The correct option is (a).
Explanation
Humus formation, or humification, is a critical phase in the decomposition process where complex organic matter is broken down into a dark, amorphous, and stable substance known as humus. This process is primarily driven by the metabolic activities of soil microorganisms.
Statement-wise Analysis:
- Statement 1 is Correct: Decomposition is an exothermic process. As microorganisms (bacteria and fungi) break down organic bonds through respiration to derive energy, a significant amount of heat is released. This metabolic heat generation causes decaying heaps, such as compost piles, to become warm.
- Statement 2 is Incorrect: Humus formation is a biological process carried out by decomposers (detritivores and microorganisms) using enzymes. It is not a chemical process driven directly by sunlight. While sunlight can cause physical degradation (photodegradation) of surface litter, the formation of humus within the soil ecosystem is biotic.
- Statement 3 is Incorrect: This statement describes photosynthesis (primary production), where plants synthesize new tissues from inorganic matter (carbon dioxide and water). In contrast, humus formation involves the breakdown (catabolism) of organic matter into simpler substances, rather than the synthesis of new plant tissues.
Key Takeaway:
Humification is a biological, heat-releasing (exothermic) decomposition process driven by microbial activity, distinct from the light-driven, anabolic process of photosynthesis.