IB Environmental systems and societies (ESS) - Old 2.3E Nitrogen Cycle Notes

The Nitrogen Cycle: Moving Nitrogen Between Living and Non-Living Stores

1. The nitrogen cycle moves nitrogen through many forms and stores in ecosystems.
2. Nitrogen exists in both organic and inorganic stores, each vital to life.

Organic Nitrogen Stores

1. Organic nitrogen is held in living tissues and recently dead matter, the biologically active pool.
2. The main stores include:
   1. Proteins and enzymes that run metabolism.
   2. Nucleic acids (DNA and RNA) that carry genetic information.
   3. Chlorophyll, the nitrogen-containing pigment for photosynthesis.
   4. Humus and leaf litter, temporary soil stores before decomposition.
3. When organisms die, decomposers break their tissues into ammonium compounds.

Inorganic Nitrogen Stores

1. Inorganic nitrogen sits in the atmosphere, in soil ammonia, and in nitrate and nitrite compounds in soil and water.
2. These forms drive nutrient cycling and plant growth.
3. They include:
   1. Nitrogen gas (N₂): about 78% of the atmosphere, inert due to a strong triple bond.
   2. Ammonia (NH₃) and ammonium (NH₄⁺): produced by fixation or decomposition.
   3. Nitrites (NO₂⁻) and nitrates (NO₃⁻): oxidised forms taken up by plants.
   4. Dissolved nitrogen compounds in lakes, rivers, and oceans.

Building a Systems Diagram of the Nitrogen Cycle

1. Identify the main stores:
   1. Organic: plants, animals, decomposers.
   2. Inorganic: soil (ammonia, nitrites, nitrates) and atmosphere (N₂).
2. Map the flows:
   1. Fixation, nitrification, assimilation, ammonification, and denitrification.
3. Label each arrow with its process and direction.

Bacteria Drive Every Step of the Nitrogen Cycle

1. Bacteria are the biological engines of the nitrogen cycle.
2. They convert nitrogen between forms, making it available to living things.

Nitrogen Fixation

1. Nitrogen-fixing bacteria convert atmospheric N₂ into ammonia (NH₃), forming ammonium ions (NH₄⁺) that plants can use.
2. Free-living bacteria (e.g. Azotobacter) fix nitrogen in the soil.
3. Symbiotic bacteria (e.g. Rhizobium) live in the root nodules of legumes.

Nitrification

1. Nitrifying bacteria convert ammonia (NH₃) into nitrite (NO₂⁻) and then nitrate (NO₃⁻), which plants absorb.
   1. Nitrosomonas convert ammonia into nitrites (NO₂⁻).
   2. Nitrobacter convert nitrites into nitrates.
2. It needs oxygen, so it happens in aerobic soils.

Decomposition and Ammonification

1. Decomposers (saprotrophic bacteria and fungi) break proteins and amino acids from dead matter into ammonia (NH₃) or ammonium (NH₄⁺).
2. This recycles organic nitrogen back into the soil for nitrification or plant uptake.

Denitrification

1. Denitrifying bacteria (e.g. Pseudomonas denitrificans) convert nitrates (NO₃⁻) back into nitrogen gas (N₂), completing the cycle.
2. It happens in anaerobic conditions like waterlogged, compacted soils, where bacteria use nitrate instead of oxygen for respiration.
3. This releases N₂ or nitrous oxide (N₂O) to the atmosphere.
4. Consequences:
   1. Loss of soil fertility as plant-available nitrates are removed.