There is no such single micro-organism that is responsible for soil fertility.

for example Decomposers they breakdown the dead plant into similar forms. They release nutrients like nitrogen, phosphorus, and potassium that become available for plants to uptake. Nitrogen fixation have a symbiotic relationship with legumes and fix atmospheric nitrogen into a form usable by plants. This process replenishes nitrogen in the soil, a vital nutrient for plant growth.

The combined activities of this diverse microbial community create a healthy and fertile soil ecosystem. By decomposing organic matter, fixing nitrogen, transforming nutrients, and promoting plant growth, microorganisms play an essential role in recycling nutrients and maintaining soil fertility, which ultimately supports plant life and the entire food chain.

Dr Mutyalu Sheshu thank you for your contribution to the discussion

Microorganisms can convert toxic elements into water, carbon dioxide, and other less toxic compounds, which are further degraded by other microbes in a process referred to as mineralization. Micro organisms play a significant role in biodegradation of organic material in waste recycling. Microbial action on dead organic matter releases nutrients in the soil thus raising the fertility status of the soil. Diazotrophic bacteria and cyanobacteria like Azotobacter, Bacillus, Beijerinckia, Clostridium, Klebsiella, Nostoc, Anabaena, Anabaenopsis, etc. Decomposers (fungi, bacteria, invertebrates such as worms and insects) have the ability to break down dead organisms into smaller particles and create new compounds. We use decomposers to restore the natural nutrient cycle through controlled composting. However, micro-organisms such as bacteria, fungi, and actinomycetes–even though they go unnoticed in your compost pile–are responsible for most of the organic material breakdown. They are chemical decomposers because they use chemicals in their bodies to break down organic matter. Bacteria break down (or decompose) dead organisms, animal waste, and plant litter to obtain nutrients. But microbes don't just eat nature's waste, they recycle it. The process of decomposition releases chemicals (such as carbon, nitrogen, and phosphorus) that can be used to build new plants and animals. Bacteria increase soil fertility through nutrient recycling such as carbon, nitrogen, sulphur and phosphorus. Bacteria also help in the decomposition of dead organic matter and then give out simple compounds in the soil, which can be used up by plants. The type of organism that recycles nutrients in a food web is decomposers. Decomposers are organisms that consume dead or decaying matter and recycle the nutrients back into the soil. Process of recycling of nutrients by chemosynthetic autotrophic bacteria: These bacteria make a great contribution to the recycling of nutrients like nitrogen, phosphorus, and sulphur. For obtaining energy they oxidize inorganic materials. Microorganisms abound in the soil and are critical to decomposing organic residues and recycling soil nutrients. Bacteria are the smallest and most hardy microbe in the soil and can survive under harsh conditions like tillage. The microbe plays an essential role of organic matter degradation in nutrient cycling; microorganism present in soil digests the organic matter including dead organisms. The nutrients get released by the breakdown of the organic molecule to make it available for plants to uptake nutrients in the soil through roots.