The correct option is Mineralisation.
Explanation
Decomposition is the ecological process where complex organic matter is broken down into simpler inorganic substances like carbon dioxide, water, and nutrients. This process is essential for nutrient cycling and involves sequential steps: fragmentation, leaching, catabolism, humification, and mineralisation.
Option Analysis
- Mineralisation is Correct: Mineralisation is the specific process wherein humus (a dark, amorphous, and colloidal substance formed during humification) is further degraded by microbes. This degradation results in the release of inorganic nutrients (such as ammonium, sulphates, and phosphates) back into the soil, making them available for plant uptake.
- Nitrification is Incorrect: Nitrification is a specific phase in the nitrogen cycle involving the oxidation of ammonia to nitrite and subsequently to nitrate by chemoautotrophic bacteria (e.g., Nitrosomonas and Nitrobacter). It is not the general process of nutrient release from humus.
- Fixation is Incorrect: Fixation (specifically Nitrogen Fixation) refers to the conversion of atmospheric nitrogen ($N_2$) into ammonia ($NH_3$) or other nitrogenous compounds usable by living organisms. This is primarily carried out by nitrogen-fixing bacteria like Rhizobium.
- Denitrification is Incorrect: Denitrification is the process of reducing soil nitrates back into gaseous nitrogen ($N_2$), which is then released into the atmosphere. This process is carried out by bacteria such as Pseudomonas and Thiobacillus.
Key Takeaway: While humification leads to the formation of humus, mineralisation is the subsequent biological process that breaks down humus to release inorganic nutrients into the ecosystem.