Welcome to Nutrient Cycling: Keeping Nature in Balance
Welcome to one of the most vital chapters in A2 Biology! In everyday life, we are constantly reminded to recycle plastics, glass, and paper. Nature, however, has been running the ultimate recycling system for billions of years.
While energy flows in one single direction through an ecosystem and is eventually lost into space as heat, nutrients and chemical elements on Earth are finite. There is no continuous delivery of new carbon or nitrogen atoms from space! Therefore, every single atom of carbon, nitrogen, and oxygen inside your body today has been recycled millions of times through living organisms (the biotic environment) and non-living reservoirs like the air, soil, and oceans (the abiotic environment).
Don't worry if this chapter feels like a lot of chemistry and Latin names at first. We will break every cycle down step-by-step with simple analogies, clear memory tricks, and direct exam guidance!
---1. Core Principles: Energy Flow vs. Nutrient Cycling
Before diving into the detailed cycles, let's understand the golden rule of ecosystems:
• Energy Flow is Unidirectional: Light energy from the Sun enters ecosystems via producers (photosynthesis), passes along food chains, and is continuously lost as metabolic heat. It cannot be recycled.
• Nutrients are Finite and Recycled: Chemical elements like Carbon (\(C\)) and Nitrogen (\(N\)) move through cyclical pathways known as biogeochemical cycles, alternating between organic forms (in living tissue) and inorganic forms (in the air, soil, and water).
The Unsung Heroes: Decomposers and Saprobionts
What happens when plants shed leaves or animals produce waste and die? This is where saprobionts (specialised fungi and bacteria) take center stage through a process called saprobiotic nutrition:
1. Saprobionts secrete extracellular digestive enzymes directly onto dead organic matter (detritus, urea, faeces).
2. These enzymes break down large, insoluble organic polymers (such as proteins and complex carbohydrates) into smaller, soluble inorganic molecules outside the microbial cells.
3. The saprobionts absorb the nutrients they need to survive, releasing remaining inorganic mineral ions back into the soil and atmosphere for plants to absorb again.
Key Takeaway: Without saprobionts, essential nutrients would stay locked away in dead biomass, and primary productivity would grind to a halt!
---2. The Nitrogen Cycle
Why do living things need nitrogen? Nitrogen is a fundamental building block for vital biological molecules: amino acids (and therefore proteins), nucleic acids (DNA and RNA), ATP, and chlorophyll.
The Big Problem: About 78% of the Earth's atmosphere is nitrogen gas (\(N_2\)). However, nitrogen gas contains an extremely strong triple covalent bond (\(N \equiv N\)). Plants and animals cannot break this bond directly, meaning atmospheric nitrogen is chemically unreactive and unavailable to them.
To enter biological systems, nitrogen must be converted into soluble forms that plants can take up through their roots via active transport—chiefly nitrate ions (\(NO_3^-\)) and ammonium ions (\(NH_4^+\)).
The nitrogen cycle consists of four main microbiological stages:
Stage 1: Nitrogen Fixation
Nitrogen fixation is the process of converting inert atmospheric nitrogen gas (\(N_2\)) into usable nitrogen-containing compounds (such as ammonium ions, \(NH_4^+\), or amino acids).
This occurs in three main ways:
• Free-living Nitrogen-Fixing Bacteria (e.g., Azotobacter): These are aerobic bacteria living freely in the soil. They fix \(N_2\) into ammonium ions, which they use to manufacture their own amino acids. When they die, these amino acids are released into the soil.
• Mutualistic / Symbiotic Nitrogen-Fixing Bacteria (e.g., Rhizobium): These bacteria live inside specialized root nodules of leguminous plants (such as peas, beans, clover, and alfalfa). This is a classic mutualistic (symbiotic) relationship:
- The bacteria provide the legume with fixed nitrogen (in the form of ammonium/amino acids).
- The host plant supplies the bacteria with carbohydrates (like sucrose and glucose from photosynthesis) and provides an oxygen-regulated microenvironment maintained by the pigment leghaemoglobin.
• Non-Biological Fixation: Lightning has enough electrical energy to force nitrogen and oxygen to react, forming nitrates (\(NO_3^-\)) that wash into soil with rain. Humans also fix nitrogen industrially using the Haber process to make artificial fertilisers.
Stage 2: Ammonification (Putrefaction)
When plants and animals die, or produce nitrogenous waste products (such as urea and faeces), their nitrogen is locked up in organic forms like proteins and nucleic acids.
