An increase in human-made carbon dioxide in the atmosphere could initiate a chain reaction between plants and microorganisms that would unsettle one of the largest carbon reservoirs on the planet—soil. In a study, it was found that the soil, which contains twice the amount of carbon present in all plants and Earth's atmosphere combined, could become increasingly volatile as people add more carbon dioxide to the atmosphere. This is largely because of increased plant growth. Although a greenhouse gas and a pollutant, carbon dioxide also supports plant growth. As trees and other vegetation flourish in a carbon dioxide-rich future, their roots could stimulate microbial activity in soil that may in turn accelerate the decomposition of soil carbon and its release into the atmosphere as carbon dioxide.

Correct Option (D)

The passage describes a potential feedback loop where elevated atmospheric carbon dioxide (CO₂) levels, resulting from human activities, stimulate increased plant growth. This enhanced vegetation, referred to as 'green cover', would lead to greater root activity. Subsequently, these roots could intensify microbial activity within the soil, accelerating the decomposition of organic matter and releasing stored soil carbon back into the atmosphere as CO₂. Therefore, an increase in green cover, driven by higher CO₂ concentrations, could inadvertently trigger the release of significant amounts of carbon trapped within the soil reservoir.

Incorrect Options:

• Option (A) states that carbon dioxide is essential for the survival of microorganisms and plants. While the passage acknowledges that CO₂ supports plant growth, it does not assert that it is essential for the survival of either plants or microorganisms.
• Option (B) claims that humans are solely responsible for the release of carbon dioxide into the atmosphere. The passage specifically mentions "human-made carbon dioxide" but does not exclude natural sources of CO₂ emissions, thus not attributing sole responsibility to humans.
• Option (C) suggests that microorganisms and soil carbon are mainly responsible for increased plant growth. The passage clearly indicates that increased plant growth is a consequence of higher atmospheric carbon dioxide levels, not primarily due to microorganisms or soil carbon. Microorganisms are depicted as agents of soil carbon decomposition, not drivers of plant growth in this context.