🦉Intro to Ecology Unit 11 – Biogeochemical Cycles

Key Concepts and Definitions

• Biogeochemical cycles describe the movement and exchange of matter and energy between the biosphere, atmosphere, hydrosphere, and geosphere
• Matter is neither created nor destroyed, but transformed from one form to another through biogeochemical processes
• Reservoirs represent the locations where matter is stored for varying lengths of time (atmosphere, oceans, soil)
• Fluxes refer to the movement of matter between reservoirs, often driven by biological, geological, or chemical processes
• Residence time indicates the average amount of time a molecule spends in a reservoir before moving to another
  • Reservoirs with longer residence times (deep ocean sediments) are more stable compared to those with shorter residence times (atmosphere)
• Limiting nutrients are essential elements that restrict biological productivity when in short supply (nitrogen, phosphorus)
• Turnover time measures the rate at which a reservoir is replenished or cycled, calculated by dividing the reservoir size by the flux rate

Types of Biogeochemical Cycles

• Gaseous cycles involve the exchange of elements between the atmosphere and other reservoirs (carbon, nitrogen, oxygen)
  • These cycles are driven by biological processes (photosynthesis, respiration) and physical processes (diffusion, dissolution)
• Sedimentary cycles involve the movement of elements through the Earth's crust and oceans (phosphorus, sulfur)
  • These cycles operate on much longer timescales compared to gaseous cycles, often millions of years
• Hydrobiogeochemical cycles combine the movement of water with the cycling of elements (water cycle, dissolved nutrients)
• Macronutrient cycles involve elements that are required in large quantities by living organisms (carbon, nitrogen, phosphorus)
• Micronutrient cycles involve elements needed in smaller amounts but still essential for biological processes (iron, zinc, copper)
• Coupled biogeochemical cycles occur when the cycling of one element influences the availability or movement of another (carbon and nitrogen, phosphorus and oxygen)

The Water Cycle

• The water cycle, also known as the hydrologic cycle, describes the continuous movement of water through Earth's reservoirs
• Evaporation occurs when water changes from a liquid to a gas, primarily from the surface of oceans, lakes, and rivers
  • Transpiration is the process by which plants release water vapor through their leaves, contributing to overall evaporation
• Condensation happens when water vapor cools and forms liquid water droplets, leading to the formation of clouds and fog
• Precipitation refers to the falling of water from the atmosphere back to Earth's surface in the form of rain, snow, sleet, or hail
• Infiltration is the process by which water moves into the soil and groundwater, replenishing aquifers and supporting plant growth
• Runoff occurs when water flows over the land surface, eventually reaching streams, rivers, and oceans
• Groundwater storage represents the water held in soil pores and rock fractures beneath the Earth's surface, which can slowly discharge into surface waters or be extracted by humans

The Carbon Cycle

• The carbon cycle involves the exchange of carbon between the atmosphere, biosphere, oceans, and geosphere
• Photosynthesis is the process by which plants and other autotrophs convert atmospheric carbon dioxide into organic compounds using sunlight energy
  • This process removes carbon from the atmosphere and stores it in biomass (leaves, roots, wood)
• Respiration releases carbon dioxide back into the atmosphere when organisms break down organic compounds for energy
• Decomposition occurs when microorganisms break down dead organic matter, releasing carbon into the soil and atmosphere
• Carbonation is the process by which atmospheric carbon dioxide dissolves in water, forming carbonic acid and contributing to ocean acidification
• Fossil fuel combustion releases stored carbon from ancient organic matter (coal, oil, natural gas) back into the atmosphere, increasing greenhouse gas concentrations
• Carbon sequestration refers to the long-term storage of carbon in reservoirs such as soil, vegetation, and the deep ocean, helping to mitigate climate change

The Nitrogen Cycle

• The nitrogen cycle describes the movement of nitrogen through the atmosphere, biosphere, and geosphere
• Nitrogen fixation converts atmospheric nitrogen gas (N2) into biologically available forms (ammonia, nitrates)
  • Biological nitrogen fixation is carried out by certain bacteria and archaea, often in symbiosis with plants (legumes)
  • Industrial nitrogen fixation occurs through the Haber-Bosch process, producing synthetic fertilizers
• Nitrification is the process by which ammonia is oxidized into nitrites and then nitrates by soil bacteria, making nitrogen available for plant uptake
• Assimilation incorporates inorganic nitrogen compounds into organic molecules (amino acids, proteins) within living organisms
• Ammonification converts organic nitrogen from dead organisms back into ammonia through decomposition
• Denitrification reduces nitrates back into atmospheric nitrogen gas, completing the cycle
  • This process is carried out by anaerobic bacteria in oxygen-poor environments (waterlogged soils, sediments)

The Phosphorus Cycle

• The phosphorus cycle involves the movement of phosphorus through the geosphere, biosphere, and hydrosphere
• Weathering of rocks releases phosphorus into the soil and water, making it available for biological uptake
• Assimilation incorporates inorganic phosphorus into organic molecules (DNA, RNA, ATP) within living organisms
• Decomposition releases organic phosphorus from dead organisms back into the soil and water
• Adsorption occurs when phosphorus ions bind to soil particles or sediments, temporarily removing them from the cycle
• Leaching is the process by which dissolved phosphorus moves through the soil profile and into groundwater or surface waters
• Sedimentation is the long-term burial of phosphorus in aquatic sediments, representing a major sink in the global phosphorus cycle
  • Uplifting and exposure of these sediments through geological processes can reintroduce phosphorus into the cycle over millions of years

Human Impacts on Biogeochemical Cycles

• Fossil fuel combustion has increased atmospheric carbon dioxide concentrations, contributing to global climate change
• Deforestation reduces the amount of carbon stored in biomass and soils, while also altering water and nutrient cycles
• Agricultural practices (tillage, fertilization) can lead to soil erosion, nutrient depletion, and eutrophication of water bodies
  • Eutrophication occurs when excess nutrients (nitrogen, phosphorus) stimulate algal blooms, leading to oxygen depletion and fish kills
• Urbanization alters local water cycles through increased runoff, decreased infiltration, and changes in precipitation patterns
• Dam construction and river regulation modify the natural flow and sediment transport of rivers, affecting downstream ecosystems
• Overfishing removes key species from marine food webs, disrupting the cycling of nutrients and energy
• Invasive species can alter biogeochemical cycles by changing plant communities, soil properties, and nutrient availability

Ecological Importance and Applications

• Biogeochemical cycles support the productivity and diversity of ecosystems by providing essential nutrients for living organisms
• Understanding biogeochemical cycles helps predict the responses of ecosystems to environmental changes (climate, land use)
• Knowledge of nutrient cycling informs sustainable agriculture practices (crop rotation, precision fertilization) to maintain soil fertility and minimize environmental impacts
• Watershed management strategies consider the movement of water and nutrients through landscapes to protect water quality and aquatic habitats
• Carbon cycle research informs climate change mitigation efforts, such as reforestation, soil carbon sequestration, and reduced fossil fuel use
• Bioremediation techniques harness microbial processes to clean up contaminated soils and water by altering biogeochemical cycles
• Ecological stoichiometry studies the balance of chemical elements in ecological interactions and processes, providing insights into nutrient limitation and ecosystem functioning