Saprobiotic fungi and bacteria decompose this nitrogen-rich dead organic matter. They secrete extracellular enzymes to digest the proteins into amino acids and break them down further, releasing ammonia (\(NH_3\)), which readily forms ammonium ions (\(NH_4^+\)) in the soil solution.
Stage 3: Nitrification
Nitrification is an aerobic, two-step oxidation reaction carried out by chemoautotrophic nitrifying bacteria in well-aerated soil. Because it is an oxidation process, it requires ample oxygen (\(O_2\)):
Step 1: Ammonium ions (\(NH_4^+\)) are oxidized into nitrite ions (\(NO_2^-\)) by the bacterium Nitrosomonas:
\(NH_4^+ \xrightarrow{\textit{Nitrosomonas}} NO_2^-\)
Step 2: Nitrite ions (\(NO_2^-\)) are oxidized into nitrate ions (\(NO_3^-\)) by the bacterium Nitrosobacter:
\(NO_2^- \xrightarrow{\textit{Nitrosobacter}} NO_3^-\)
Plant Assimilation: Plants absorb these soluble nitrate ions (\(NO_3^-\)) from the soil water via active transport in their root hair cells. Once inside the plant, nitrates are assimilated to build amino acids, proteins, and nucleic acids.
Memory Trick: Alphabetical order helps you remember the bacteria and stages!
Ammonium (\(NH_4^+\)) \(\to\) Nitrite (\(NO_2^-\)) by Nitrosomonas (comes first alphabetically).
Nitrite (\(NO_2^-\)) \(\to\) Nitrate (\(NO_3^-\)) by Nitrobacter (comes second).
Stage 4: Denitrification
Denitrification is the opposite of fixation: it converts soil nitrates (\(NO_3^-\)) back into atmospheric nitrogen gas (\(N_2\)).
• Organisms involved: Anaerobic denitrifying bacteria (e.g., Pseudomonas species).
• Conditions required: Anaerobic conditions (low or zero oxygen), such as waterlogged, compacted, or poorly aerated soils.
• Consequence: Denitrification reduces soil fertility because it depletes the pool of usable nitrates available for crop growth.
Summary of the 4 Nitrogen Cycle Stages
1. Nitrogen Fixation: \(N_2 \to NH_4^+\) or amino acids (by Azotobacter, Rhizobium, lightning).
2. Ammonification: Organic nitrogen (proteins, DNA, urea) \(\to NH_4^+\) (by saprobionts).
3. Nitrification: \(NH_4^+ \to NO_2^-\) (by Nitrosomonas) \(\to NO_3^-\) (by Nitrobacter) [AEROBIC].
4. Denitrification: \(NO_3^- \to N_2\) (by Pseudomonas) [ANAEROBIC].
3. The Carbon Cycle
Carbon is the core structural element of all organic macromolecules: carbohydrates, lipids, proteins, and nucleic acids. The carbon cycle tracks the flow of carbon dioxide (\(CO_2\)) between the atmosphere/hydrosphere and living organisms.
Key Processes in the Carbon Cycle:
• 1. Photosynthesis (Carbon Fixation): Autotrophs (green plants, algae, and phytoplankton) absorb atmospheric \(CO_2\) (or dissolved hydrogen carbonate in aquatic ecosystems) and reduce it into organic carbohydrates (such as glucose, starch, and cellulose) using light energy.
• 2. Respiration: All living organisms—producers, consumers, and saprobiontic decomposers—carry out cellular respiration. Aerobic and anaerobic respiration break down organic carbon compounds and release \(CO_2\) back into the atmosphere or water.
• 3. Feeding (Ingestion and Assimilation): Carbon moves along food webs when herbivores consume producers, and carnivores consume herbivores, assimilating plant biomass into animal biomass.
• 4. Fossilisation: In certain environments (such as waterlogged peat bogs, swamps, and deep marine sediments), conditions are extremely acidic or anaerobic. This prevents decomposers from breaking down dead organic material completely. Over millions of years, heat and immense pressure convert this undecomposed organic matter into fossil fuels (coal, crude oil, natural gas, peat) or sedimentary rock like limestone (\(CaCO_3\)).
• 5. Combustion: The burning of biomass (wood) and fossil fuels releases stored carbon rapidly back into the atmosphere in the form of \(CO_2\).
Key Takeaway: Atmospheric \(CO_2\) levels represent a balance between processes that remove \(CO_2\) (photosynthesis) and processes that release \(CO_2\) (respiration and combustion).
