# The Nitrogen Cycle ## Introduction to Nutrient Cycling - In natural ecosystems, **nutrients** (such as nitrogen and phosphorus) are **recycled** through food webs - Nutrients pass from producers to primary consumers during feeding, then from primary consumers to secondary consumers - **Microorganisms** play a critical role in ensuring nutrients stored in dead organisms and waste products are recycled and made available to producers again - This recycling process is known as **decomposition**

Decomposition is the breakdown of dead organic matter and waste products by microorganisms, releasing nutrients back into the ecosystem

## Saprobionts: Nature's Recyclers - True decomposers are called **saprobionts** - Saprobionts mainly come from the **fungi** and **bacteria** kingdoms - Saprobionts **secrete enzymes** onto their food (dead organisms and waste material), which digest the material externally - The products of this external digestion are then **absorbed** by the saprobionts

Saprobiotic nutrition is the method of obtaining nutrients from dead or waste organic matter via extracellular digestion

### How Saprobionts Work - Saprobionts secrete a wide range of digestive enzymes that hydrolyse (break down) many different biological molecules - This releases a large range of products (nutrients) - Important mineral ions such as **ammonium** and **phosphate ions** are released during saprobiotic nutrition - These ions are crucial for the growth and function of producers (like plants)

Not all products of extracellular digestion are absorbed by saprobionts; many remain in the environment and are available to other organisms

### Why Saprobionts Are Essential - Without saprobionts, nutrients locked in dead and waste matter would never be made available again - Producers such as plants would not have access to sufficient nutrients - Some saprobionts even excrete important nutrient mineral ions as waste products from their own metabolism - Think of saprobionts as nature's waste management system - they prevent organic matter from piling up endlessly ## Mycorrhizae: Plant-Fungal Partnerships - Many plants have evolved **symbiotic** (mutually beneficial) relationships with fungi - The fungi are composed of long, thin filaments called **hyphae**, which interact with plant roots - These hyphae greatly increase the **surface area** of root systems - This increases the amount of **water** and **mineral ions** (like nitrates and phosphates) that plants can absorb

Mycorrhizae (singular: mycorrhiza) are mutual, symbiotic relationships between fungi and plant roots

### Benefits for Both Partners - **Plants** receive: - Increased water absorption - Enhanced mineral ion uptake (especially nitrates and phosphates) - **Fungi** receive: - Organic compounds (like glucose) from the plant - This is a win-win situation where both organisms benefit from the partnership - Consider it like having an extended plumbing network that dramatically expands water and nutrient access ## The Nitrogen Cycle: Converting the Unreactive - Nitrogen is the most abundant gas in the atmosphere (approximately 78%) - Despite its abundance, nitrogen gas ($N_2$) is relatively **inert** (unreactive) - Nitrogen plays a vital role in biology because it can be converted (fixed) into biomass - Bacteria and other microorganisms play a **vital role** in the nitrogen cycle ### Key Nitrogen-Related Ions | Ion | Formula | Importance | |-----|---------|------------| | Ammonium | $NH_4^+$ | First product of nitrogen fixation; used by plants | | Nitrite | $NO_2^-$ | Intermediate product in nitrification | | Nitrate | $NO_3^-$ | Highly soluble; readily absorbed by plant roots | ## Nitrogen Cycle Processes ### 1. Nitrogen Fixation - **Nitrogen-fixing bacteria** (like *Azotobacter* and *Rhizobium* species) convert gaseous nitrogen into ammonium ions ($NH_4^+$) - *Azotobacter* species live free in the soil - *Rhizobium* species live in root nodules of leguminous plants (like peas and beans)

Nitrogen fixation is the conversion of atmospheric nitrogen gas into nitrogen-containing compounds that plants can use

### 2. Nitrification - Ammonium ions are converted by **nitrifying bacteria** into nitrites and then nitrates - This is a two-step process: 1. **Step 1**: *Nitrosomonas* bacteria convert ammonium ($NH_4^+$) into nitrite ($NO_2^-$) 2. **Step 2**: *Nitrobacter* bacteria convert nitrite ($NO_2^-$) into nitrate ($NO_3^-$) - Nitrates are highly **soluble** and can be readily absorbed by plant roots - Imagine this as a two-stage purification process that makes nitrogen increasingly accessible to plants

