The soil beneath our feet is a complex, living ecosystem teeming with immense biodiversity. This hidden world supports nearly all terrestrial life. The vast array of organisms, from the smallest microbes to the most familiar burrowing animals, drive essential global processes. Understanding what lives in soil requires examining the different size classes of organisms and the specific roles they play.
The Unseen Architects: Microorganisms
The smallest inhabitants of the soil—microorganisms—are the foundational drivers of soil chemistry and fertility. These organisms include bacteria, archaea, and fungi, which exist in staggering numbers, often comprising the largest fraction of living mass in the soil.
Bacteria and archaea are primarily responsible for mineralization, the process where complex organic molecules are broken down into simpler, plant-available inorganic nutrients. For instance, certain bacteria and archaea convert atmospheric nitrogen gas into ammonium and nitrate through nitrogen fixation and nitrification, making this element usable by plants.
Fungi, including yeasts and molds, are major decomposers, extending vast networks of hyphae to break down tough materials like cellulose and lignin. Mycorrhizal fungi form symbiotic relationships with plant roots, effectively extending the root system’s reach by many times. In exchange for plant sugars, these fungi scavenge for nutrients, particularly phosphorus, and transport them back to the host plant. This microbial activity is fundamental to the formation of stable soil organic matter, known as humus, which binds nutrients and improves water retention.
The Intermediate Residents: Soil Mesofauna
Bridging the gap between the microscopic world and larger visible organisms is the soil mesofauna, defined by body sizes ranging from 0.1 millimeters to about 2 millimeters. This group includes organisms such as nematodes, mites, and Collembola, commonly known as springtails. Their main functional role is fragmentation, where they graze on organic debris and microorganisms, breaking down larger pieces of organic matter into smaller fragments.
Nematodes are extremely abundant and occupy diverse roles, with some feeding on bacteria and fungi, while others are micro-predators or herbivores that feed on plant roots. By consuming microbes, they help regulate microbial populations and accelerate the release of nutrients, effectively stimulating the overall rate of decomposition. Mites and springtails are micro-arthropods that primarily consume decaying plant material and fungal hyphae.
Springtails play a significant role in regulating fungal communities through selective grazing. Their feeding activity prevents any single fungal species from dominating and helps disperse fungal spores throughout the soil profile. By shredding and partially digesting organic matter, the mesofauna create a larger surface area for microbial colonization.
The Soil Engineers: Macrofauna
The macrofauna are organisms larger than 2 millimeters, often visible to the naked eye, and are best described as the “soil engineers”. This category includes earthworms, various insect larvae and adults, and other invertebrates. Their primary contribution is physical alteration of the soil structure rather than chemical decomposition.
Earthworms are the most recognized soil engineers, creating extensive networks of burrows that can extend several meters deep. These tunnels improve soil aeration and enhance the infiltration of water, reducing surface runoff and erosion. As earthworms feed on organic matter and soil, they excrete “casts,” which are highly stable, nutrient-rich aggregates that contribute significantly to soil structure and fertility.
Different types of earthworms perform different functions; anecic worms, for example, create permanent vertical burrows and incorporate surface litter deep into the soil profile. Larger insects, such as beetle larvae and ants, also contribute to soil mixing and pore creation through their burrowing and nesting activities. This collective physical movement, known as bioturbation, mixes soil horizons and distributes organic matter and nutrients throughout the soil.
The Critical Role of Soil Life in Ecosystems
The combined actions of microorganisms, mesofauna, and macrofauna provide a suite of services that are fundamental to global ecosystems. These groups work together in a complex food web to maintain soil fertility and stability.
Soil life plays a large role in regulating the carbon cycle by facilitating the decomposition of plant residues and the formation of stable soil organic carbon. Healthy, biologically active soils act as one of the planet’s largest carbon reservoirs, sequestering carbon that would otherwise be released into the atmosphere. Furthermore, the improved soil structure created by macrofauna enhances water retention and purification, filtering water as it moves through the soil profile.
The biodiversity within the soil also provides a form of disease suppression, where a diverse community of microbes and mesofauna can outcompete or prey upon harmful pathogens and pests. By supporting these functions—from recycling nutrients to mitigating erosion—the living components of the soil maintain the health and resilience of the entire environment.