Biogeochemical Cycles Flashcards

Fundamentals of Biogeochemical Cycles

• Biogeochemical Cycles: Defined as the movement of matter within and between ecosystems, encompassing biological, chemical, and geological processes.
• Conservation of Matter: Matter is conserved across all ecosystems; molecules are continually recycled and reconstructed into new chemical forms.
• Reservoirs: Components of biogeochemical cycles that contain matter (e.g., air, water, living organisms, soil, and rocks).
  • Source: Term used to describe the phenomenon of molecules leaving a reservoir.
  • Sink: Term used to describe the phenomenon of molecules entering a reservoir.

The Carbon Cycle and Climate System

• Carbon Cycle: The movement of carbon around the biosphere. Carbon is held in or associated with living organisms for relatively short periods, while being stored in rocks, soil, and fossil fuels for long periods lasting millions of years.

• Core Processes in the Carbon Cycle:

  • Photosynthesis: Plants and algae use solar energy to convert atmospheric carbon dioxide ($\text{CO}_2$) and water into glucose ($\text{C}_6\text{H}_{12}\text{O}_6$) and oxygen. The chemical energy in glucose is utilized by plants for metabolic functions and passed along food webs to consumers.
  • Respiration: A cellular process performed by organisms that converts glucose ($\text{C}_6\text{H}_{12}\text{O}_6$) and oxygen into gaseous carbon dioxide ($\text{CO}_2$), water, and energy in the form of adenosine triphosphate (ATP).
  • Exchange: Carbon continuously travels back and forth between the atmosphere and aquatic ecosystems (such as oceans) through physical mechanisms like dissolution and evaporation.
  • Burial: Dead organic biomass is buried, incorporated into ocean sediments, and over geological time scales, fossilized into rock and fossil fuels.
  • Extraction: The human process of removing buried fossil fuels (coal, oil, and natural gas) from deep within the Earth.
  • Combustion: The burning of fossil fuels by human activity or natural carbon burning (such as forest fires), both of which release stored carbon back into the atmosphere as gaseous $\text{CO}_2$.

• Greenhouse Gases and Thermal Regulation:

  • Greenhouse Gases: Atmospheric gases that trap heat near Earth's surface, helping regulate Earth's global temperature to create conditions essential for life.
  • Role of Carbon Dioxide: $\text{CO}_2$ contributes the most to atmospheric warming.
  • Impact of Industrialization: Industrial activities add carbon to the atmosphere at a significantly quicker rate than what would naturally occur.

• Deforestation and Global Warming:

  • Trees store large amounts of carbon within their structural wood.
  • Clear-Cutting and Burning: Removing trees destroys a vital natural carbon sink and releases stored $\text{CO}_2$ back into the atmosphere, upsetting the natural balance of carbon dioxide.
  • Global Warming: Refers to increases in overall global temperatures resulting from human activities producing excess greenhouse gases.

The Nitrogen Cycle and Limiting Nutrients

• Chemical Profile of Nitrogen:

  • Atomic number: $7$
  • Chemical symbol: $\text{N}$
  • Atomic weight: $14.007$
  • Biological significance: Nitrogen is a key component required for building nucleic acids (DNA and RNA) and proteins.

• Nitrogen Cycle Dynamics:

  • Defined as the movement of nitrogen around the biosphere among reservoir sources and sinks.
  • Most reservoirs hold nitrogen for relatively short periods of time compared to geological carbon reservoirs.
  • Limiting Nutrient: A nutrient that is required for the growth of organisms but is available in lower quantities than other nutrients. Limited nitrogen directly restricts plant growth in terrestrial and aquatic ecosystems.

• Key Biological and Chemical Processes of the Nitrogen Cycle:

  • Nitrogen Fixation:
  • Definition: The process of converting gaseous atmospheric nitrogen ($\text{N}_2$) into forms that plants can utilize (such as ammonia or nitrates).
  • Source: Atmosphere (gaseous $\text{N}_2$).
  • Destination / Key Agents: Soil cyanobacteria, free-living nitrogen-fixing soil bacteria, and symbiotic nitrogen-fixing bacteria living in legume root nodules; lightning also fixes atmospheric nitrogen directly into nitrates ($\text{NO}_3^-$).
  • Nitrification:
  • Definition: Biological conversion of ammonia/ammonium ($\text{NH}_4^+$) into nitrite ($\text{NO}_2^-$) and subsequently into nitrate ($\text{NO}_3^-$).
  • Source: Cyanobacteria and ammonium ($\text{NH}_4^+$) in soil.
  • Destination / Key Agents: Nitrifying bacteria residing in soil surrounding plant roots.
  • Assimilation:
  • Definition: Process where plants and algae uptake inorganic nitrogen forms (primarily $\text{NO}_3^-$ and $\text{NH}_4^+$) through their roots and incorporate them into organic plant tissues.
  • Source: Nitrates ($\text{NO}_3^-$) produced by nitrifying bacteria in root-zone soil.
  • Destination: Plant and algal tissues, which are subsequently consumed by animals.
  • Mineralization / Ammonification:
  • Definition: Decomposers break down dead organic matter and waste products, converting organic nitrogen back into inorganic compounds like ammonium ($\text{NH}_4^+$).
  • Source: Decomposing organic matter from dead plants and animals.
  • Destination: Ammonium ($\text{NH}_4^+$) in soil and surrounding aquatic environments.
  • Denitrification:
  • Definition: Biological conversion of nitrates ($\text{NO}_3^-$) into nitrous oxide ($\text{N}_2\text{O}$) gas and ultimately back into gaseous nitrogen ($\text{N}_2$).
  • Source: Nitrates in soil or water.
  • Destination: Specialized denitrifying bacteria convert it back to nitrogen gas ($\text{N}_2$) in the atmosphere.

