Multiple Choice
Which organisms are primarily responsible for transforming atmospheric nitrogen ($N_2$) into a form that is usable by plants?
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Nitrogen Fixation
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Which of the following pathways indicates how nitrogen is added to the largest nitrogen reservoir, the atmosphere?
A
Nitrogen fixation by bacteria converting atmospheric $\mathrm{N_2}$ to ammonia ($\mathrm{NH_3}$)
B
Ammonification of organic nitrogen to ammonia ($\mathrm{NH_3}$)
C
Assimilation of nitrate ($\mathrm{NO_3^-}$) by plants
D
Denitrification by bacteria converting nitrate ($\mathrm{NO_3^-}$) to nitrogen gas ($\mathrm{N_2}$)
Verified step by step guidance1
Step 1: Understand the nitrogen cycle, which is the process by which nitrogen moves through the atmosphere, biosphere, and geosphere in different chemical forms. The atmosphere is the largest nitrogen reservoir, primarily containing nitrogen gas ($\mathrm{N_2}$).
Step 2: Recognize the role of denitrification in the nitrogen cycle. Denitrification is a process carried out by specific bacteria in anaerobic conditions, where nitrate ($\mathrm{NO_3^-}$) is reduced to nitrogen gas ($\mathrm{N_2}$), returning nitrogen to the atmosphere.
Step 3: Analyze the other processes mentioned in the problem. Nitrogen fixation converts atmospheric nitrogen gas ($\mathrm{N_2}$) into ammonia ($\mathrm{NH_3}$), which is the opposite direction of adding nitrogen to the atmosphere. Ammonification and assimilation involve transformations of nitrogen within the biosphere, not directly contributing to the atmospheric reservoir.
Step 4: Identify that denitrification is the only process listed that adds nitrogen back to the atmosphere, making it the correct answer.
Step 5: Conclude that the correct pathway for adding nitrogen to the atmosphere is denitrification by bacteria converting nitrate ($\mathrm{NO_3^-}$) to nitrogen gas ($\mathrm{N_2}$).
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