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Overview of soil biological properties and associated activities affecting soil biological properties and processes. Click on links to go to a specific section.
Property Effects Desired value1 Management strategies

Soil biological properties, unlike chemical and physical properties, are difficult to define. Various sources provide different lists of what comprises soil biological properties, ranging from considering only soil biota to including biological processes such as respiration, nutrient cycling, mineralization, and organic matter cycling.

We've chosen to list only soil biota and microorganisms as soil biological properties, but we provide a discussion of the processes affected by soil organisms and methods for measuring these processes. We include a discussion of effects of human activities, discussion of stormwater applications, and links to related topics, including information on sampling, testing, and soil health assessments.

Soil biota

There are different ways of classifying soil biota, such as classification based on function, size, or method of producing energy (i.e. autotroph or heterotroph). The following discussion classifies biota by size, using the scheme given by de Neergaard. The three soil biotic groups are microbiota (microorganisms), mesobiota, and macrobiota.

Soil microbiota (microorganisms)

Microbiota are less than 0.2mm and consist of bacteria, actinomycetes, fungi, algae and protozoa. The following table summarizes environmental conditions favorable for each group.

Environmental factor Bacteria Fungi Algae Protozoa Actinomycetes
Carbon source1 Low C:N ratio materials that are easily degraded Stable organic residues with high C:N ratio Carbon dioxide (photosynthesis) Bacteria Stable organic residues with high C:N ratio
Moisture Attach to materials with a film of water Can grow to seek water Moist but well oxygenated Require moist soil Can tolerate a range of moisture conditions
Aeration Generally aerobic but some are anaerobic Generally aerobic Well-oxygenated Aerobic Aerobic
Temperature Optimal at 25-30oC but can tolerate higher temperatures Optimal at 25-30oC but can tolerate lower temperatures Optimal at 25-35oC Optimal at 18-22oC Can tolerate a range of soil temperatures
pH2 5-9; optimum 7 2-7; optimum 5 6-9; optimum >7 5-8; Optimum >7 6.5-9.5; Optimum 8

Bacteria

Bacteria are the most diverse and abundant biotic group in soil. Bacteria are usually in the range of 0.4-2 μm and vary in shape. Some bacteria have flagella, making them mobile, but most often they are attached to surfaces by ion exchange. Bacteria include archaebacteria and eubacteria. Archaebacteria are considered an ancient class of bacteria, often found in harsh environments. Eubacteria are a large group typically having simple cells with rigid cell walls and often flagella for movement. The eubacteria comprises the “true” bacteria and cyanobacteria. Eubacteria play important roles in nutrient and chemical cycling, including nitrogen fixation and organic matter decomposition. Actinomycetes are bacteria, but they have traditionally been considered as an intermediate group between bacteria and fungi. Actinomycetes in soil are particularly specialized towards decomposition of organic matter, including more complex substrates as chitin and hemicellulose, particularly under adverse conditions as high pH, temperatures and water stress.

Considering functionality, the United Stated Department of Agriculture (USDA) divides bacteria fall into four functional groups. Decomposers consume simple carbon compounds, such as root exudates and fresh plant litter. In doing this, bacteria convert energy in soil organic matter into forms useful to other organisms in the soil food web. A second group of bacteria, called mutualist, form partnerships with plants. These include nitrogen-fixing bacteria. A third group of bacteria is the pathogens. A fourth group, called lithotrophs or chemoautotrophs, obtain energy from compounds of nitrogen, sulfur, iron or hydrogen instead of from carbon compounds.

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Fungi

Fungi are eukaryotes, organisms with a true cell nucleus. Most fungi are aerobic heterotrophs. Although bacteria are more numerous in soil, the total mass of fungi and bacteria in soil is similar. The fungi are divided into five main groups, all of which occur in soil as plant and insect pathogens. The majority of soil fungi consist of a filamentous mycelia made up of hyphen, and sexual reproduction takes place via fruiting bodies produced from the mycelium.

