Final answer:
The majority of organic matter in soils comes from vascular plants, which are primary producers along with algae and cyanobacteria. These photoautotrophs create their own food through photosynthesis and contribute significantly to the organic content in soils through dead plant material.
Step-by-step explanation:
Most organic matter found in soils results from primary producers such as vascular plants. These organisms, including plants, algae, and cyanobacteria, are capable of performing photosynthesis; they are known as photoautotrophs (self-feeders using light). Vascular plants significantly contribute to the soil's organic matter through the decomposition of plant material which includes leaves, stems, and roots.
Participants in the carbon cycle are divided among producers like plants and photosynthetic bacteria, consumers, and decomposers. Plants are the largest contributors to organic carbon compounds found in soils, as they contribute both living biomass and decomposing matter that forms humus. Cyanobacteria played an essential role in the history of life on Earth by being among the first organisms to oxygenate the atmosphere, and they are still important in ecosystems, especially aquatic ones, but vascular plants are predominantly responsible for the organic matter in soil in present-day terrestrial environments.