2.3E Nitrogen cycle (HL) Notes

The Nitrogen Cycle: Organic and Inorganic Nitrogen Stores

1. The nitrogen cycle involves the movement of nitrogen through various forms and stores within ecosystems.
2. Nitrogen exists in both organic and inorganic stores, each playing a vital role in sustaining life on Earth.

1. Organic Nitrogen Stores

1. Organic nitrogen refers to nitrogen incorporated within living tissues or recently dead organic matter.
2. It forms the biologically active part of the nitrogen pool.
3. The stores include:
   1. Proteins and enzymes that regulate metabolic reactions in plants and animals.
   2. Nucleic acids (DNA and RNA) that encode genetic information.
   3. Chlorophyll, the nitrogen-containing pigment vital for photosynthesis.
   4. Humus and leaf litter in soils, which act as temporary nitrogen stores before decomposition.
4. When organisms die, their tissues become part of the detrital pool, where bacteria and fungi break down organic matter into ammonium compounds during decomposition.

2. Inorganic Nitrogen Stores

1. Inorganic nitrogen stores include nitrogen in the atmosphere, ammonia in the soil, and nitrate (NO₃⁻) and nitrite (NO₂⁻) compounds in soil and water.
2. These inorganic forms of nitrogen are important for nutrient cycling and plant growth.
3. These include:
   1. Nitrogen gas (N₂): constitutes ~78% of the atmosphere; biologically inert due to strong triple bonds.
   2. Ammonia (NH₃) and ammonium (NH₄⁺): produced by nitrogen fixation or decomposition.
   3. Nitrites (NO₂⁻) and nitrates (NO₃⁻): oxidized forms taken up by plants.
   4. Dissolved nitrogen compounds: in lakes, rivers, and oceans

Creating a Systems Diagram of the Nitrogen Cycle

1. Identify the Main Stores:
   1. Organic: Plants, animals, decomposers.
   2. Inorganic: Soil (ammonia, nitrites, nitrates), atmosphere (N₂).
2. Map the Flows:
   1. Nitrogen Fixation
   2. Nitrification
   3. Assimilation
   4. Ammonification
   5. Denitrification
3. Label Processes and Arrows:
   1. Use arrows to show the direction of flows.
   2. Label each process (e.g., nitrification, assimilation).

Bacteria’s Essential Roles in the Nitrogen Cycle

1. Bacteria are the biological engines of the nitrogen cycle.
2. They convert nitrogen between its different forms, making it accessible to living organisms.

1. Nitrogen Fixation

1. Nitrogen-fixing bacteria convert atmospheric nitrogen (N₂) into ammonia (NH₃), which dissolves in water to form ammonium ions (NH₄⁺) usable by plants.
2. Free-living bacteria (e.g., Azotobacter) fix nitrogen independently in the soil.
3. Symbiotic bacteria (e.g., Rhizobium) live in root nodules of legumes (peas, beans, clover).

2. Nitrification

1. Nitrifying bacteria convert ammonia (NH₃) into nitrite (NO₂⁻) and then into nitrate (NO₃⁻), which plants can absorb.
   1. Nitrosomonas bacteria convert ammonia into nitrites (NO₂⁻).
   2. Nitrobacter bacteria convert nitrites into nitrates, which plants can easily absorb.
2. This process requires oxygen and thus occurs in aerobic soils.
3. Nitrification helps transform ammonia into a usable form for plants.
4. Nitrates are a primary nitrogen source for plant growth.

3. Decomposition and Ammonification

1. When plants and animals die or produce waste, decomposer organisms (saprotrophic bacteria and fungi) break down proteins and amino acids into ammonia (NH₃) or ammonium ions (NH₄⁺).
2. This process recycles organic nitrogen back into the soil, making it available again for nitrification or plant uptake.

4. Denitrification

1. Denitrifying bacteria (e.g., Pseudomonas denitrificans) convert nitrates (NO₃⁻) back into nitrogen gas (N₂), completing the nitrogen cycle.
2. This occurs in anaerobic conditions, such as waterlogged soils, where oxygen is scarce.

Denitrification in Waterlogged, Anaerobic Soils

1. Denitrification primarily occurs in oxygen-poor environments, such as swamps, flooded fields, and compacted soils.
2. In these conditions, bacteria use nitrate ions (NO₃⁻) as an alternative to oxygen for respiration, releasing N₂ or N₂O gases.

Consequences for Ecosystems

1. Loss of soil fertility: Denitrification removes plant-available nitrates.
2. Reduced plant growth: Waterlogging prevents oxygen diffusion, stunting roots.
3. Leaching: Nitrates are washed out of the soil, contaminating groundwater.

