Biogeochemical Cycles: Carbon, Nitrogen, Phosphorus, and Water

The Carbon Cycle

  • Definition: The movement of molecules that contain carbon between sources and sinks in the biosphere.

  • Carbon reservoirs/sinks (stores more carbon than it releases): ocean, atmosphere, old‑growth forests, sediments.

  • Carbon sources (release more carbon than stored): fossil fuel combustion, animal agriculture, deforestation.

  • Carbon reservoirs vs. sources are dynamic; fluxes connect biotic and abiotic components of the biosphere.

  • Carbon forms that matter here: CO$2$, glucose (C$6$H${12}$O$6$), CH$_4$ (methane).

  • Major concept: carbon cycles between living organisms and the abiotic environment through multiple processes and timescales.

  • 7 processes in the carbon cycle (categorized as fast vs slow):

    • Fast processes (biotic; living organisms involved):
      1) Photosynthesis
      2) Respiration
      3) Exchange
      4) Combustion
    • Slow processes (carbon stored for long timescales):
      5) Sedimentation
      6) Burial
      7) Extraction

  • Photosynthesis (producers take in CO$_2$ to make sugars):

    • Overall representation: 6\ CO2 + 6\ H2O \rightarrow C6H{12}O6 + 6\ O2
    • Significance: Fixes inorganic carbon into organic molecules; basis of biomass.

  • Respiration (living organisms release CO$_2$):

    • General form: C6H{12}O6 + 6\ O2 \rightarrow 6\ CO2 + 6\ H2O + \text{energy}
    • Significance: Returns carbon to the atmosphere; part of short-term cycling.

  • Exchange (atmosphere ↔ ocean):

    • CO$2$ in the atmosphere dissolves in ocean water at the surface and can be released back to the air; the two pools exchange CO$2$ continuously.
    • Fast process that helps balance CO$_2$ between air and seawater; contributes to global carbon flux.
    • Consequence: Increasing atmospheric CO$2$ leads to more CO$2$ dissolved in the ocean, contributing to ocean acidification.
    • Role of marine organisms:
    • Algae & phytoplankton take up CO$2$ via photosynthesis (reducing local CO$2$ in water/air).
    • Coral reefs and marine organisms with calcium carbonate shells use dissolved CO$2$ to form CaCO$3$ exoskeletons.
    • Calcium carbonate precipitation: Sedimentation of CaCO$_3$ occurs as sediments on the ocean floor.

  • Sedimentation & Burial (long-term storage):

    • Sedimentation: CaCO$_3$ and other carbonates precipitate and settle to the ocean floor as sediments.
    • Burial: Over long timescales and under pressure, carbon-containing sediments become rock (e.g., limestone) or fossil fuels; this is a long-term carbon reservoir.
    • Weathering and volcanic activity can release buried carbon back to the atmosphere, completing cycles.

  • Extraction & Combustion (human influence):

    • Extraction: Humans mine fossil fuels (coal, oil, natural gas) and bring stored carbon to Earth’s surface.
    • Combustion: Burning fossil fuels releases CO$_2$ to the atmosphere, increasing atmospheric carbon and driving rapid changes in the cycle.
    • Result: These processes can shift the balance toward higher atmospheric CO$_2$ on centennial timescales, influencing climate.

  • No net increase in atmospheric CO$2$ from photosynthesis–respiration cycles alone; however, human activities (extraction/combustion) introduce additional CO$2$ input that disrupts the natural balance.

  • Ocean acidification: Increased atmospheric CO$2$ elevates dissolved CO$2$ in the ocean, lowering pH and impacting calcifying organisms (e.g., corals).

  • Human Impacts (summary):

    • Fossil fuel combustion releases large amounts of CO$_2$ into the atmosphere, contributing to global warming.
    • Tree harvesting/deforestation reduces the biosphere’s capacity to absorb CO$2$, further enhancing atmospheric CO$2$ levels.

  • FRQ Practice – 1.4 (carbon cycle related):

    • Task: Identify one process that happens quickly and one that happens slowly in the carbon cycle diagram.
    • Question: Explain how the rate at which fossil fuels are transferred into the atmosphere (as shown) has altered the carbon cycle during the past 250\ \text{years}.

The Nitrogen Cycle

  • Definition: The movement of nitrogen around the biosphere; essential for building blocks like RNA and DNA.

  • Macronutrient: One of the six key elements needed in relatively large amounts (N, P, K, Ca, Mg, S).

