Unit 5: Biogeochemical Cycles and Nutrient Cycling

Biogeochemical Cycles

Introduction

Definition: A biogeochemical cycle is the pathway by which a chemical substance (element or molecule) moves through the biotic (living; bio-) and abiotic (non-living; geo-) compartments of Earth.

In contrast to the one-way flow of energy, nutrients and matter are cycled and conserved within the biosphere.

Types of Cycles:

1. Gaseous Cycles:
   • The main reservoir is the atmosphere or hydrosphere.
   • Cycles are relatively fast.
   • Examples: Carbon, Nitrogen, Water (Hydrological).
2. Sedimentary Cycles:
   • The main reservoir is the Earth's crust (soil and rocks).
   • Cycles are very slow, often relying on geological processes like weathering.
   • Examples: Phosphorus, Sulphur.

Carbon Cycle

Carbon is the backbone of all organic molecules.

• Reservoirs:
  1. Atmosphere: As CO₂ (Carbon Dioxide).
  2. Ocean: As dissolved CO₂ and bicarbonates (HCO₃^-). This is the largest active reservoir.
  3. Lithosphere (Land): In organic matter (living and dead), soil, and sedimentary rocks (limestone - CaCO₃).
  4. Fossil Fuels: Coal, oil, and natural gas (carbon stored from ancient dead organisms).
• Key Processes (Fluxes):
  • Photosynthesis: Plants and algae pull CO₂ from the atmosphere/water and convert it into organic carbon (glucose).
    (CO₂ → Organic C)
  • Respiration: Organisms (plants, animals, decomposers) break down organic carbon to get energy, releasing CO₂ back into the atmosphere.
    (Organic C → CO₂)
  • Decomposition: Decomposers (bacteria, fungi) respire as they break down dead organic matter, releasing CO₂.
  • Combustion: Burning of fossil fuels and biomass (wood) by humans rapidly releases vast amounts of stored carbon into the atmosphere as CO₂, causing global warming and climate change.
  • Ocean-Atmosphere Exchange: CO₂ dissolves into and is released from the oceans.

Nitrogen Cycle

Nitrogen is essential for proteins and nucleic acids (DNA, RNA). The atmosphere is ~78% N₂ gas, but this form is unusable by most organisms.

The nitrogen cycle is almost entirely driven by microorganisms (bacteria).

Key Processes (Fluxes):

1. Nitrogen Fixation: The conversion of N₂ gas into a usable form, ammonia (NH₃) or ammonium (NH₄⁺).
   • Biological: By nitrogen-fixing bacteria (e.g., Rhizobium in legume root nodules, Azotobacter in soil, cyanobacteria in water).
   • Atmospheric: Lightning has enough energy to split N₂.
   • Industrial: Haber-Bosch process for making fertilizers.
   (N₂ → NH₃ / NH₄⁺)
2. Nitrification: A two-step process by nitrifying bacteria.
   1. Nitrosomonas bacteria convert ammonium to nitrite (NO₂^-).
   2. Nitrobacter bacteria convert nitrite to nitrate (NO₃^-).
   (NH₄⁺ → NO₂^- → NO₃^-)
   Plants can readily absorb both NH₄⁺ and NO₃^- (preferred).
3. Assimilation: Plants absorb nitrate or ammonium from the soil and incorporate it into their tissues (proteins, DNA). Animals get nitrogen by eating plants.
4. Ammonification (Mineralization): Decomposers (bacteria, fungi) break down dead organic matter and waste products (urea, feces), converting organic nitrogen back into ammonium (NH₄⁺).
   (Organic N → NH₄⁺)
5. Denitrification: Denitrifying bacteria (e.g., Pseudomonas) convert nitrate back into N₂ gas, which returns to the atmosphere. This occurs in anaerobic (low-oxygen) conditions, such as waterlogged soils.
   (NO₃^- → N₂)

Phosphorus Cycle

Phosphorus is essential for ATP (energy), DNA, RNA, and cell membranes. This is a sedimentary cycle with no major atmospheric (gas) phase.

• Reservoir: The largest reservoir is phosphate rock (lithosphere).
• Key Processes (Fluxes):
  • Weathering: Rain and erosion slowly break down phosphate rocks, releasing phosphate (PO₄^3-) into the soil and water. This is a very slow process, making phosphorus a common limiting nutrient.
  • Assimilation: Plants absorb dissolved phosphate from the soil. Animals get it by eating plants.
  • Decomposition (Mineralization): Decomposers break down dead organic matter, returning phosphate to the soil.
  • Sedimentation: Phosphates in water can settle to the bottom of lakes and oceans, forming new sedimentary rock over geological time. This removes it from the cycle for a long time.

