IB Environmental systems and societies 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.
   2. Reduced plant growth as waterlogging starves roots of oxygen.
   3. Leaching washes nitrates into groundwater.

Mutualistic Nitrogen Fixation: A Partnership for Nitrogen

1. Atmospheric N₂ is stable and unavailable to plants directly.
2. Some plants form mutualistic partnerships with nitrogen-fixing bacteria to get it.
3. The bacteria convert N₂ into ammonia (NH₃) the plant can use.
4. In return, the plant supplies carbohydrates and a protected home in its root nodules.
5. Both legumes (beans, peas, clover) and some non-legumes (alders, cycads) form these partnerships.

Benefits to Plants and Bacteria

1. Plants benefit by:
   1. Getting nitrogen in nitrogen-poor soils, so they grow where others cannot.
   2. Depending less on soil nitrogen lost to leaching or competition.
   3. Enriching soil nitrogen for nearby plants after they die, as with legumes in crop rotation.
2. Bacteria benefit by:
   1. Receiving carbohydrates from the plant as an energy source.
   2. Living in a stable, protected root-nodule environment.

Nitrogen Flows: Transfers Move It, Transformations Change It

1. Transfers move nitrogen between locations without changing its form; transformations change its chemical form.

Transfer Flows

1. These move nitrogen without changing its chemical form:
   1. Mineral uptake: plants absorb nitrate (NO₃⁻) and ammonium (NH₄⁺) through roots.
   2. Consumption: nitrogen passes along the food chain as animals feed.
   3. Excretion: animals release nitrogen in waste (urea, uric acid, ammonia).
   4. Death and decomposition: organic nitrogen passes to decomposers.

Transformation Flows

1. These change nitrogen from one form to another:
   1. Nitrogen fixation: N₂ → NH₃ or NH₄⁺ (bacteria or lightning).
   2. Nitrification: NH₄⁺ → NO₂⁻ → NO₃⁻.
   3. Assimilation: plants build NH₄⁺ and NO₃⁻ into amino acids and proteins.
   4. Ammonification: organic nitrogen → NH₄⁺.
   5. Denitrification: NO₃⁻ → N₂.

Human Activities Overload the Nitrogen Cycle

1. Deforestation, agriculture, aquaculture, and urbanisation have dramatically altered the nitrogen cycle.
2. These disrupt natural nitrogen flows, causing eutrophication, air pollution, and biodiversity loss.

Deforestation

1. Trees act as nitrogen stores, so clearing them reduces nitrogen uptake.
2. Without roots to hold soil, nitrates leach away, depleting soil and polluting water.

Agriculture

1. Fertiliser use raises soil nitrogen but runs off into rivers and lakes, causing eutrophication.
2. Livestock release ammonia (NH₃) and methane (CH₄) through manure and digestion.
3. Ploughing speeds nitrification and denitrification, losing nitrogen as N₂ and nitrous oxide (N₂O).

Aquaculture

1. Fish farms produce ammonia-rich waste that depletes oxygen in water.
2. Extra nitrogen fuels toxic algal blooms that harm marine life.

Urbanisation

1. Untreated sewage releases nitrogen compounds into rivers and lakes.
2. Vehicles and factories emit nitrogen oxides (NOₓ), causing acid rain.
3. Roads and buildings increase runoff and water pollution.

The Haber Process: Making Nitrogen Fertiliser

1. The Haber process transformed agriculture by supplying nitrogen fertiliser at scale, but it also drives environmental harm.
2. Its ammonia becomes fertilisers like ammonium nitrate and urea.

How It Works

1. Nitrogen (N₂) reacts with hydrogen (H₂) at high pressure (200 to 300 atm) and temperature (400 to 500°C) over an iron catalyst.
2. This makes ammonia (NH₃) for fertilisers like ammonium nitrate (NH₄NO₃) and urea.

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Advantages

1. Boosts global food production by raising crop yields.
2. Reduces reliance on natural nitrogen fixation.
3. Its ammonia also supplies pharmaceuticals, explosives, and refrigeration.
4. Without synthetic fertilisers, global food production would fall by nearly 50%.

Disadvantages

1. Energy-intensive, relying heavily on fossil fuels.
2. CO₂ emissions from hydrogen production add to climate change.
3. Nitrate leaching from over-fertilising causes eutrophication.
4. Creates nitrogen imbalance in ecosystems.

Crossing the Planetary Boundary for Nitrogen

1. The planetary boundary concept sets safe operating limits for Earth systems.
2. Excessive nitrogen fertiliser use has pushed the nitrogen cycle past its safe limit.

Evidence of Boundary Crossing

1. The safe limit for human nitrogen fixation is about 62 Tg N per year.
2. The current rate is over 150 Tg N per year, mostly from fertiliser and fossil fuels.
3. Excess reactive nitrogen builds up in soils, freshwater, oceans, and air.

Environmental Impacts

1. Eutrophication: nutrient enrichment leads to algal blooms, oxygen depletion, and fish kills.
2. Air pollution: nitrogen oxides (NOₓ) form smog and acid rain.
3. Soil acidification: excess nitrates lower soil pH and harm microbes.
4. Biodiversity loss: fast-growing species outcompete natives, reducing richness.

Restoring Nitrogen Balance Needs Global Cooperation

1. Bringing nitrogen back within safe limits needs coordination across farming, industry, and policy.
2. Nitrogen is a transboundary pollutant whose air and water pathways cross borders.

Sustainable Agriculture

1. Precision farming applies fertiliser by GPS to cut waste.
2. Crop rotation with legumes restores nitrogen naturally.
3. Organic farming uses compost and manure instead of synthetic inputs.
4. Cover crops prevent erosion and leaching.

Integrated Nutrient Management

1. Combines organic and inorganic sources for soil fertility and microbial health.
2. Uses biofertilisers (Rhizobium, Azospirillum) to boost natural fixation.
3. Recycles crop residues and manure.

Improved Wastewater Treatment

1. Secondary and tertiary treatment removes nitrogen before discharge.
2. Constructed wetlands mimic natural denitrification to filter nitrates.
3. Anammox bacteria cut nitrogen in effluent using less energy.

Sustainable Urban Design and Policy

1. Green infrastructure like green roofs and riparian buffers captures nitrogen runoff.
2. Electric vehicles cut NOₓ emissions.
3. The UNEP Global Nitrogen Campaign aims to halve nitrogen waste by 2030.
4. Economic incentives and education spread nitrogen-efficient technology.