Multiple Choice
Which of the following best describes what happens during nitrogen fixation?
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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
Assimilation of nitrate ($\mathrm{NO_3^-}$) by plants
B
Denitrification by bacteria converting nitrate ($\mathrm{NO_3^-}$) to nitrogen gas ($\mathrm{N_2}$)
C
Ammonification of organic nitrogen to ammonia ($\mathrm{NH_3}$)
D
Nitrogen fixation by bacteria converting atmospheric $\mathrm{N_2}$ to ammonia ($\mathrm{NH_3}$)
Verified step by step guidance1
Step 1: Understand the nitrogen cycle, which describes how nitrogen moves through the environment, including its conversion between different chemical forms by biological and physical processes.
Step 2: Identify the largest nitrogen reservoir, which is the atmosphere, primarily composed of nitrogen gas ($\mathrm{N_2}$). This is key to understanding how nitrogen is added to this reservoir.
Step 3: Examine the process of denitrification. Denitrification is carried out by specific bacteria that convert nitrate ($\mathrm{NO_3^-}$) into nitrogen gas ($\mathrm{N_2}$), releasing it back into the atmosphere. This process directly adds nitrogen to the atmospheric reservoir.
Step 4: Compare denitrification with other processes mentioned in the problem. Assimilation of nitrate by plants, ammonification, and nitrogen fixation do not add nitrogen to the atmosphere; instead, they involve transformations of nitrogen within the biosphere or soil.
Step 5: Conclude that denitrification is the pathway responsible for adding nitrogen to the atmosphere, as it converts nitrate ($\mathrm{NO_3^-}$) into nitrogen gas ($\mathrm{N_2}$), replenishing the atmospheric nitrogen reservoir.
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