Soil biodiversity
Soil biodiversity is the variety of life forms, animal, plant, and microbial, present in a soil for at least part of their biological cycle. Soil biodiversity includes inhabitants of the soil matrix as well as those of the “soil annexes” (litter, decomposing dead wood, animal carcasses, excrement). These organisms play a major role as carbon sinks, soil producers, and in terrestrial biogeochemical cycles. The identification, counting, and characterization of the diversity of living soil organisms allow defining indicators (or bioindicators) of soil quality and the underground (and sometimes aerial) environment.
What is its role?
Its role is considerable and very varied: humification and mineralization, mycorrhization, atmospheric nitrogen fixation, plant defense by endophytic fungi.
The activity of these organisms underpins many essential ecosystem services for human societies:
- Soil fertility: Soil organisms indirectly support the quality and abundance of plant production by renewing soil structure, enabling the decomposition of organic matter, and facilitating the assimilation of mineral nutrients available to plants.
- Crop protection: Having high soil biodiversity increases the likelihood that soils host a natural enemy of crop diseases. Maintaining or promoting soil organism diversity thus helps limit pesticide use.
- Water cycle regulation and soil erosion control: The presence of earthworms promotes water infiltration into the soil by increasing the permeability of surface horizons. -Water and soil decontamination: Microorganisms can immobilize and degrade pollutants.
For more information on the subject: https://agriculture-de-conservation.com/sites/agriculture-de-conservation.com/IMG/pdf/atlas-diversite-sol.pdf
Different soil organisms
Various organisms participate in soil life and are generally classified by size.
Megafauna
Salamanders, moles, mice, shrews and squirrels. They benefit from soil biodiversity to feed, shelter, defend themselves, and rest. In turn, they provide organic matter that ensures soil health and mechanically fragment matter into smaller particles.
Macrofauna
Soil macrofauna is mainly represented by earthworms (endogeic, epigeic, and anecic) and beetles. Earthworm species are grouped into three categories:
- endogeic (small geophagous worms living permanently in the first centimeters of soil in a network of horizontal galleries).
- anecic (a term for large saprophagous worms living in large galleries, which can descend to 3 m depth, the “ploughers” of the soil).
- epigeic (small red worms living in litter, on the surface, and generally not digging galleries).
Earthworms create holes and tunnels that aerate the soil and allow water and air entry. They cut waste into smaller particles usable by organisms at the lower trophic level.
Mesofauna
Mesofauna consists of animals about 0.2 to 4 mm in size. Springtails and mites are the two main representatives of this group.
- Springtails: a group with high diversity (about 8,000 known species, including more than 2,000 in Europe). Springtails are mainly decomposers, feeding on mycelial hyphae and organic matter, thus mostly found in soil and litter (leaves and dead wood). They play a major role in litter decomposition processes by fragmenting and mixing organic matter and help regulate microbial populations.
- Mites: they colonize many habitats, especially those rich in organic matter (peat, decomposing wood, litter, etc.). Like springtails, most mites contribute to fragmenting organic matter by producing fecal pellets and thus indirectly regulate microbial communities. Some are predators (mainly parasitiformes), others parasites (of plants or animals), or decomposers (mainly acariformes). Some species carry bacteria and fungi on their surface, aiding their dissemination.
Microfauna
Fungi and mycorrhizae
Fungi are commonly classified into two groups:
- yeasts: unicellular
- fungi: multicellular, forming branches called hyphae (thin white filaments observed on the soil or leaf surface). These filaments constitute a very significant biomass (several tons per hectare) and also an impressive network circulating through the soil over long distances: one square meter of grassland or forest soil contains several kilometers of hyphae.
Fungi can be classified according to their feeding mode in the soil: saprophyte, mycorrhizal, endophytic, and pathogenic.