How Does Denitrification Work?

1. Anaerobic Conditions: When soil becomes waterlogged, oxygen is depleted, creating anaerobic conditions.
2. Bacterial Activity: Denitrifying bacteria, such as Pseudomonas denitrificans, thrive in these conditions and use nitrates as an alternative to oxygen for respiration.
3. Conversion Process: Nitrates are reduced to nitrogen gas or nitrous oxide, which escapes into the atmosphere.

Mutualistic Nitrogen Fixation

1. Atmospheric nitrogen (N₂) is chemically stable and unavailable to plants.
2. To access nitrogen, certain plants form mutualistic relationships with nitrogen-fixing bacteria.
3. The bacteria convert atmospheric nitrogen (N₂) into ammonia (NH₃), which plants can use to grow.
4. In return, the plant provides carbohydrates and a protective environment for the bacteria inside its root nodules.

Why Do Plants Have Mutualistic Nitrogen Fixation?

1. Plants develop these relationships because most cannot absorb nitrogen directly from the atmosphere and must rely on nitrogen compounds in the soil.
2. In nutrient-poor environments, forming a mutualistic association gives plants a competitive advantage by ensuring a direct nitrogen supply.
3. They enrich soil nitrogen content after death and decomposition, benefiting nearby plants.

Benefits to Plants and Bacteria

1. Plants Benefit By:
   1. Accessing nitrogen in nitrogen-poor soils, allowing them to grow in environments where other plants may struggle.
   2. Reducing dependence on soil nitrogen, which may be limited or unavailable due to leaching or competition.
   3. Improving soil fertility, benefiting nearby plants over time (e.g., legumes in crop rotation enrich soil nitrogen for future crops).
2. Bacteria Benefit By:
   1. Receiving carbohydrates from the plant, which they use as an energy source.
   2. Living in a stable, protected environment within plant root nodules, shielding them from harsh soil conditions.

Flows in the Nitrogen Cycle: Transfers vs. Transformations

1. The nitrogen cycle consists of flows that move nitrogen between different stores.
2. These flows can be categorized into transfers (where nitrogen moves between locations without chemical changes) and transformations (where nitrogen changes form through biological or chemical processes).

Transfer Flows

1. These processes move nitrogen from one part of the ecosystem to another without altering its chemical form.
   1. Mineral uptake: Plants absorb nitrate (NO₃⁻) and ammonium (NH₄⁺) through their roots.
   2. Consumption: Nitrogen passes along the food chain as animals feed on plants or other animals.
   3. Excretion: Animals release nitrogen in waste products (urea, uric acid, or ammonia).
   4. Death and decomposition: Organic nitrogen from dead organisms is transferred to decomposers.

Transformation Flows

1. These involve biological or chemical reactions that change nitrogen from one form to another.
   1. Nitrogen fixation: Atmospheric N₂ → NH₃ or NH₄⁺ (biological or lightning).
   2. Nitrification: NH₄⁺ → NO₂⁻ → NO₃⁻ (by nitrifying bacteria).
   3. Assimilation: Plants incorporate NH₄⁺ and NO₃⁻ into amino acids and proteins.
   4. Ammonification (decomposition): Organic nitrogen → NH₄⁺.
   5. Denitrification: NO₃⁻ → N₂ (by anaerobic bacteria).
   6. Photosynthesis link: Provides organic energy (carbohydrates) that fuels nitrogen assimilation in producers.

Human Impacts on the Nitrogen Cycle

1. Human activities such as deforestation, agriculture, aquaculture, and urbanization have dramatically altered the global nitrogen cycle.
2. These actions disrupt natural nitrogen flows and stores, resulting in eutrophication, air pollution, and biodiversity loss.

1. Deforestation

1. Loss of nitrogen stores: Trees and plants act as nitrogen stores, absorbing nitrates from the soil. Deforestation removes these plants, reducing nitrogen uptake.
2. Increased leaching: Without plant roots to hold soil, nitrates (NO₃⁻) are easily washed away by rain, leading to soil nutrient depletion and water pollution.

2. Agriculture

1. Fertilizer Use: Synthetic nitrogen fertilizers increase soil nitrogen levels but can lead to runoff into rivers and lakes, causing eutrophication (excessive algae growth).
2. Livestock Farming: Cattle release large amounts of ammonia (NH₃) and methane (CH₄) through manure and digestion, contributing to nitrogen pollution.
3. Soil Disruption: Plowing and tilling increase nitrification and denitrification, accelerating nitrogen loss as nitrogen gas (N₂) and nitrous oxide (N₂O) escape into the atmosphere.

