Biogeochemical Cycles: Water, Carbon, Nitrogen, and Phosphorous

Biogeochemical Cycles

Biogeochemical cycles describe the movement of elements and compounds between living (biotic) and non-living (abiotic) forms within Earth's ecosystems. These cycles are critical for sustaining life.

Key biogeochemical cycles that support life include:

  • Water Cycle

  • Carbon Cycle

  • Nitrogen Cycle

  • Phosphorous Cycle

The Water Cycle

Water is an indispensable component for all living organisms.

Processes of the Water Cycle

  • Evaporation and Transpiration: Most water molecules are converted into vapor and rise into the atmosphere to form clouds. Evaporation occurs from bodies of water and moist surfaces, while transpiration is the release of water vapor from plants.

  • Precipitation: Water returns to the Earth's surface in the form of rain, snow, or hail, replenishing oceans, lands, and lakes.

  • Runoff: Water that falls on land and enters the soil eventually flows over the surface or through groundwater, returning to oceans and other large water bodies.

The Carbon Cycle

Carbon is a fundamental element for life, forming the backbone of all organic molecules.

Carbon Storage and Fluxes

Carbon is stored in various reservoirs (GIC = Gigatons of Carbon) and moves between them through fluxes (GIC/yr).

Major Carbon Storage Reservoirs:

  • Atmosphere (CO_2):

    • 750 GtC

  • Vegetation:

    • 610 GtC

  • Soils:

    • 1,580 GtC

  • Fossil Fuels & Cement Production:

    • 4,000 GtC (a significant human-influenced reservoir)

  • Surface Ocean:

    • 1,020 GtC

  • Marine Biota:

    • 3 GtC

  • Dissolved Organic Carbon:

    • <700 GtC

  • Deep Ocean:

    • 38,100 GtC (the largest single reservoir of carbon)

  • Sediments:

    • 150 GtC

Examples of Carbon Fluxes (GIC/yr):

  • Fossil Fuels & Cement Production to Atmosphere: 5.5 GtC/yr

  • Surface Ocean to Atmosphere: 92 GtC/yr

  • Atmosphere to Surface Ocean: 50 GtC/yr

  • Surface Ocean to Deep Ocean: 40 GtC/yr

  • Deep Ocean to Surface Ocean: 100 GtC/yr

  • Surface Ocean to Marine Biota (and vice versa): 60 GtC/yr for each direction.

  • Inputs to Deep Ocean from Sediments/Rivers: 121.3 GtC/yr total.

Movement of Carbon Through the Biosphere

Carbon effectively moves through the biosphere via four primary mechanisms:

  1. Biological Processes:

    • Photosynthesis: Plants and other producers take up atmospheric CO_2 to synthesize sugars and other organic compounds.

    • Cellular Respiration: Living organisms release CO_2 back into the atmosphere and oceans as they break down organic compounds for energy.

    • Decomposition: Decomposers (bacteria and fungi) break down dead organic matter, returning carbon to the soil and atmosphere.

  2. Geochemical Processes: Natural geological activities release CO_2. Examples include:

    • Erosion: Weathering of rocks can release carbon compounds.

    • Volcanic Activity: Volcanoes release substantial amounts of CO_2 into the atmosphere and ocean.

  3. Formation and Release from Fossil Fuels:

    • Burial and Decomposition Under Pressure: Over millions of years, dead organisms subjected to intense pressure and heat are converted into fossil fuels (e.g., coal, petroleum). This process stores large amounts of carbon underground.

  4. Human Activities: Human actions significantly impact the carbon cycle, primarily by releasing stored carbon into the atmosphere.

    • Mining: Extraction of fossil fuels.

    • Forest Fires: Release carbon stored in biomass.

    • Burning of Fossil Fuels: Combustion of coal, oil, and natural gas for energy releases large quantities of CO_2 into the atmosphere, contributing to climate change.

Key Considerations for Carbon

  • While the deep ocean holds the largest amount of carbon, a significant portion of active carbon flux occurs with atmospheric CO_2. The statement