Welcome to Nutrient Cycles!
Hi there! This chapter might sound complex, but it’s actually about one of the most fundamental concepts in all of ecology: recycling. Think of the Earth as one giant organism—it can't just keep consuming new resources; it has to continuously reuse the elements it already has.
In these notes, we will explore how crucial elements like Carbon (C) and Nitrogen (N) move between the living (biotic) and non-living (abiotic) parts of an ecosystem. Understanding these cycles is vital because human activity has a huge impact on them, leading to major global issues like climate change and pollution.
Ready to see how nature recycles its most valuable ingredients? Let's dive in!
3.3.5 Principles of Nutrient Cycling
In any ecosystem, energy flows (usually starting from the sun) but chemical elements must be cycled. They are constantly being reused.
Key Roles of Chemical Elements
Different chemical elements are essential building blocks for life. For example:
- Carbon is needed for all organic substances (carbohydrates, lipids, proteins).
- Nitrogen is needed for proteins (amino acids) and nucleic acids (DNA/RNA).
- Phosphorus is needed for DNA, RNA, and ATP.
The Central Role of Decomposers
The entire nutrient cycle relies on microscopic organisms to unlock elements stored in dead biomass. Without them, nutrients would be permanently locked away, and life would stop!
Decomposers (mainly bacteria and fungi, known as saprophytic microorganisms) are organisms that break down dead organic matter and waste products (like faeces) back into simple inorganic forms.
Step-by-step Recycling:
- Organisms die, or produce waste. This contains complex organic molecules (e.g., proteins, cellulose).
- Decomposers secrete digestive enzymes onto the dead material. This is extracellular digestion.
- The large organic molecules are broken down into smaller, soluble inorganic molecules (like ions or simple molecules, e.g., nitrate ions, ammonium ions).
- These inorganic molecules/ions are released back into the soil, water, or atmosphere.
- Producers (plants) take up these simple inorganic molecules (e.g., absorbing nitrates through their roots).
- These elements are incorporated into new organic molecules in the producer.
- Substances containing these elements are then passed along food chains through digestion and assimilation when consumers eat the producers or other consumers.
Think of decomposers as the essential clean-up crew—they ensure that the ingredients for new life are never permanently lost.
Quick Review: Principles
Nutrients are recycled, starting when decomposers break down complex organic substances into inorganic ions, which are then absorbed by producers and moved along food chains.
3.3.5.2 The Carbon Cycle
Carbon is the backbone of life, moving mainly as carbon dioxide (\(CO_2\)) in the atmosphere and dissolved in oceans.
Key Biological Processes in the Carbon Cycle
- Photosynthesis: Producers (plants, algae) remove \(CO_2\) from the atmosphere to make organic compounds (glucose). This moves carbon from the abiotic environment into the biotic world.
- Respiration: All living organisms (plants, animals, microorganisms) release \(CO_2\) into the atmosphere when they break down organic molecules to release energy. This moves carbon from the biotic world back to the abiotic environment.
- Decomposition: When decomposers break down dead biomass, they respire, releasing \(CO_2\).
Fluctuations in Carbon Dioxide Concentration
Short-term Fluctuations (Daily/Seasonal)
These changes are mainly due to the balance between photosynthesis and respiration:
- During the day (or summer): Photosynthesis rates are high, so plants are taking in lots of \(CO_2\). Concentration tends to fall.
- During the night (or winter): Photosynthesis stops, but respiration continues. Plants and animals release \(CO_2\). Concentration tends to rise.
Long-term Changes (Global)
These are primarily driven by human activity:
- Burning Fossil Fuels: Fossil fuels (coal, oil, gas) store carbon that was locked away millions of years ago. Burning them releases huge amounts of \(CO_2\) rapidly into the atmosphere.
- Deforestation: Trees are carbon sinks (they store carbon). Cutting down and burning forests releases stored carbon as \(CO_2\), and reduces the amount of future photosynthesis that can happen.
The Greenhouse Effect and Climate Change
The syllabus requires you to know the roles of \(CO_2\) and Methane (\(CH_4\)) in enhancing the greenhouse effect.
- The Greenhouse Effect: This is a natural process where gases in the atmosphere (like water vapour, \(CO_2\), and \(CH_4\)) trap some of the heat radiated from the Earth's surface. This keeps the planet warm enough for life.
- Enhancing the Effect: When human activities increase the concentration of these gases, more heat is trapped, leading to global warming and subsequent climatic change.
Did you know? Methane is a far more potent greenhouse gas than \(CO_2\), though it lasts for a shorter time in the atmosphere. It is mainly released from agricultural sources (like livestock farming) and anaerobic decomposition (like landfill sites).
Impacts of Climate Change: Students should be able to evaluate data relating to effects like:
— Decreased yield of crop plants (due to extreme weather or drought).
— Changes in the life cycles and numbers of insect pests (warmer temperatures allow pests to survive better or reproduce faster, extending their range).
Key Takeaway: Carbon
Carbon cycles rapidly between the atmosphere and organisms via photosynthesis and respiration. Human burning of fossil fuels creates a massive long-term imbalance, leading to an enhanced greenhouse effect and climate change.
