RG

🌿 APES Topic 1.5: The Nitrogen Cycle

The movement of nitrogen atoms and nitrogen containing molecules between sources and sinks defines the nitrogen cycle.

1. Why Nitrogen Matters

Essential for all living things (proteins, DNA, chlorophyll) but atmospheric N_2 is inert. Its availability often limits plant growth.

2. Major Nitrogen Reservoirs

The atmosphere is a major reservoir of nitrogen, but most organisms cannot use it directly; it's mostly inert N_2 gas (the largest reservoir).

  • Soil: inorganic (NH4^+, NO3^-$) and organic forms.

  • Living organisms: in tissues.

  • Water bodies: dissolved compounds, sediments.

3. Key Processes in the Nitrogen Cycle

  • Nitrogen Fixation: Converts atmospheric N2 into usable NH3 or NH_4^+.

    • I. Abiotic Fixation: Through lightning and cosmic radiation, nitrogen in the atmosphere (N2) becomes NO or NO2 which will combine with rain to form HNO_3 (nitric acid).

    • II. Biotic Fixation: Through soil microorganisms (e.g., Rhizobium, free-living, cyanobacteria), N2 in the soil is converted to NH3 (ammonia). Some plants (like legumes - soy, chickpeas) have nodules in their roots that perform this, making nitrogen easily accessible to the plant.

  • Nitrification: Ammonia (NH3) or ammonium (NH4^+) is converted to nitrites (NO2^-$) which are then converted to nitrates (NO3^-$) by nitrifying bacteria in aerobic conditions. Nitrates are the most usable form for plants.

  • Assimilation: Plants absorb nitrates (NO3^-$) or ammonium (NH4^+) through root hairs into organic molecules; animals get nitrogen by consuming other organisms (nitrogen is essential for our genetic material and proteins).

  • Ammonification (Deamination): Decomposers like bacteria and fungi break down organic nitrogen from dead organic matter into ammonium ions (NH_4^+) that can be absorbed by plants.

  • Denitrification: Nitrates (NO3^-$) are converted back to gaseous nitrogen (N2) (or N_2O) by anaerobic denitrifying bacteria, returning it to the atmosphere.

4. Human Impacts & Alterations

  • Haber-Bosch process: produces fertilizers, adding excess reactive nitrogen to ecosystems.

  • Fertilizer runoff: leads to eutrophication, algal blooms, and "dead zones" in water bodies.

  • Burning fossil fuels: releases NO_x, contributing to air pollution, acid rain.

  • Increased denitrification: produces N_2O, a potent greenhouse gas.

5. Short vs Long Nitrogen Cycles

Most reservoirs in the nitrogen cycle hold nitrogen atoms or nitrogen containing molecules for a relatively short period of time. Nitrogen in living biomass, soil, and dissolved water cycles relatively quickly. Atmospheric N_2, geological sediments, and deeply buried organic matter are stable, long-term reservoirs, with the atmospheric pool being the largest but largely inert.

6. Key Terms You Should Be Able To Define / Recognize

  • Nitrogen fixation

  • Nitrification

  • Denitrification

  • Ammonification

  • Assimilation

  • Haber-Bosch process

  • Reactive nitrogen

  • Eutrophication

7. How the Nitrogen Cycle Connects to Other Topics

  • Primary productivity: Nitrogen limits plant growth and carbon uptake.

  • Water quality: Excess nitrogen causes eutrophication.

  • Atmospheric chemistry & climate: N2O is a greenhouse gas; NOx causes smog and acid rain.

  • Biodiversity: Altered nitrogen favors some plant species, changing ecosystems.

8. Very Important Takeaways (Exam-Relevant)

  • Know the process order: fixation
    \rightarrow
    assimilation
    \rightarrow
    ammonification
    \rightarrow
    nitrification
    \rightarrow$$
    denitrification.

  • Know the agents: bacteria types, lightning, industrial processes.

  • Understand human effects: fertilizer, fossil fuels, pollution.

  • Explain consequences of nitrogen imbalance: eutrophication, greenhouse gases, species shifts.