  • Limiting nutrient: A nutrient required for growth but available in lower quantity than other nutrients; nitrogen often acts as a limiting factor in ecosystems.

  • Major reservoir/sink: Atmosphere (as N$_2$ gas).

  • Nitrogen transformations (chemical transformations): Nitrogen fixation, nitrification, assimilation, mineralization (ammonification), denitrification.

  • Key transformations:

    • Nitrogen fixation: converts atmospheric N$2$ into usable forms (NH$3$ and NH$4^+$ or NO$3^-$ via abiotic routes).
    • Biotic fixation: carried out by bacteria (e.g., cyanobacteria) and root-associated bacteria; converts N$2$ to NH$3$ then NH$_4^+$.
    • Abiotic fixation: lightning and fires convert N$2$ to nitrates NO$3^-$.
    • Representations: \mathrm{N}2 \xrightarrow{\text{fixation}} \mathrm{NH}3 \rightarrow \mathrm{NH}_4^+ \; (\text{ammonium}) or directly to nitrates in some abiotic pathways.
    • Nitrification: biological oxidation of ammonium to nitrite and then nitrate by bacteria.
    • Sequence: \mathrm{NH}4^+ \rightarrow \mathrm{NO}2^- \rightarrow \mathrm{NO}_3^-
    • Assimilation: producers take up nitrogen in forms such as NH$4^+$, NH$3$, NO$3^-$, or NO$2^-$ and incorporate it into organic molecules.
    • Mineralization (Ammonification): decomposers convert organic nitrogen from dead organisms and waste products into inorganic NH$_4^+$.
    • Simplified: Organic-N -> NH$_4^+$
    • Denitrification: bacteria reduce nitrate (NO$3^-$) in oxygen-poor soils or stagnant water to NO$2^-$, then to N$2O, N$2$ gas, which is released back to the atmosphere.
    • Pathway: NO$3^- \rightarrow NO2^- \rightarrow N2O \rightarrow N2

  • Additional processes:

    • Leaching: movement of dissolved nitrogen species through soil to groundwater; can alter species composition and ecosystem dynamics.
    • Anthropogenic input: use of nitrogen-containing fertilizers can increase atmospheric and soil nitrogen; may lead to ecological shifts where high-nitrogen-tolerant species outcompete natives adapted to low nitrogen.

  • Key species/forms:

    • Ammonia: NH$_3$
    • Ammonium: NH$_4^+$
    • Nitrite: NO$_2^-$
    • Nitrate: NO$_3^-$
    • Nitrous oxide: N$_2$O
    • Dinitrogen: N$_2$
    • Fixation processes: biotic (cyanobacteria, root-associated bacteria) and abiotic (lightning, combustion)
    • Assimilation: producers take up inorganic N forms for growth; consumers acquire N by eating producers.

The Phosphorus Cycle

  • Key characteristics:

    • Phosphorus cycle involves movement of phosphorus around the biosphere with no gaseous phase under natural conditions.
    • Main form: phosphate, PO$_4^{3-}$, which is essential for DNA, RNA, ATP, and cell membranes.
    • Major reservoir/sink: sediments (and rocks) rather than the atmosphere.
    • Phosphorus is a limiting nutrient in many aquatic systems and soils because it is not very soluble and readily leached.

  • Five major processes:

    • Assimilation
    • Mineralization
    • Sedimentation
    • Geologic uplift
    • Weathering
    • This cycle is relatively slow compared with carbon and nitrogen cycles.

  • Assimilation & Mineralization (biotic/biochemical steps):

    • Producers take up inorganic phosphate (PO$_4^{3-}$) and assimilate it into organic phosphorus.
    • Decomposers (fungi/bacteria) decompose waste and dead organisms, mineralizing organic phosphorus back to inorganic phosphate (PO$_4^{3-}$).

  • Sedimentation, Geologic Uplift and Weathering (abiotic steps):

    • Because phosphate is not very soluble, much of it precipitates as minerals and sediments.
    • Over geological timescales, uplift brings these phosphate-containing rocks to land.
    • Weathering of rocks releases phosphate back into soils and waters.
    • Because of limited mobility, phosphorus is often a limiting nutrient in aquatic systems.

  • Human impacts:

    • Phosphate mining for fertilizers increases the input of phosphate to soils and waterways.
    • Excess phosphate in aquatic systems leads to eutrophication and algal blooms.
    • Algal blooms can cause hypoxic conditions and dead zones when algae die and bacteria decompose them, consuming oxygen.