3.3.5.3 The Nitrogen Cycle
Nitrogen is essential for making proteins and nucleic acids, but atmospheric nitrogen gas (\(N_2\)) is very unreactive and cannot be used directly by most plants or animals.
The nitrogen cycle relies entirely on bacteria to convert nitrogen into usable forms (ions) through a series of four key processes.
Memory Aid for the Nitrogen Cycle Processes: N.A.N.D
Nitrogen fixation, Ammonification, Nitrification, Denitrification.
1. Nitrogen Fixation (N2 to Ammonia/Ammonium)
This is the process where nitrogen gas (\(N_2\)) from the atmosphere is converted into nitrogen-containing compounds, specifically ammonia (\(NH_3\)), which quickly becomes ammonium ions (\(NH_4^+\)) in the soil.
- Role of Bacteria: Performed by Nitrogen-fixing bacteria (e.g., Rhizobium, which live symbiotically in the root nodules of legumes, or free-living bacteria in the soil).
- Importance: This step makes atmospheric nitrogen accessible to living things.
2. Ammonification (Organic N to Ammonium)
This is the decomposition stage—releasing nitrogen stored in dead organisms and waste.
- Process: Decomposers (bacteria and fungi) break down organic nitrogen compounds (like proteins and urea) in dead matter and waste.
- Product: They release ammonium ions (\(NH_4^+\)) into the soil.
3. Nitrification (Ammonium to Nitrate)
Nitrification is a two-step process that converts the ammonium ions into the most easily absorbed form for plants: nitrate ions.
This process requires oxygen (it is aerobic).
Step 3a: Ammonium ions (\(NH_4^+\)) are converted to Nitrite ions (\(NO_2^-\)).
— Bacteria involved: Nitrosomonas.
Step 3b: Nitrite ions (\(NO_2^-\)) are converted to Nitrate ions (\(NO_3^-\)).
— Bacteria involved: Nitrobacter.
The nitrate ions (\(NO_3^-\)) are readily taken up by producers (plants) via active transport through their roots.
4. Denitrification (Nitrate to N2 Gas)
This process reverses the cycle, taking usable nitrogen and returning it to the atmosphere as unusable nitrogen gas.
- Process: Denitrifying bacteria use nitrate ions instead of oxygen for respiration (under anaerobic conditions).
- Condition: Occurs primarily in waterlogged soil where there is little oxygen.
- Result: Nitrates (\(NO_3^-\)) are converted back into nitrogen gas (\(N_2\)).
Quick Review: The Nitrogen Cycle
The cycle is dominated by bacteria. Plants absorb nitrogen mainly as nitrates (\(NO_3^-\)) which are produced by nitrification. Loss of nitrogen occurs through denitrification in waterlogged conditions.
Human Impact on Nutrient Cycles: Fertilisers and Pollution
Replacing Lost Nutrients
When we harvest crops or remove livestock, we are taking nutrients (like N and P) out of the ecosystem that would normally be recycled through decomposition. To ensure future plant growth, farmers must replace these lost nutrients using fertilisers.
- Natural Fertilisers: Include manure and compost. They release nutrients slowly as organic matter decomposes (ammonification).
- Artificial (Inorganic) Fertilisers: Synthesised chemicals (e.g., ammonium nitrate) containing high concentrations of soluble inorganic ions, such as nitrates. They provide nutrients immediately, boosting crop yield.
The Problem with Soluble Fertilisers
While effective, the excessive use of artificial fertilisers can lead to two major environmental problems:
1. Leaching
Leaching occurs because artificial fertilisers contain highly soluble inorganic ions, especially nitrates (\(NO_3^-\)).
If these ions are applied when plants are not growing fast (e.g., heavy rain after application), the water dissolves the ions, and they are carried down through the soil, washing them into surrounding rivers and lakes.
Consequence: This wastes the farmer's money and can contaminate drinking water (high nitrate levels are toxic to babies).
2. Eutrophication (The Result of Leaching)
Eutrophication is the process by which a body of water becomes overly enriched with nutrients, leading to devastating ecological effects.
Step-by-step Eutrophication:
- Nutrient Enrichment: Nitrates and phosphates leached from fields enter a river or lake.
- Algal Bloom: These nutrients cause an enormous, rapid growth of algae (an algal bloom) on the water's surface.
- Light Blockage: The dense surface layer of algae prevents sunlight from reaching the aquatic plants underneath. These plants die.
- Decomposition and Oxygen Depletion: The dead plants (and eventually the algae itself) are broken down by saprophytic bacteria (decomposers). These bacteria respire aerobically, consuming vast amounts of dissolved oxygen in the water.
- Death of Aquatic Life: The lack of oxygen leads to the death of other aerobic organisms (fish, invertebrates). The ecosystem is severely damaged.
Analogy: Imagine throwing too much food into a fish tank. The fish can't eat it all, the bacteria eat the excess, and their booming population suffocates the fish by stealing all the oxygen.
Final Key Takeaway
Human practices like harvesting remove nutrients. We replace them with fertilisers, but if overused, the soluble nutrients can be leached into waterways, causing eutrophication, where excessive growth and subsequent bacterial decomposition deplete oxygen, killing aquatic life.
Keep practising these cycles, focusing on the names of the processes and the specific role of the bacteria involved—you've got this!