Soil biological condition

Soil Organisms

Forest soils are complex habitats for diverse bacteria, fungi, archaea, protozoa and viruses and fauna. Bacteria, fungi and fauna have an important role in the food chain.

Bacteria are the most abundant group of microorganisms in forest soils. By decomposing organic matter, including leaf litter and dead wood, they release nutrients (e.g. carbon, nitrogen, and phosphorus) back into the soil, supporting tree growth and maintaining soil health.

Fungi play a major role in plant health, nutrient availability, and soil structure. Fungi are involved in plant community dynamics, belowground trophic interactions, and biogeochemical cycles. Fungi can make up between a third to half of the soil biomass. Fungal diversity in forests is heavily influenced by the type of vegetation and forest age, with old-growth forests typically supporting more complex fungal networks. Some fungi can also threaten tree health through infections.

The three main types of fungi in forest soils are mycorrhizal, saprophytic and pathogenic fungi.

Mycorrhizal fungi

Mycorrhizal fungi form symbiotic relationships with host plant roots. This symbiosis increases water and nutrient uptake for the plant, while the fungi receive carbohydrates produced by the plant through photosynthesis. In forests, mycorrhizal fungi are particularly important for nutrient exchange. In European forests, over 90% of tree species, including beech, oak, and pine, form symbiotic relationships with these fungi, allowing trees to thrive even in nutrient-poor soils. This symbiosis is also key for seedling establishment and forest regeneration.

Saprophytic fungi

Saprophytic fungi are important decomposers of forest litter. They break down complex carbon structures such as lignin and cellulose, making nutrients and minerals to become available to other (micro)organisms and plants. Their activity accelerates nutrient turnover, particularly during spring and summer.

Pathogenic fungi

Pathogenic fungi cause plant diseases which can alter forest composition by selectively affecting certain species, thereby influencing biodiversity and ecosystem stability. Examples include the worldwide spread disease ash dieback caused by Hymenoscyphus fraxineus; honey fungus (Armillaria spec.) causing oak dieback,  Cryphonectria parasitica, responsible for chestnut blight, and blue stain fungi (Ophiostomataceae), which discolour wood and disrupt water flow in coniferous trees, often in association with bark beetle infestations.

Soil fauna can be divided into microfauna (e.g. nematodes, protozoans), mesofauna (e.g. microarthropods and enchytraeids), macrofauna (e.g. earthworms, termites and millipedes) and megafauna with soil impact (e.g. moles, mice, snakes, deer and wild boar). Soil fauna break down leaf litter and recycle nutrients. They aerate the soil through burrowing, improving root growth and water flow. Soil fauna mix and redistribute soil layers through their movement, influencing soil horizons and promoting organic matter incorporation deeper into the soil. By feeding on microbes and pests, soil fauna help control harmful insect larvae and root pests. Some species help suppress soil-borne diseases by competing with or consuming pathogens.

The food chain links all soil organisms together, as one species group consumes the other

Soil biodiversity in different forest types

Across Europe, forest types vary widely, from the cold boreal zones to the dry Mediterranean, and each supports distinct soil communities shaped by climate, vegetation, and soil conditions.

Northern Europe

Cold, with long winters and short, cool summers

Often acidic, nutrient-poor, and rich in organic layers

Soil life is less diverse but adapted to harsh conditions. Dominated by fungi (especially mycorrhizal fungi), enchytraeids (tiny worms), and cold-tolerant microarthropods. Decomposition is slow, so litter accumulates, and microbial activity is lower

Central and Western Europe

Moderate temperatures, well-distributed rainfall

Generally fertile, loamy, or clay-rich

These forests host rich and diverse soil communities, including earthworms, beetles, springtails, mites, and diverse microbial populations. Deciduous trees provide nutrient-rich litter, supporting high decomposition rates and biological activity.

Southern Europe

Hot, dry summers and mild, wet winters

Often shallow, dry, and nutrient-limited

Soil biodiversity is adapted to drought and nutrient stress. Fauna includes drought-tolerant ants, mites, and microbial communities that can remain dormant during dry periods. Mycorrhizal associations are critical for water and nutrient uptake in these conditions.

Mountains of Europe

Cool temperatures, high precipitation, and elevation gradients

Often acidic and shallow, with organic accumulation

Biodiversity features: Soil biodiversity changes with altitude, where lower zones resemble temperate forests, while higher zones support cold-adapted fungi, springtails, and mites. Steep slopes and erosion risks mean that soil fauna often play key roles in soil stability and aggregation.

Drivers of soil biodiversity

Forest soil biodiversity is influenced by climate, vegetation, soil properties, and human activity. Climate affects temperature and moisture, which directly impact the survival and activity of soil organisms. The type and diversity of vegetation determine the amount and composition of organic matter, shaping food resources and habitat conditions in the soil. Soil characteristics  like pH, texture, and nutrient levels influence which species can thrive, favouring some communities over others. Human actions such as logging, pollution, and land use changes (e.g. conversion to plantations, drainage of forested wetlands) can reduce habitat quality and disrupt ecological balance, often leading to a decline in soil biodiversity.

Ecosystem functions of soil biota

Soil formation is one of the most important functions of soil biota. Soil organisms’ build soil structure and influence its chemical properties by weathering bedrock and decomposing materials, aggregating mineral and organic components.

Functions of soil organisms

Soil respiration
Soil respiration refers to the release of CO₂ from the soil into the atmosphere as a result of both heterotrophic and autotrophic respiration.

Respiration differs between peat soils and mineral soils. Undisturbed peat soils have lower microbial activity which lower respiration. The respiration in mineral soils is more dynamic and influenced by temperature, soil moisture and organic matter available.