Nutrient Cycles

Nutrients are constantly recycled through ecosystems by the combined actions of living organisms and abiotic processes. The carbon and nitrogen cycles are two of the most important biogeochemical cycles studied at A-Level. Understanding these cycles — and the roles of microorganisms within them — is essential for topics on ecosystems, biodiversity, and the impact of human activity.

Key Definition: A nutrient cycle is the movement and exchange of organic and inorganic matter back into the production of living matter. Nutrients are recycled — they are not created or destroyed, but move between biotic and abiotic components of ecosystems.

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The Carbon Cycle

Carbon is the backbone of all organic molecules (carbohydrates, lipids, proteins, nucleic acids). The carbon cycle describes how carbon moves between the atmosphere, biosphere, hydrosphere, and lithosphere.

Key Processes in the Carbon Cycle

1. Photosynthesis

• Green plants, algae, and cyanobacteria absorb CO₂ from the atmosphere and use light energy to convert it into organic compounds (glucose and other carbohydrates).
• Overall equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
• This is the primary route by which inorganic carbon (CO₂) enters the biotic (living) component of the ecosystem.
• Carbon is incorporated into organic molecules and passed along food chains via feeding.

2. Respiration

• All living organisms (plants, animals, fungi, bacteria) carry out aerobic respiration, releasing CO₂ back into the atmosphere.
• Overall equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
• Respiration returns carbon from the biotic to the abiotic component.

3. Decomposition

• When organisms die, their organic remains are broken down by decomposers (saprobiont bacteria and fungi).
• Saprobionts secrete extracellular enzymes that digest organic matter outside the cell, then absorb the soluble products.
• Decomposers respire, releasing CO₂ into the atmosphere.
• The rate of decomposition depends on temperature, moisture, oxygen availability, and the nature of the organic material.

4. Combustion

• Burning of fossil fuels (coal, oil, natural gas) and biomass (wood, crop residues) releases CO₂ into the atmosphere.
• Fossil fuels are the remains of organisms that died millions of years ago; their carbon was locked away in sedimentary rocks and is now released by human activity.

5. Fossil Fuel Formation

• Over millions of years, the remains of dead organisms that were not fully decomposed (e.g., in anaerobic conditions at the bottom of oceans or in swamps) were compressed and heated, forming fossil fuels.
• This represents a long-term carbon sink — carbon removed from the cycle for geological timescales.

6. Dissolving in Oceans

• CO₂ dissolves in ocean water, forming carbonic acid (H₂CO₃), which dissociates into bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions.
• Marine organisms use carbonate to build calcium carbonate (CaCO₃) shells and skeletons. When these organisms die, their remains form limestone — another long-term carbon sink.
• The oceans are a major carbon reservoir, absorbing approximately 25% of atmospheric CO₂.

Human Impact on the Carbon Cycle

• Burning fossil fuels releases stored carbon, increasing atmospheric CO₂.
• Deforestation reduces photosynthesis (less CO₂ removed from the atmosphere) and burning/decomposition of felled trees releases CO₂.
• Increased atmospheric CO₂ enhances the greenhouse effect, contributing to global warming and climate change.
• CO₂ dissolving in oceans leads to ocean acidification, which threatens coral reefs and organisms with calcium carbonate shells.

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The Nitrogen Cycle

Nitrogen is essential for the synthesis of amino acids, proteins, nucleotides, and nucleic acids (DNA and RNA). Although nitrogen gas (N₂) makes up approximately 78% of the atmosphere, most organisms cannot use it directly because of the strong triple covalent bond (N≡N). The nitrogen cycle describes how nitrogen is converted between different forms and made available to living organisms.

Key Processes in the Nitrogen Cycle

1. Nitrogen Fixation

The conversion of atmospheric N₂ into a usable form (NH₃ or NH₄⁺ — ammonia or ammonium):

• Biological nitrogen fixation:

  • Carried out by nitrogen-fixing bacteria (diazotrophs).
  • Free-living nitrogen-fixing bacteria: e.g., Azotobacter in soil.
  • Mutualistic nitrogen-fixing bacteria: e.g., Rhizobium species that live in root nodules of leguminous plants (peas, beans, clover). The bacteria fix N₂ into NH₃ using the enzyme nitrogenase. In return, the plant provides the bacteria with organic molecules (sugars) as an energy source. The relationship is mutualistic — both organisms benefit.
  • The enzyme nitrogenase is inhibited by oxygen, so root nodules contain leghaemoglobin, which binds oxygen and maintains low O₂ conditions.

• Industrial nitrogen fixation:

  • The Haber process converts N₂ and H₂ into NH₃ under high temperature (450 °C) and pressure (200 atm) with an iron catalyst.
  • NH₃ is used to manufacture fertilisers (e.g., ammonium nitrate).

• Lightning fixation:

  • Lightning provides enough energy to combine N₂ with O₂ to form nitrogen oxides, which dissolve in rainwater and enter the soil as nitrate.

2. Ammonification (Mineralisation)