Final answer:
Decomposition in landfills involves aerobic processes in the upper layers, producing CO₂, and anaerobic processes in deeper, oxygen-depleted layers, producing CH₄. The presence of oxygen determines the type of gases produced during the decomposition of organic matter.
Step-by-step explanation:
The decomposition of carbon-based matter in a landfill involves both aerobic (with oxygen) and anaerobic (without oxygen) processes. In the upper layers of a landfill, oxygen is present, and decomposers like bacteria and fungi break down organic matter, resulting in the production of carbon dioxide (CO₂). However, as waste gets buried under more layers, oxygen becomes scarce, and anaerobic bacteria begin to decompose the waste, producing methane (CH₄). This methane contributes significantly to global CH₄ emissions, which is a potent greenhouse gas.
The correct statement describing the process is: Oxygen is present on the upper-most part of the landfill, where CO₂ is formed as a result of decomposition. In deeper parts of landfill, where oxygen is not present, CH₄ forms as a result of decomposition, which corresponds to the first option in the question posed by the student.