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Difficulty: MediumLogical Inferences

During boreal forest fires, the combustion of organic matter produces pyrogenic carbon (charcoal), which is highly resistant to microbial decomposition. Some ecologists hypothesized that this carbon represents a long-term sink that helps offset carbon dioxide emissions from the fire. However, researcher Maya Lin and her team demonstrated that pyrogenic carbon particles adsorb dissolved organic compounds from the soil, concentrating easily degradable nutrients. This process stimulates local soil microbe activity, which in turn increases the decomposition rate of nearby unburned soil organic matter. Therefore, while pyrogenic carbon itself remains stable, its presence in the soil may ______

Which choice most logically completes the text?

  1. A
    gradually decompose over time as microbial populations adapt to using it as a primary nutrient source.
  2. B
    reduce the overall microbial diversity of the boreal forest soil after a fire event.
  3. indirectly accelerate the loss of other carbon pools within the forest ecosystem.Answer
  4. D
    prevent dissolved organic compounds from leaching into local waterways during heavy rainfall.

Answer

The correct answer is the option stating that pyrogenic carbon may indirectly accelerate the loss of other carbon pools within the forest ecosystem.
The correct option is supported because the passage states that pyrogenic carbon stimulates microbial activity, which increases the decomposition rate of nearby unburned soil organic matter. Because this unburned matter contains carbon, accelerating its decomposition leads to a loss of carbon from other pools in the forest ecosystem.

Step-by-Step Solution

1
Identify the key characteristics of pyrogenic carbon described in the passage.
Pyrogenic carbon is highly stable and resistant to decomposition, but it adsorbs dissolved organic compounds and concentrates nutrients.
Establishing the baseline properties of the carbon helps define what it does directly.
2
Determine the ecological consequence of this nutrient concentration.
The concentrated nutrients stimulate soil microbes, which then decompose nearby unburned organic matter at a higher rate.
This establishes the chain of cause and effect resulting from the presence of pyrogenic carbon.
3
Synthesize these findings to draw a logical conclusion.
Even though pyrogenic carbon is a stable sink, its presence causes the breakdown and loss of other organic matter (another carbon pool) in the soil.
This completes the logical gap in the text by connecting the stable carbon's effect to the loss of surrounding carbon pools.

Key Concept

Logical Inferences
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Logical Inferences

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