Biology (0610) | Nutrient cycles

🌱 Nutrient Cycles: The Earth's Recycling System

Welcome to one of the most important topics in environmental biology! If you've ever wondered how plants keep getting the resources they need, or where all the carbon dioxide goes, this chapter explains it all.

Nutrient cycles show how essential chemical elements—like Carbon and Nitrogen—are continuously recycled between living things (organisms) and the non-living parts of the environment (air, soil, water).

Why is this important? Without these cycles, all life on Earth would stop! We need to understand them to manage our impact on the environment, especially concerning climate change and agriculture.

1. The Carbon Cycle (Core Content)

The Carbon Cycle describes the movement of carbon atoms (often in the form of carbon dioxide, CO₂) through the atmosphere, oceans, soil, and living things.

How Carbon Moves Around

The carbon cycle is often easiest to understand by tracking how CO₂ moves into and out of the atmosphere:

A. Moving Carbon OUT of the Atmosphere (Storage)

This is primarily done by producers (plants).

• Photosynthesis: Plants (producers) take in atmospheric carbon dioxide (\(CO_2\)) and use light energy to convert it into organic carbon compounds (like glucose/sugars). This stores carbon in the biomass of the plant.
• Formation of Fossil Fuels: This is long-term storage. When dead plants and animals are buried under intense pressure and heat over millions of years, the carbon in their bodies forms fossil fuels (coal, oil, gas).

B. Moving Carbon INTO the Atmosphere (Release)

This happens through several key processes:

• Respiration: All living organisms (plants, animals, and microorganisms) break down carbon-containing compounds (like glucose) to release energy. A waste product of this process is carbon dioxide, which is released back into the atmosphere.
• Feeding (Consumption): When an animal eats a plant (or another animal), the carbon stored in the food is transferred up the food chain. This carbon will eventually be released back into the environment through respiration or decomposition.
• Decomposition: When organisms die or produce waste (e.g., faeces), decomposers (like bacteria and fungi) break down the dead organic material. They use the carbon compounds for their own respiration, releasing CO₂ back into the air and soil.
• Combustion: This is the burning of materials. When wood (plant biomass) or fossil fuels are burned, the stored carbon is rapidly released back into the atmosphere as CO₂. This is the main way humans disrupt the cycle.

2. The Nitrogen Cycle (Extended/Supplement Content)

Nitrogen is crucial! It is needed to make amino acids (which build proteins) and nucleic acids (like DNA). Although 78% of the air is nitrogen gas (\(N_2\)), plants cannot absorb it in this form—it is inert (unreactive).

The Nitrogen Cycle is complex because nitrogen must be converted into soluble, reactive forms (like nitrates) before plants can use it.

Analogy: Imagine nitrogen gas is locked in a secure vault. Bacteria are the specialized workers that convert the unusable gas into soil forms, and then "unlock" the best form (nitrates) that plants can easily absorb.

The Key Chemical Conversions and Processes

The cycle relies heavily on specific bacteria living in the soil.

Step 1: Nitrogen Fixation

This process converts inert atmospheric nitrogen gas (\(N_2\)) into usable nitrogen compounds (ammonia/ammonium ions).

• Agents:
  1. Nitrogen-fixing bacteria: These bacteria live freely in the soil or in the root nodules of certain plants (like peas and beans).
  2. Lightning: High energy from lightning causes nitrogen and oxygen to react, forming nitrates that dissolve in rainwater and enter the soil.

Step 2: Decomposition (Ammonification)

This process handles nitrogen recycling from dead material.

• When plants and animals die, or animals excrete waste (containing urea or faeces), this organic material contains proteins and amino acids.
• Decomposition (or Ammonification) occurs: Decomposers (bacteria and fungi) break down the plant and animal protein to ammonium ions (\(NH_4^+\)).

Step 3: Nitrification (The Conversion Steps)

Ammonium ions are toxic to plants in high concentrations and are still not the preferred form. The process of Nitrification involves two stages, both carried out by different types of bacteria:

• Conversion 1: Ammonium ions are converted to Nitrite ions (\(NO_2^-\)).
• Conversion 2: Nitrite ions are converted to Nitrate ions (\(NO_3^-\)).
• Key point: Nitrates (\(NO_3^-\)) are the chemical form most easily absorbed by plants.

Step 4: Uptake and Assimilation

This is where life uses the nitrogen.

• Absorption: Plants take up nitrate ions from the soil through their roots (often using active transport, requiring energy).
• Production of Amino Acids and Proteins: Once inside the plant, the nitrates are used to synthesise (make) amino acids, which are then built into proteins.
• Feeding and Digestion of Proteins: Animals get their required nitrogen by feeding on plants or other animals. They digest the proteins into amino acids, and then assimilate (use) these amino acids to build their own proteins.

Step 5: Deamination and Denitrification (Removing Excess and Returning to Air)

Two processes remove nitrogen from the usable pool:

1. Deamination:

• If animals eat too much protein, they have excess amino acids.
• The liver performs Deamination: it removes the nitrogen-containing part of the amino acids to form urea (an excretory product).
• The remaining non-nitrogen part is converted into carbohydrate and used for respiration.

2. Denitrification:

• Denitrifying bacteria, usually found in waterlogged (anaerobic) soil, convert nitrates (\(NO_3^-\)) back into atmospheric nitrogen gas (\(N_2\)).
• This is the step that completes the cycle, returning the nitrogen to the air.

The Crucial Roles of Microorganisms (Bacteria and Fungi)

You must understand the role of microorganisms (bacteria and fungi) in enabling this cycle (Syllabus 19.3.3). They are the engine of the nitrogen cycle:

• Decomposition (Ammonification): Fungi and bacteria break down dead organic matter and waste, releasing ammonium ions.
• Nitrogen Fixation: Bacteria convert atmospheric nitrogen (\(N_2\)) into ammonium ions.
• Nitrification: Bacteria convert ammonium ions into nitrites, and then nitrites into the crucial nitrates.
• Denitrification: Bacteria convert nitrates back into atmospheric nitrogen gas (\(N_2\)).