Biogeochemical Cycles

Carbon cycle

• Reservoirs: atmosphere (CO₂), ocean (HCO₃⁻), biosphere (living tissue), fossil fuels, sediments / limestone.
• Processes: photosynthesis (CO₂ → glucose), respiration, combustion, decomposition, ocean dissolution.
• Human disruption: burning fossil fuels and deforestation → ↑ atmospheric CO₂ → climate change & ocean acidification.

            ATMOSPHERE (CO₂)
                  ↑↓
       photosynthesis  respiration · combustion · decomposition
                  ↓↑
   PRODUCERS  →  CONSUMERS  →  DECOMPOSERS  →  FOSSIL FUELS / SEDIMENT

Nitrogen cycle (most tested!)

• N₂ gas dominates the atmosphere (78%) but is unusable by most organisms — the bonds are too strong.
• Nitrogen fixation — bacteria (Rhizobium in legume root nodules, free-living cyanobacteria) and lightning convert N₂ → NH₃ / NH₄⁺. The Haber-Bosch industrial process does the same with high heat & pressure.
• Nitrification — soil nitrifying bacteria convert NH₄⁺ → NO₂⁻ → NO₃⁻. Plants prefer to absorb nitrate.
• Assimilation — plants take up NO₃⁻ to build proteins, DNA. Animals get N by eating plants/animals.
• Ammonification — decomposers turn organic nitrogen (proteins, urea) back into NH₄⁺.
• Denitrification — anaerobic denitrifying bacteria convert NO₃⁻ → N₂ (gas), returning N to the atmosphere. Occurs in waterlogged or oxygen-poor soils.

     ATMOSPHERE (N₂, 78%)
              ↑                  ↓
       Denitrification     Fixation (bacteria, lightning, Haber-Bosch)
              ↑                  ↓
            NO₃⁻ ←── Nitrification ── NH₄⁺
              ↑                       ↑
        (assimilation)          Ammonification
              ↓                       ↑
           PLANTS  →  ANIMALS  →  DECOMPOSERS

Human disruption of N cycle: synthetic fertilizers (Haber-Bosch produces ~150 Mt N/yr), animal waste, fossil-fuel combustion. Excess N causes eutrophication, acid rain, and N₂O greenhouse gas (GWP ~265).

Phosphorus cycle

• NO atmospheric component — there is no P gas phase. The cycle is slow.
• Reservoirs: rocks, soil, sediments, water. Weathering of rock releases PO₄³⁻.
• P is often the limiting nutrient in freshwater ecosystems — adding P (fertilizer, detergent) drives eutrophication.
• Human disruption: mining for fertilizer and detergents; runoff to surface waters.

Sulfur cycle

• Reservoirs: rocks, ocean, atmosphere (SO₂).
• Volcanic eruptions and decomposition naturally release SO₂; sulfur-fixing bacteria convert it.
• Human disruption: burning coal and oil releases SO₂ → atmospheric H₂SO₄ → acid rain.

Water (hydrologic) cycle

• Processes: evaporation, transpiration (combined: evapotranspiration), condensation, precipitation, runoff, infiltration, percolation to groundwater.
• Powered by solar energy.
• Human disruption:
  • Deforestation → less transpiration → drier local climate.
  • Impervious surfaces (pavement) → more runoff, less infiltration → flooding & polluted stormwater.
  • Aquifer depletion (Ogallala, California Central Valley).
  • Damming alters flow, sediment, fish migration.

Example FRQs

FRQ Identify the role of denitrifying bacteria in the nitrogen cycle.

Answer: Denitrifying bacteria convert nitrate (NO₃⁻) in soil into atmospheric N₂ gas, returning nitrogen to the atmosphere. The reaction occurs in anaerobic conditions — waterlogged soils, sediments, or wetlands.

FRQ Describe TWO ways humans disrupt the carbon cycle.

Answer: (1) Burning fossil fuels (coal, oil, natural gas) releases CO₂ that has been stored underground for millions of years, raising atmospheric CO₂ concentrations and intensifying the greenhouse effect. (2) Deforestation reduces the biotic carbon sink — fewer trees mean less CO₂ removed via photosynthesis. When forests are burned to clear land, the stored carbon in biomass is also released back to the atmosphere as CO₂.

FRQ Why is phosphorus often a limiting nutrient in freshwater ecosystems?

Answer: Phosphorus has no atmospheric (gaseous) form, so it cycles slowly through weathering of rock into soil and water. Its naturally low concentrations are low relative to plant demand, so even modest additions of P (from fertilizer runoff, detergents, or sewage) cause rapid algal blooms. Nitrogen tends to be more readily replaced via N-fixing bacteria, so freshwater is usually phosphorus-limited.

Long FRQ Explain how excess nitrogen fertilizer in the US Midwest causes a Gulf of Mexico dead zone.

Answer: Excess synthetic-fertilizer nitrogen runs off Midwestern croplands into the Mississippi River system, which empties into the Gulf of Mexico. The added nitrogen (often with phosphorus) acts as a fertilizer for marine algae, triggering a massive algal bloom. When the algae die, decomposer bacteria break them down, consuming dissolved oxygen at a high rate (BOD spikes; DO drops to hypoxic levels < 2 mg/L). Aerobic organisms (fish, shrimp, crabs) die or flee, leaving a hypoxic dead zone that can exceed 20,000 km² in summer.

Drill flashcards