Nitrification requires oxygen; these bacteria are aerobic and work best in well-aerated soils

### 3. Ammonification - Saprobionts decompose waste and dead matter via **extracellular digestion** - This releases inorganic mineral ions, including ammonium ions - Nitrogen compounds in waste (like urea) and dead matter (like proteins) are converted into ammonia - Ammonia then forms **ammonium ions** in soil

Ammonification is the conversion of nitrogen compounds in waste and dead matter into ammonia, which then forms ammonium ions in soil

### 4. Denitrification - **Denitrifying bacteria** (like *Thiobacillus denitrificans*) use nitrates for respiration - This returns gaseous nitrogen to the atmosphere - Denitrifying bacteria are **anaerobic** and live in waterlogged or compacted soils - This process essentially reverses nitrogen fixation

Denitrification reduces soil fertility by removing nitrates; farmers can reduce this by improving soil drainage and aeration

## Roles of Microorganisms in Recycling Minerals | Microorganism | Role in Recycling Minerals | |---------------|----------------------------| | **Saprobionts** | • Decompose waste and dead matter via extracellular digestion, making inorganic (mineral) ions available to other organisms
• Carry out **ammonification**: convert nitrogen compounds in waste and dead matter into ammonia, which then forms ammonium ions in soil | | **Nitrogen-fixing bacteria** | • Convert atmospheric nitrogen gas into nitrogen-containing compounds, such as ammonia, which then forms ammonium ions in soil that can be used by plants | | **Nitrifying bacteria** | • Convert ammonium ions in soil into nitrogen compounds that can be used by plants, known as **nitrates**
• Some nitrifying bacteria convert ammonium ions into **nitrites**; different nitrifying bacteria then convert nitrites into **nitrates** | | **Denitrifying bacteria** | • Use **nitrates** during respiration, releasing nitrogen gas in the process | | **Mycorrhizal fungi** | • Increase surface area of root systems, helping plants to absorb water and scarce mineral ions from soil | ## The Nitrogen Cycle in Context ### Uptake and Assimilation by Plants - Plants absorb nitrate ions from the soil through their roots - Nitrates are used to synthesize amino acids, which are then used to make proteins - Plants also use nitrates to make nucleotides for DNA and RNA - Animals obtain nitrogen by eating plants (or other animals) ### Death and Decomposition - When organisms die, saprobionts break down their remains - Proteins and nucleic acids are decomposed, releasing nitrogen compounds - These compounds undergo ammonification to produce ammonium ions - The cycle continues as these ions are nitrified back to nitrates ### Lightning - High-energy lightning strikes can fix atmospheric nitrogen - This converts $N_2$ directly into nitrogen oxides - These oxides dissolve in rainwater and enter the soil as nitrates - However, this contributes a relatively small amount compared to biological nitrogen fixation ## Summary of the Nitrogen Cycle Flow ```text Atmospheric Nitrogen (N₂) ↓ (nitrogen fixation by bacteria) Ammonium ions (NH₄⁺) ↓ (nitrification by Nitrosomonas) Nitrites (NO₂⁻) ↓ (nitrification by Nitrobacter) Nitrates (NO₃⁻) ↓ (uptake by plants) Plant proteins ↓ (feeding) Animal proteins ↓ (death and waste) Dead organic matter ↓ (decomposition and ammonification) Ammonium ions (NH₄⁺) [Denitrification returns N₂ to atmosphere from nitrates] ```

The nitrogen cycle is essentially a continuous loop with nitrogen constantly changing forms between atmosphere, soil, and living organisms

## Key Species to Remember | Bacterial Species | Function | |-------------------|----------| | *Azotobacter* | Free-living nitrogen-fixing bacteria in soil | | *Rhizobium* | Nitrogen-fixing bacteria in legume root nodules | | *Nitrosomonas* | Nitrifying bacteria: $NH_4^+ \rightarrow NO_2^-$ | | *Nitrobacter* | Nitrifying bacteria: $NO_2^- \rightarrow NO_3^-$ | | *Thiobacillus denitrificans* | Denitrifying bacteria: $NO_3^- \rightarrow N_2$ |