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