Fungi have several important specialized functions in the soil, as plant pathogens, decomposers and as mycorrhizal symbionts with plants. The soil hosts numerous fungi that are plant pathogens, primarily attacking the roots. Some are very host specific and can be suppressed by avoiding susceptible crops for a number of years. Certain decomposition processes of highly complex and recalcitrant structures such as lignin are almost exclusively carried out by a limited group of fungi.

Fungi are capable of moving to their substrate by hyphen growth. Consequently, colonization and decomposition of litters on soil surfaces is mainly carried out by fungi. Fungal decomposers are also capable of acquiring nutrients from one part of their hyphen and carbon substrates from other parts, thereby enabling them to grow on physically separated nutrient and carbon substrates. Some fungi form symbiotic relationships with plants by attaching themselves to the roots. The plants supply the fungi with carbohydrates, and the fungi hyphen act as an extension of the plant roots, enhancing uptake of soil nutrients.

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Algae

Cyanobacteria and algae are photosynthetic organisms, though cyanobacteria are prokaryotic and algae eucaryotic. Cyanobacteria can also fix nitrogen. Algae are among the first organisms to colonize soil and excrete material important to soil aggregation. Soil algae are an important food source and are important in nutrient cycling.

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Protozoa

Protozoa are single-celled animals that feed primarily on bacteria, but also eat other protozoa, soluble organic matter, and sometimes fungi. They are much larger than bacteria and are mobile in soil. Protozoa include ciliates, flagellates, and amoebae. The most prevalent type depends on the primary food source (e.g. bacteria, fungi).

Protozoa play important roles in nutrient cycling through their predation. In particular, they release large amounts of nitrogen. They are also important in controlling bacteria populations in soil. They compete with nematodes.

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Actinomycetes

Actinomycetes are prokaryotes classified as bacteria but unique enough to be discussed as an individual group. Actinomycete numbers are generally one to two orders of magnitude smaller than the total bacterial population. They are an important component of the bacterial community, especially under conditions of high pH, high temperature or water stress.

Actinomycetes possess elongated cells that branch into filaments or hyphae. They are able to utilize a variety of substrates in soil. They can degrade complex organic compounds and are capable of nitrogen fixation. They produce important soil enzymes such as amylase, lipase, and cellulases.

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Soil mesobiota

Soil organisms of intermediate size, including small, invertebrate animals found in the soil. Examples include annelids, arthropods, nematodes, and molluscs. These organisms are readily removed from a soil sample using a tullgren funnel or similar device ([3]). de Neergaard defines mesobiota as organisms ranging from 0.2 to 10 mm in size.

Mesobiota play an important role in nutrient cycling in the soil, primarily through their predation on bacteria and fungi. They can suppress degradation of organic material and reduce nutrient cycling if too abundant, but they generally have a positive effect in soil. Mesobiota tend to be most abundant in the top soil layers due to the greater abundance of organic material, but their activity is also increased in the soil layer surrounding the roots (rhizosphere). Most species are susceptible to soil disturbance and cultivation due to physical disruption, disturbance of pores and pathways, and soil drying. Mesobiota biodiversity decreases as soil disturbance increases.

Soil macrobiota

Larger soil organisms which may be hand-sorted from a soil sample. Examples include burrowing vertebrate animals (e.g. rabbits and moles), larger plant material (e.g. tree roots), and larger insects and earthworms ([4]).

de Neergard defines macrobiota as organisms larger than 10mm. They comprise a smaller fraction of soil biomass and abundance, but are important in soil physical processes and nutrient cycling. Because of their mobility, they affect soil structure and pore geometry and move organic material within the soil profile.

Earthworms are the most important macrobiotic functional group. They facilitate litter turnover by drawing it into the soil and breaking organic material into smaller components, and by mixing litter with soil and soil organisms in their gut. Earthworms increase dispersal of microorganisms and create soil macropores, which are important in water and air transport.

Soil biological processes

  • Carbon and nutrient cycling
  • Biotic interactions (e.g. pests, pest control)
  • Soil formation
  • Effects on soil structure, aggregation, and stability
  • Effects on soil hydrology
  • Pollutant fate and transport

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Measuring soil biology

Suggested reading

This page was last edited on 6 August 2021, at 21:26.

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