Sulphur Cycle

Sulphur is essential for some amino acids (e.g., cysteine, methionine) and proteins.

• Reservoirs: Rocks (gypsum - CaSO₄), fossil fuels, and the atmosphere (as SO₂ - sulfur dioxide).
• Key Processes (Fluxes):
  • Weathering: Rocks release sulfates (SO₄^2-), which are absorbed by plants.
  • Decomposition: Decomposers release hydrogen sulfide (H₂S - rotten egg smell) under anaerobic conditions.
  • Volcanic Eruption: Volcanoes release SO₂ into the atmosphere.
  • Combustion: Burning high-sulfur coal and oil releases large amounts of SO₂.
  • Atmospheric Processes: SO₂ in the atmosphere reacts with water (H₂O) to form sulfuric acid (H₂SO₄), which falls as acid rain.

Hydrological (Water) Cycle

The cycle of water (H₂O) through the biosphere, powered by solar energy.

• Evaporation: Solar energy heats liquid water in oceans, lakes, and rivers, turning it into water vapor (gas).
• Transpiration: The release of water vapor from plants through their leaves. (Often combined with evaporation as Evapotranspiration).
• Condensation: As warm, moist air rises, it cools, and the water vapor condenses into tiny droplets, forming clouds.
• Precipitation: When the water droplets in clouds become too heavy, they fall as rain, snow, sleet, or hail.
• Collection: Water that falls on land either:
  • Surface Runoff: Flows over the land into streams, rivers, and eventually the ocean.
  • Infiltration/Percolation: Seeps into the ground to become groundwater.

Nutrient Cycling

This refers to the movement and exchange of nutrients (like N and P) within an ecosystem, primarily between the soil, plants, and decomposers. (This is a smaller-scale loop within the larger biogeochemical cycles).

Nutrient Uptake

Also known as Assimilation. This is the process where plants absorb inorganic nutrients from the soil through their roots.

• Nutrients must be in a dissolved, inorganic form (e.g., nitrate NO₃^-, ammonium NH₄⁺, phosphate PO₄^3-).
• The plant then incorporates these into its own organic molecules (e.g., proteins, DNA).

Nutrient Immobilization

Definition: The process where inorganic nutrients are taken up by organisms (plants or microbes) and converted into organic forms (their biomass).

When a nutrient is "immobilized," it is locked up in living or dead organic matter and is temporarily unavailable for other organisms to use.

• Example: A nitrogen atom in a protein molecule inside a plant leaf is immobilized. A decomposer microbe using nitrate for its own growth also immobilizes it.

Nutrient Mineralization

Definition: The process (a part of decomposition) where decomposers (bacteria, fungi) break down organic matter and release nutrients back into the soil in an inorganic, plant-available form.

This is the opposite of immobilization.

• Example: A decomposer breaks down a dead leaf, releasing the nitrogen from its proteins as ammonium (NH₄⁺). This process is also called ammonification.

Nutrient Supply to Roots

Plants can't "go and get" nutrients. Nutrients must get to the plant's roots. This happens in three main ways:

1. Mass Flow (or Bulk Flow):
   • This is the most important mechanism for many nutrients (like nitrate).
   • As plants absorb water through their roots (transpiration), a "flow" of water is created in the soil, which carries dissolved nutrients along with it to the root surface.
2. Diffusion:
   • This is important for nutrients like phosphate and potassium.
   • Nutrients move from an area of high concentration (in the soil) to an area of low concentration (at the root surface, where the plant has absorbed them).
   • This is a very slow process and only works over short distances.
3. Root Interception:
   • As a root grows through the soil, it physically "bumps into" and comes into direct contact with soil particles (like clay) that have nutrients adsorbed to their surface.
   • This is generally a minor pathway compared to mass flow and diffusion.

Mycorrhizae: Many plants have a mutualistic relationship with mycorrhizal fungi. The fungi's thin hyphae extend far into the soil, acting as an extension of the plant's root system. They are extremely efficient at absorbing nutrients (especially phosphorus) and water and delivering them to the plant in exchange for sugars.