3. Aquaculture (Fish Farming)

1. Excess Nitrogen Waste: Fish farms produce large amounts of waste rich in ammonia (NH₃), leading to oxygen depletion in water bodies.
2. Algal Blooms: Increased nitrogen levels promote excessive phytoplankton growth, causing toxic algal blooms that harm marine life.

4. Urbanization

1. Sewage and Wastewater: Urban areas generate large amounts of untreated sewage, which releases nitrogen compounds into rivers and lakes.
2. Industrial Emissions: Factories and vehicles emit nitrogen oxides (NOₓ), contributing to acid rain, which damages soil and water ecosystems.
3. Impermeable Surfaces: Roads and buildings prevent nitrogen from cycling naturally in soil, increasing runoff and water pollution.

The Haber Process: Producing Ammonia for Fertilizers

1. It revolutionized global agriculture by providing a steady supply of nitrogen fertilizer, but it also contributes to environmental degradation.
2. This ammonia is then used to create fertilizers like ammonium nitrate and urea, which are essential for modern agriculture.

How the Haber Process Works

1. Nitrogen gas (N₂) from the atmosphere reacts with hydrogen gas (H₂) under high pressure (200-300 atm) and high temperature (400-500°C) in the presence of an iron catalyst.
2. This produces ammonia (NH₃), which is then used in fertilizers like ammonium nitrate (NH₄NO₃) and urea (CO(NH₂)₂).

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Advantages

1. Increases global food production by enhancing crop yields.
2. Enables agricultural independence, reducing reliance on natural nitrogen fixation.
3. Ammonia produced is also used for pharmaceuticals, explosives, and refrigeration.

Disadvantages

1. Energy intensive as it relies heavily on fossil fuels.
2. CO₂ emissions from hydrogen production contribute to climate change.
3. Nitrate leaching from over-fertilization leads to eutrophication.
4. Creates nitrogen imbalance in ecosystems.

Crossing the Planetary Boundary for the Nitrogen Cycle

1. The planetary boundary concept defines safe operating limits for Earth’s systems.
2. Human activities, particularly the excessive use of nitrogen fertilizers, have pushed the nitrogen cycle beyond its natural limits, causing severe environmental consequences.

Evidence of Boundary Crossing

1. The safe limit for anthropogenic nitrogen fixation is estimated at 62 Tg N/year.
2. Current global rate: >150 Tg N/year, mostly from fertilizer production and fossil fuel combustion.
3. Excess reactive nitrogen accumulates in soils, freshwater, oceans, and the atmosphere, disrupting all Earth spheres.

Environmental Impacts

1. Eutrophication: Nutrient enrichment of aquatic ecosystems → algal blooms → oxygen depletion → fish kills.
2. Air Pollution: Nitrogen oxides (NOₓ) from combustion form smog and acid rain, harming forests and human lungs.
3. Soil Acidification: Accumulated nitrates increase H⁺ ions, decreasing soil pH and harming microbial diversity.
4. Biodiversity Loss: Nitrogen favors fast-growing species (e.g., grasses) that outcompete slow-growing natives, reducing species richness.

Global Collaboration to Restore Nitrogen Balance

1. Bringing the nitrogen cycle back within safe limits requires international coordination across agriculture, industry, and policy.
2. Nitrogen is a transboundary pollutant.
3. Its atmospheric and hydrological pathways cross borders.

1. Sustainable Agriculture

1. Precision Farming: GPS-guided fertilizer application reduces waste.
2. Crop Rotation with Legumes: Restores nitrogen naturally via symbiotic fixation.
3. Organic Farming: Uses compost and manure instead of synthetic inputs.
4. Cover Crops: Prevent soil erosion and nitrogen leaching during fallow periods.

2. Integrated Nutrient Management

1. Combines organic and inorganic sources to enhance soil fertility and microbial health.
2. Utilizes biofertilizers (Rhizobium, Azospirillum) to increase biological nitrogen fixation.
3. Encourages recycling of crop residues and animal manure.

3. Improved Wastewater Treatment

1. Secondary and tertiary treatment removes nitrogen compounds before discharge.
2. Constructed wetlands replicate natural denitrification zones, filtering nitrates.
3. Technologies like anammox bacteria reduce nitrogen in effluent with less energy.

4. Sustainable Urban Design

1. Expands green infrastructure (green roofs, riparian buffers) to capture nitrogen runoff.
2. Transitioning to electric vehicles reduces NOₓ emissions from combustion.

5. Global Policy and Education

1. UNEP Global Nitrogen Campaign (2019-present): Aims to cut global nitrogen waste by 50% by 2030.
2. Promotes capacity-building in developing nations.
3. Encourages economic incentives for nitrogen-efficient technology.