Choose the correct statements with respect to decomposition from the…

The correct answer is Option (3) → (B) and (C) only

Decomposition is the breakdown of detritus (dead leaves, bark, flowers, dead animals and fecal matter) by decomposers into simpler inorganic substances such as CO₂, water and nutrients. Its steps are fragmentation, leaching, catabolism, humification and mineralisation.

(A) Decomposition is an anaerobic process. False. Decomposition is largely an oxygen-requiring process. Lack of oxygen (anaerobiosis) slows it down, and organic matter then builds up.

(B) Decomposition rate of detritus depends upon the chemical nature of it. True. Detritus rich in lignin and chitin breaks down slowly. Detritus rich in nitrogen and water-soluble substances like sugars breaks down quickly. So the rate depends on what the detritus is made of, along with temperature and soil moisture.

(C) Water-soluble inorganic nutrients go into the soil and get precipitated in the process of leaching. True. This is the leaching step of decomposition.

(D) Humification follows mineralisation. False. The order is the other way round. Humification first forms humus, a dark substance that breaks down very slowly. Some microbes then degrade humus and release inorganic nutrients, which is mineralisation. So mineralisation follows humification.

Note: The original NTA paper prints statement (C) as “Water-soluble organic nutrients go into the soil …”. Leaching moves inorganic nutrients, so the word has been changed to “inorganic”. With this change, Option (3), the answer given by NTA, is correct.

NCERT Extract:
Decomposition – “… Detritivores (e.g., earthworm) break down detritus into smaller particles. This process is called fragmentation. By the process of leaching, watersoluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts. Bacterial and fungal enzymes degrade detritus into simpler inorganic substances. This process is called as catabolism. It is important to note that all the above steps in decomposition operate simultaneously on the detritus …. Humification and mineralisation occur during decomposition in the soil. Humification leads to accumulation of a dark coloured amorphous substance called humus that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. Being colloidal in nature it serves as a reservoir of nutrients. The humus is further degraded by some microbes and release of inorganic nutrients occur by the process known as mineralisation.
Decomposition is largely an oxygen-requiring process. The rate of decomposition is controlled by chemical composition of detritus and climatic factors. In a particular climatic condition, decomposition rate is slower if detritus is rich in lignin and chitin, and quicker, if detritus is rich in nitrogen and water-soluble substances like sugars.”