Mycorrhizal fungi form symbiotic associations with plant roots. These plant-fungus symbioses are extremely widespread and concern 80 to 90% of plant species. In this mutually beneficial relationship, the fungus receives from the plant elements necessary for its growth such as sugars and vitamins. It absorbs various soil elements, including phosphorus, which it transfers back to the plant, greatly increasing the soil volume explored by plants for their growth.
Two main groups of mycorrhizal fungi are distinguished: ectomycorrhizal and endomycorrhizal.
- Ectomycorrhizal fungi form a network of hyphae on the surface of colonized plant roots. The plant hosts of these fungi are mainly shrubs and trees.
- Endomycorrhizal fungi form intracellular structures, with hyphae entering root cells to perform close exchanges with the plant.
Nematodes
Nematodes are animals with a very simple organization found in all environments: marine, freshwater, soils, animals, or aerial parts of plants. These organisms are very abundant in all soil types.
Many species are plant parasites (phytophagous) and can cause serious problems for crops.
There are also many free-living nematode species that promote organic matter decomposition, including bacterivores, fungivores, omnivores, and even predators.
Microorganisms
Bacteria are the most numerous and diverse soil microorganisms. They exhibit a wide variety of shapes and sizes (mostly under 2 µm). It is estimated that one gram of soil contains about one billion bacteria and between 2,000 and 10,000 bacterial species depending on soil health.
Bacteria play a key role notably in nutrient recycling, development (through symbiosis formation), and soil structuring. They also contribute to disease regulation and the depollution of contaminated soils. They interact in the plant rhizosphere (the soil region directly formed and strongly influenced by the association of roots and soil microorganisms) with which they can form symbioses with plants and can also be pathogenic to both animals and plants.
Primarily through their mineralization functions (mineralization of organic matter, redox of inorganic compounds, solubilization or precipitation of minerals, transformation of more or less recalcitrant organic compounds…), bacteria play an essential role in soil functioning. They are thus at the base of regulating the main biogeochemical cycles of soils (carbon, nitrogen, phosphorus, sulfur…) and are capable of:
- reducing sulfates to sulfites and sulfides (sulfate-reducing bacteria),
- oxidizing sulfur (e.g., Thiobacillus),
- fixing atmospheric nitrogen (diazotrophy alone or in symbiosis with plants),
- producing nitrates (nitrifying bacteria),
- making phosphorus available (e.g., via alkaline phosphatase)
Moreover, some bacteria, notably those called PGPR (Plant Growth Promoting Rhizobacteria), can develop symbioses with plants, increasing nutrient availability for their growth.
How to improve soil biodiversity?
The abundance and diversity level vary from soil to soil, depending on factors such as organic matter content, soil texture, pH, and soil management practices.
Some examples of practices favorable to soil biodiversity
Increase soil organic matter content
Regular inputs of organic matter improve soil structure, increase water and nutrient retention capacity, protect soil against erosion and compaction, and support the development of a healthy soil organism community. Practices such as maintaining crop residues on the soil surface, rotations including plants with high residue rates, cover crops, reduced or no tillage systems, or spreading compost or other organic waste products increase soil organic matter content.
Limit agrochemical inputs and soil contamination
The use of pesticides and chemical fertilizers boosts yields but active substances can harm soil organisms. Additionally, inputs of voluntary contaminants (e.g., Bordeaux mixture based on copper) or involuntary ones (e.g., cadmium in fertilizers, mercury in sewage sludge, zinc in slurry) can influence soil organisms, leading to biodiversity changes.
Prevent soil compaction
Soil compaction by repeated machinery passes, especially on wet soil, reduces air, water, and space available for roots and soil organisms. Since remediation is difficult or impossible, prevention is essential (e.g., use of low-pressure tires, reducing the number of passes).
Minimize erosion risk
Bare soil is sensitive to Soil erosion by wind and water, drying, and crusting. The presence of vegetation cover or crop residues protects the soil, provides habitats for soil organisms, and can improve water and nutrient availability.
