Which of the following locations is the main reservoir for nitrogen in Earth’s nitrogen cycle?
A) soil
B) atmosphere
C) sedimentary bedrock
D) plant and animal biomass
E) fossilized plant and animal remains (coal, oil, and natural gas)
The correct answer and explanation is :
Correct Answer: B) Atmosphere
Explanation:
The main reservoir for nitrogen in Earth’s nitrogen cycle is the atmosphere, which contains about 78% nitrogen gas (N₂) by volume. This makes it the largest and most significant source of nitrogen on Earth.
Nitrogen is a critical element for all living organisms, as it is a major component of amino acids (the building blocks of proteins), nucleic acids (like DNA and RNA), and other biological molecules. However, the nitrogen found in the atmosphere exists in a diatomic, inert form (N₂) that most organisms cannot use directly. This is due to the strong triple bond between the nitrogen atoms, which makes N₂ relatively unreactive.
To become biologically available, atmospheric nitrogen must first be “fixed” into forms like ammonia (NH₃) or nitrate (NO₃⁻) through processes such as:
- Nitrogen fixation by bacteria (e.g., Rhizobium in legumes)
- Lightning, which converts N₂ into nitrates
- Industrial processes (e.g., the Haber-Bosch process)
Once fixed, nitrogen can be taken up by plants and passed through the food web to animals and decomposers. It cycles through ecosystems in various forms—ammonium, nitrate, nitrite, and organic nitrogen compounds—but eventually, much of it is returned to the atmosphere through denitrification, where specialized bacteria convert nitrates back into N₂ gas.
Other reservoirs like soil (A) and plant and animal biomass (D) hold much smaller quantities of nitrogen and serve more as temporary storage or pathways rather than primary sources. Sedimentary bedrock (C) and fossil fuels (E) are more relevant in the phosphorus and carbon cycles, respectively, and are not significant nitrogen reservoirs.
Thus, due to its vast quantity and central role in the global nitrogen cycle, the atmosphere is the main nitrogen reservoir.