The largest reservoir of nitrogen in the global nitrogen cycle is the atmosphere. Options A‐D below represent important pathways in the removal of nitrogen from the atmosphere at different rates. A. Biological fixation in oceans B. Fixation by lightning C. Biological fixation in natural terrestrial systems D. Industrial nitrogen fixation Arrange the above pathways from the lowest to the highest rate.
B < C < D < A
Nitrogen Cycle and Atmospheric Nitrogen Fixation
The atmosphere is the largest reservoir of nitrogen, primarily in the form of nitrogen gas ($\text{N}_2$). However, organisms cannot directly use atmospheric nitrogen. It must be converted into usable forms like ammonia ($\text{NH}_3$) or nitrate ($\text{NO}_3^-$) through a process called nitrogen fixation.
Nitrogen fixation removes nitrogen from the atmosphere. Several pathways contribute to this removal at different rates:
- A. Biological fixation in oceans: Certain marine microorganisms, particularly cyanobacteria, fix significant amounts of nitrogen in the vast oceanic environment.
- B. Fixation by lightning: Lightning provides the energy to convert atmospheric nitrogen and oxygen into nitrogen oxides, which dissolve in rain and fall to the ground as nitrates.
- C. Biological fixation in natural terrestrial systems: Nitrogen-fixing bacteria in soil, many of which live in symbiotic relationships with plants (especially legumes), convert atmospheric nitrogen into ammonia.
- D. Industrial nitrogen fixation: The Haber-Bosch process is a human-engineered method to produce ammonia from nitrogen and hydrogen under high pressure and temperature. This process is crucial for manufacturing synthetic fertilizers.
Comparing Nitrogen Fixation Rates
These pathways contribute to global nitrogen fixation at different scales. To arrange them from the lowest to the highest rate, we consider typical global estimates:
- Fixation by lightning (B) is a relatively small contribution to global nitrogen fixation compared to biological and industrial processes.
- Biological fixation in natural terrestrial systems (C) is a major natural process occurring in soils and associated with many plant species.
- Industrial nitrogen fixation (D), driven by human activities for fertilizer production, has dramatically increased fixed nitrogen inputs into ecosystems, often exceeding natural terrestrial fixation in many regions, but its global scale is still often considered less than total biological fixation (including oceanic).
- Biological fixation in oceans (A) is estimated to be the largest single contributor to global nitrogen fixation due to the vast area covered by the oceans and the activity of marine diazotrophs.
Based on typical estimates of global nitrogen fixation rates (often measured in teragrams of nitrogen per year, Tg N/yr), the relative order from lowest to highest rate is generally accepted as:
| Pathway | Description | Relative Rate (Approx.) |
|---|---|---|
| B | Fixation by lightning | Lowest |
| D | Industrial nitrogen fixation | Medium-Low |
| C | Biological fixation in natural terrestrial systems | Medium-High |
| A | Biological fixation in oceans | Highest |
Arranging Pathways by Rate
Therefore, arranging the given pathways from the lowest to the highest rate of nitrogen removal from the atmosphere gives the order:
Lightning (B) < Industrial (D) < Terrestrial Biological (C) < Oceanic Biological (A)
This corresponds to the sequence B < D < C < A.
Important Questions from Ecosystem Ecology
Which of the following describes the littoral zone?
1. The zone at the edge of a lake or ocean which is alternatively exposed to air and immersed in water
2. The zone at the middle of a lake or ocean which is alternatively exposed to air and immersed in water
3. The zone at the both the ends of a lake which is alternatively exposed to air and immersed in water
Which of the following constitute the largest reservoir of carbon in the global carbon cycle?
Which one of the following represents the largest outflux of nitrogen from the atmospheric reservoir?
In which of the following ecosystems would the largest percentage of Net Primary Productivity (NPP) be taken up by the grazing food chain?
Which one of the following correctly shows the total estimated biomass of the lifeforms on Earth given here, in increasing order?
