What is the dark-colored amorphous substance formed during decomposition called?

Humus

The correct answer is Option (2) - Humus.

Decomposers break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients and the process is called decomposition. Dead plant remains such as leaves, bark, flowers and dead remains of animals, including fecal matter, constitute detritus, which is the raw material for decomposition. The important steps in the process of decomposition are fragmentation, leaching, catabolism, humification and mineralisation.

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.

Humus is the dark-colored amorphous substance that forms during decomposition. It is a highly decomposed organic material that results from the breakdown of plant and animal residues by microbial activity. Humus is rich in carbon and is composed of complex organic compounds.

During the decomposition process, microorganisms, such as bacteria and fungi, break down the organic matter into simpler substances. However, some components of the organic matter, such as lignin, chitin, and other complex compounds, are more resistant to microbial degradation. These resistant materials accumulate over time and form humus.

The humus is further degraded by some microbes and release of inorganic nutrients occur by the process known as mineralisation.