---4. Human Impacts on Nutrient Cycles & Agricultural Practices
Modern farming practices intentionally manipulate nutrient cycles to maximize crop yield, but improper management can lead to severe environmental damage.
Agricultural Practices That Benefit Soil Fertility
• Ploughing and Aerating Soil: Breaking up compacted soil introduces oxygen (\(O_2\)) into the soil pore spaces. This creates ideal aerobic conditions for nitrifying bacteria (Nitrosomonas and Nitrobacter) and free-living nitrogen fixers (Azotobacter), while suppressing anaerobic denitrifying bacteria (Pseudomonas).
• Installing Drainage Systems: Agricultural drainage prevents fields from becoming waterlogged. This stops anaerobic pockets from forming, thereby preventing rapid denitrification and root rot.
• Crop Rotation with Legumes: Periodically planting leguminous crops (like clover or peas) allows symbiotic Rhizobium bacteria to naturally replenish the soil's nitrogen reserves, reducing the need for costly synthetic chemical fertilisers.
The Problem: Excessive Fertilisers, Leaching, and Eutrophication
To boost plant growth, farmers apply artificial inorganic fertilisers containing soluble mineral ions (NPK: Nitrogen, Phosphorus, Potassium). However, nitrate ions are highly soluble in water and do not bind tightly to soil particles.
• Leaching: When heavy rainfall occurs, excess soluble nitrates and phosphates wash out of the soil and drain into nearby freshwater streams, rivers, and lakes.
• Eutrophication: An ecological crisis triggered by nutrient enrichment. Here is the step-by-step sequence tested in A2 exams:
Step 1: High Nutrient Influx: Leached nitrates and phosphates enter the aquatic ecosystem, causing rapid growth and reproduction of algae at the water surface.
Step 2: Algal Bloom: A dense layer of algae forms over the surface of the water, blocking sunlight from penetrating deeper layers.
Step 3: Death of Submerged Plants: Submerged aquatic plants can no longer absorb light for photosynthesis, so they die.
Step 4: Exponential Growth of Saprobionts: Decomposers (saprobiotic bacteria) feed on the abundant dead plant and algal tissues, multiplying rapidly.
Step 5: Increased Biochemical Oxygen Demand (BOD): The huge population of saprobionts respires aerobically, consuming large quantities of dissolved oxygen from the water.
Step 6: Anoxia and Death of Aquatic Animals: The water becomes severely deoxygenated (anoxic). Fish, invertebrates, and other aerobic organisms die due to suffocation.
5. Common Exam Pitfalls & Misconceptions
Avoid these frequent mistakes in your A2 1 examination:
• Pitfall 1: Confusing Nitrogen Fixation with Nitrification.
Incorrect: "Nitrifying bacteria convert atmospheric nitrogen into nitrates."
Correct: Nitrogen-fixing bacteria convert \(N_2\) gas into ammonium/organic nitrogen. Nitrifying bacteria oxidize ammonium ions into nitrites, and then into nitrates.
• Pitfall 2: Confusing Aerobic and Anaerobic Stages.
Incorrect: "Denitrification happens when soil is well aerated."
Correct: Nitrification requires oxygen (aerobic). Denitrification requires low/no oxygen (anaerobic, e.g., waterlogged soil).
• Pitfall 3: Misunderstanding the Cause of Death in Eutrophication.
Incorrect: "The algae use up all the oxygen and kill the fish directly."
Correct: Algal blooms block light, killing submerged plants. It is the aerobic respiration of multiplying saprobiontic bacteria decomposing the dead biomass that depletes the dissolved oxygen!
• Pitfall 4: How Plants Absorb Nitrogen.
Incorrect: "Plants take in nitrogen gas directly through their stomata."
Correct: Plants absorb nitrogen exclusively through their roots from the soil solution as soluble ions (mainly \(NO_3^-\) and \(NH_4^+\)) via active transport.
Quick Revision Checklist
Can you confidently explain each of these key terms and processes?
• The difference between unidirectional energy flow and circular nutrient cycling.
• The role of saprobionts and extracellular enzyme secretion.
• Why atmospheric \(N_2\) is unavailable to most living organisms (triple covalent bond \(N \equiv N\)).
• The roles and conditions for Azotobacter, Rhizobium, Nitrosomonas, Nitrobacter, and Pseudomonas.
• The role of leghaemoglobin in root nodules.
• The main steps of the carbon cycle: photosynthesis, respiration, feeding, fossilisation, and combustion.
• Why farmers plough and drain soils.
• The 6-step sequence of eutrophication and the concept of Biochemical Oxygen Demand (BOD).