Q.Which step of decomposition involves detritivores such as earthworms physically breaking detritus into smaller particles?

Saprotrophs (decomposers) obtain energy and nutrients by breaking down dead organic matter (detritus).

In an ecosystem, organisms occupy different nutritional/functional roles. Saprotrophs (decomposers), such as many fungi and bacteria, feed on dead and decaying organic matter (detritus) — dead plant and animal remains, fallen leaves, faeces, etc. — by secreting digestive enzymes onto the material and absorbing the resulting simpler organic molecules; in doing so, they meet their own energy and nutrient requirements while also recycling nutrients back into the ecosystem. Omnivores and carnivores obtain energy by consuming living organisms/other food sources, and autotrophs manufacture their own food via photosynthesis or chemosynthesis, so none of these describe an organism whose primary nutrition is degrading detritus.

Fragmentation is the mechanical breakdown of detritus into smaller pieces by detritivores.

Decomposition of dead organic matter (detritus) begins with fragmentation, in which detritivores such as earthworms and other soil organisms physically break down the detritus into much smaller particles, increasing the surface area available for microbial action. This is followed by leaching (water-soluble inorganic nutrients percolate down and are lost into the soil), catabolism (enzymatic breakdown of organic compounds by bacteria and fungi into simpler molecules), and finally humification (formation of dark, resistant humus). Since the question specifically asks about breakdown into smaller particles, the correct term is fragmentation.

Humification converts organic detritus into humus, a stable, nutrient-rich soil component.

Humification is a stage of decomposition in which soil microorganisms (bacteria and fungi) act on plant and animal remains (detritus) that have already been fragmented, chemically transforming them into humus — a dark-coloured, amorphous, colloidal substance. Humus is highly resistant to further rapid microbial breakdown, so it accumulates in the soil, acting as a reservoir of nutrients that are slowly released by further mineralisation. It also improves soil structure, water-holding capacity and fertility, making it essential for plant growth and nutrient cycling within an ecosystem.