • In soil, phosphates can be in various forms: inorganic (plant-available), organic (not plant-available), adsorbed (bound to soil particles, not readily available), and mineralized (not readily available).

  • Mineralization and Immobilization: Conversion of organic phosphorous into plant-accessible forms by microbes.

  • Water Runoff: Carries phosphorous from fertilizers and manure to waterways and sediments.

  • Biotic and Abiotic Cycling: Plants absorb phosphorous, transferring it through the food chain and back to soil via decomposition.

Sulphur Cycle

  • Importance of Sulphur: Component of amino acids and various vitamins and enzymes.

  • Sources: Found as sulphates in soils and rocks; leaching into soil and water.

  • Contributions to Ecosystem: Decomposing organisms mineralize sulphur into accessible forms.

  • Processes:

    • Oxidation of hydrogen sulfide to sulphates by bacteria, contributing to nutrient cycling.

    • Release into Atmosphere: From natural occurrences (volcanic eruptions, combustion of fossil fuels).

Oxygen Cycle

  • Role of Oxygen: Essential for respiration, protective O3 layer shields Earth from UV radiation.

  • Storage Metrics:

    • Mainly contained in atmosphere, biological matter, and lithosphere.

  • Processes in the Cycle:

    • Photosynthesis: Plants convert CO2 and water into glucose, producing oxygen.

    • Respiration: Animals take in oxygen and release CO2, driving the cycle.

    • Natural Weathering: Releases oxygen from minerals through chemical reactions.

Water Cycle (Hydrologic Cycle)

  • Importance of Water: Fundamental for life; comprises a significant portion of living organisms' body weight.

  • Distribution:

    • Oceans (96.5%), glaciers (1.7%), groundwater (1.7%), and trace amounts in the atmosphere.

  • Key Processes:

    • Evaporation, Precipitation, and Transpiration which maintain the cycle in both terrestrial and aquatic systems.

    • Global Water Cycle: Movement of water among oceans, land, and atmosphere.

    • Biological Water Cycle: Water uptake and return in terrestrial and aquatic ecosystems.

Carbon Cycle

  • Importance of Carbon: Fundamental building block of life; critical for organic compounds.

  • Main Sources: Atmospheric CO2 and dissolved gases in aquatic environments.

  • Biological Processes:

    • Photosynthesis: Converts CO2 and water into glucose, releasing oxygen.

    • Respiration: Transfer of carbon through food chains and release back into environment.

  • Role of Oceans: Major carbon reservoir that dissolves atmospheric CO2 and facilitates cycling through marine life.

Summary of Biogeochemical Cycles

  • Key Elements: Water, Nitrogen, Carbon, Phosphorous, Sulphur, Oxygen are critical for life.

  • Complex Interactions: These cycles are interconnected pathways that regulate the flow and transformation of matter across Earth’s systems.