• Human Impacts on the Nitrogen Cycle:

  • Leaching: The process where dissolved nitrate molecules travel downward through soil and groundwater.
  • Synthetic Fertilizers: Applying nitrogen-rich fertilizers to agricultural land increases nitrogen concentrations in soil and atmosphere, allowing nitrogen to be transported to non-target ecosystems.
  • Ecosystem Alterations: Excess nitrogen alters plant community composition by favoring fast-growing species adapted to high-fertility soils, outcompeting native species.

The Phosphorus Cycle and Aquatic Impacts

• Phosphorus Cycle Overview:

  • The movement of phosphorus around the biosphere between sources and sinks, operating primarily between land and water ecosystems.
  • Unlike carbon, nitrogen, or water cycles, the phosphorus cycle has no gaseous phase under normal Earth surface conditions.

• Five Major Processes in the Phosphorus Cycle:

  • Assimilation:
  • Definition: Plants and animals take up inorganic phosphate from their surroundings and assimilate it into living tissues as organic phosphorus.
  • Source: Environmental soil and water solutions.
  • Destination: Plant and animal tissues.
  • Mineralization:
  • Definition: Excreted waste products and dead organic tissues are decomposed by fungi and bacteria, converting organic phosphorus back into inorganic phosphate.
  • Source: Organic material from plants and animals.
  • Destination: Surrounding soil and aquatic environment.
  • Sedimentation:
  • Definition: In water bodies, dissolved phosphorus precipitates out of solution and forms sedimentary layers at the ocean floor.
  • Source: Water containing dissolved phosphorus solutions.
  • Destination: Ocean floor sediments and newly formed rock layers.
  • Geologic Uplift:
  • Definition: Geological tectonic forces lift deep ocean sedimentary rock layers upward over long timeframes until they form terrestrial mountains.
  • Source: Ocean sediment layers.
  • Destination: Mountainous rock formations.
  • Weathering:
  • Definition: Physical and chemical weathering of mountain rocks by natural forces (rain, wind, ice) slowly releases stored phosphate back into ecosystems.
  • Source: Mountain rocks.
  • Destination: Terrestrial soil and aquatic habitats.

• Human Perturbations and Environmental Degradation:

  • Phosphate Mining: Humans mine phosphorus reserves from mountain rocks to manufacture agricultural fertilizers.
  • Algal Blooms: Runoff containing leached agricultural phosphorus enters rivers and lakes, triggering rapid exponential population growth of algae. Certain algal blooms produce toxins harmful to wildlife and humans.
  • Hypoxia: As massive algal blooms die, decomposers consume large quantities of dissolved oxygen during decay, leaving water bodies hypoxic (low in oxygen).
  • Dead Zones: Extreme hypoxia creates biological dead zones where oxygen levels drop too low to sustain fish and aquatic animals (e.g., the massive dead zone in the Gulf of Mexico where the Mississippi River drains).
  • Detergents and Regulation:
  • In the early 1900s, household laundry detergents contained high levels of phosphorus, serving as a primary source of phosphorus pollution in waterways.
  • In the early 1990s, the United States banned phosphorus in commercial detergents to mitigate aquatic ecosystem degradation.

The Hydrologic Cycle and Human Disturbances

• Biological Importance of Water:

  • Water constitutes over $\frac{1}{2}$ of a typical animal's body weight.
  • Essential for organism survival: acts as a cellular solvent enabling internal molecular movement, facilitates nutrient transport through plant vascular systems, and dissolves and removes metabolic toxins.

• Processes of the Hydrologic Cycle:

  • Transpiration: The release of water vapor from plant leaves into the atmosphere during photosynthesis.
  • Source: Plant leaves.
  • Destination: Atmosphere.
  • Evaporation: The conversion of liquid water into water vapor through thermal heating.
  • Source: Aquatic biomes (oceans, lakes, rivers).
  • Destination: Atmosphere.
  • Evapotranspiration: The combined total amount of water entering the atmosphere through simultaneous evaporation and transpiration.
  • Source: Aquatic biomes and terrestrial vegetation.
  • Destination: Atmosphere.
  • Runoff: Surface water that moves across land terrain into freshwater streams, rivers, and ocean basins.
  • Source: Terrestrial land surfaces.
  • Destination: Streams, rivers, and oceans.
  • Infiltration: Water moving downward through soil layers to collect in underground groundwater sinks known as aquifers.
  • Source: Ground surface.
  • Destination: Underground aquifers / groundwater sinks.
  • Condensation: Atmospheric water vapor cooling and transforming back into liquid water droplets, forming clouds.
  • Source: Atmosphere.
  • Destination: Clouds.
  • Precipitation: Condensed liquid water dripping or falling from clouds in the atmosphere back to Earth's surface.
  • Source: Clouds.
  • Destination: Earth's surface.

• Human Impacts on Hydrologic Processes:

  • Deforestation and Tree Harvesting:
  • Harvesting trees drastically reduces evapotranspiration rates.
  • Loss of vegetative cover leads to proportional increases in surface runoff and soil infiltration.
  • Tree removal destroys root networks that anchor soil, leading to increased soil erosion.
  • Urban Paving and Impervious Surfaces:
  • Paving land with impermeable surfaces reduces water infiltration into soil aquifers.
  • Increases surface runoff volume and elevates localized surface evaporation rates.