  • Detergents and mining notes:

    • Detergents historically contained phosphates, contributing to eutrophication when released in wastewater.
    • Phosphate rocks and fertilizers are major anthropogenic sources of phosphorus in the environment.

The Water Cycle

  • Definition: The movement of water through the biosphere; water is essential for life and facilitates transport, dissolution of nutrients, and toxin removal.

  • Major reservoir/sink: the hydrosphere.

  • Major processes:

    • Evaporation: heat from the sun causes liquid water to become water vapor.
    • Transpiration: plants release water from their leaves during respiration (often summarized as part of evapotranspiration).
    • Condensation: water vapor cools and forms clouds.
    • Precipitation: rain, snow, hail, etc.

  • Evapotranspiration (ET): combined process of evaporation from surfaces and transpiration from plants; a key flux from land to atmosphere.

  • Infiltration and percolation:

    • Infiltration: water from precipitation infiltrates the soil and percolates downward to recharge groundwater.
    • Groundwater: water stored beneath the surface; part of the cycling loop.

  • Runoff and atmospheric return:

    • Surface runoff: water flows across land into streams and rivers.
    • Water eventually reaches the ocean, where the cycle can start again with evaporation.

  • Plant uptake:

    • Plants take up water and dissolved nutrients from the soil; water supports photosynthesis and growth.

  • Solar energy role:

    • Solar heating drives evaporation from bodies of water; energy input powers the cycle.

  • Human Impacts – Water Cycle:

    • Tree harvesting reduces evapotranspiration, leading to more surface runoff and potential erosion and flooding, especially on steep slopes.
    • Paving and urbanization reduce percolation, increasing surface runoff and flood risk.
    • Changes in ET and runoff also influence evaporation rates and groundwater recharge.

Connections, Implications, and Key Concepts

  • Conservation of matter: These cycles move matter, not create or destroy it; energy from the sun powers the cycles but matter is conserved.
  • Energy vs matter: The sun primarily drives energy flows; biogeochemical cycles describe matter flow (C, N, P, H2O) through ecosystems.
  • Real-world relevance: Human activities (fossil fuel combustion, fertilizer use, deforestation, phosphate mining, watershed alteration) alter cycle fluxes and can cause climate change, eutrophication, soil degradation, and water security issues.
  • Ethical/practical implications:
    • Balancing energy needs with climate goals (carbon cycle).
    • Sustainable fertilizer use to limit nutrient pollution and dead zones (nitrogen and phosphorus cycles).
    • Land-use decisions (forestry, agriculture, urbanization) impact the water cycle and erosion risk.

Notable Figures and Concepts Mentioned in the Transcript

  • Figure 7.2: Diagram illustrating exchange between atmospheric CO$2$ and ocean CO$2$, sedimentation, organisms' roles in photosynthesis and respiration, and fossil-fuel–related emissions.
  • Figure 7.3: Diagram outlining nitrogen fixation, nitrification, assimilation, mineralization (ammonification), denitrification, and leaching.
  • Figure 7.4: Diagram showing phosphate cycling with detergents, phosphate rocks, weathering, leaching, runoff, and sedimentation.

Quick Reference: Key Terms and Symbols

  • CO$_2$: carbon dioxide
  • CH$_4$: methane
  • CaCO$_3$: calcium carbonate
  • NH$_3$: ammonia
  • NH$_4^+$: ammonium
  • NO$_2^-$: nitrite
  • NO$_3^-$: nitrate
  • N$_2$: dinitrogen (atmospheric nitrogen)
  • N$_2$O: nitrous oxide
  • PO$_4^{3-}$: phosphate
  • POPs: not specifically mentioned, but phosphorus in DNA/RNA/ATP relevance
  • ET: evapotranspiration

Summary of Critical Timelines and Fluxes

  • Fast biotic processes connect living organisms with the atmosphere and oceans on short timescales (hours to years): photosynthesis, respiration, exchange, decomposition/breakdown.
  • Slow abiotic processes store carbon/nitrogen/phosphorus for millions of years (sedimentation, burial, rock formation, weathering, uplift).
  • Human activities have accelerated the transfer of carbon from long-term stores (fossil fuels) to the atmosphere, contributing to climate change.
  • Nutrient cycles (N and P) are sensitive to fertilizer use and land management, with impacts including eutrophication, algal blooms, hypoxic zones, and shifts in plant communities.
  • The water cycle integrates all cycles by transporting dissolved substances and supporting weathering, erosion, and biological processes; land-use changes affect ET, runoff, and groundwater recharge.