The Role of Organisms in Soil Functions and Processes

Soil organisms play key roles in ecosystems through their effects on physical properties and processes, and the biological contributions to carbon and energy fluxes and cycling of nutrients. The importance of soil fauna for soil physical

Microflora and microfauna Mesofauna

Bacteria 100 lm

Fungi

Macro- and megafauna 2 mm 20 mm

Nematoda

Protozoa

Rotifera

Acari

Collembola Protura

Diplura

Symphyla Enchytraeidae Chelonethi

Isoptera

Opiliones Isopoda

Amphipoda Chilopoda

Diplopoda

>lopi Me egadrili (earthworms)

Coleoptera

Araneida

Mollusca

Body width

Microflora

Microfauna

Body diameter

Trophic interaction actual

Energy flow

"Engineering" long-term

Habitat formation

Predation

Mesofauna 0.1-2 mm Predation

Macrofauna >2 mm

Grazing

Predation

Substrate processing

Pore formation Litt^r fragment.ation, M-► Pore formation

Pore formation Litt^r fragment.ation, M-► Pore formation

Litter fragmentation, bioturbation

Figure 3 (a) Size classification of soil organisms by body width. (b) The interactions of soil organisms are dependent on size of the organism. Depicted here are trophic interactions and ecosystem engineering effects. Note that with increasing size the relative effects on energy flow decrease, while the effects on habitat formation increase. (a) Swift MJ, Heal OW, and Anderson JM (1979) Decomposition in Terrestrial Ecosystems. Oxford: Blackwell Scientific Publications. (b) From Scheau S and Setala H (2002) The soil food web: Structure and perspectives. In: Tscharntke B and Hawkins BA (eds.) Multitrophic Level Interactions, pp. 223-264. Cambridge, UK: Cambridge University Press.

mm properties generally increases with larger body sizes. Soil macrofauna, such as earthworms, ants, and termites, can have dramatic effects on soil porosity, creating macropores and tunnels that allow for preferential flow of water into the soil profile. The movement of macrofauna through the soil profile (such as some species of earthworms) can mix mineral particles from one horizon into another, and can bring fragments of leaf litter from the surface to mineral soil horizons, thus affecting soil texture, bulk density, and organic matter contents. A notable exception to this size relationship is the role of microbes in the formation of soil aggregates. The activity of microbes, particularly myccor-hizal fungi, produces exudates that help to bind together soil particles into aggregates.

Formation Food Chain Ecosystem

Figure 4 A conceptual model of a shortgrass prairie soil food web. From Bardgett RD (2005) The Biology of Soil: A Community and Ecosystem Approach (Biology of Habitats). Oxford: Oxford University Press. Adapted from deRuiter PC, Neutel A-M, Moore JC (1995) Energetics, patterns of interaction strengths, and stability in real ecosystems. Science 269(5228): 1257-1260.

Figure 4 A conceptual model of a shortgrass prairie soil food web. From Bardgett RD (2005) The Biology of Soil: A Community and Ecosystem Approach (Biology of Habitats). Oxford: Oxford University Press. Adapted from deRuiter PC, Neutel A-M, Moore JC (1995) Energetics, patterns of interaction strengths, and stability in real ecosystems. Science 269(5228): 1257-1260.

Roughly 80-90% of net primary production enters the soil as dead plant material (e.g., leaf litter, stems, roots) or organic exudates from roots. This is decomposed primarily by bacteria and fungi in the soil and surface litter layer. It is the physiological processes of, and enzymatic digestion by the microbes, as well as the trophic interactions in the soil food web that control the cycling of nutrients in the soil. Decomposition is the transformation of an organic substrate from one form to another, and is the source of CO2 that is respired from soils, and the recycling of nutrients.

Decomposition is controlled by the interactions of the physical environment, the quality of the substrate, and the community decomposer organisms. Temperature and soil moisture are the dominant controls on decomposition, with warmer, wetter soils generally having faster rates of organic matter decomposition. Substrate quality is a function of the stoichiometry of the plant material and the composition of the litter in terms of structural components (lignin, cellulose) and secondary metabolites (tannins, phenolics), which can decrease the palatability of litter and make it more recalcitrant. The composition of the community of soil organisms mediates decomposition because different organisms have different functional roles in soils. For example, the fungal community will display successional patterns as leaf chemistry is changed during the process of decomposition, matching the suite of active fungi to substrate quality. Exclusion studies have demonstrated that soil fauna also play a role in controlling the decomposition process. The microbial loop refers to the stimulation of nutrient availability via the trophic interactions between bacteria and their consumers

(protozoa and nematodes), where predation on microbes stimulates their population growth and enhances rates of nutrient cycling. Earthworm casts provide a physical and chemical environment that is more suited to microbial activity than the surrounding bulk soil. So, while decomposition processes are driven by the extracellular enzymes of microbes, the production of enzymes and the environment in which they function is regulated by soil fauna.

Soils are notoriously heterogeneous habitats when viewed from the perspective of the soil flora and fauna. Over the scale of micro- and millimeters organisms may encounter films of water clinging to soil particles, air-filled soil pores, concentrations of organic material, aggregates of soil, and plant root hairs. One conceptual construct for managing the complexity of soils in ecological studies is to think of 'hot spots' of activity, where because of concentrations of available resources and/or microclimatic conditions, organism growth and activity is concentrated. These hot spots in soils make up a small portion of the total volume of soil (up to 10%) but will contain the majority of biological activity in soils (over 90%). The hot spots are located in (1) the drillosphere (area influenced by earthworm burrows), (2) the rhizo-sphere (area influenced by plant roots), (3) the aggregatusphere (soil aggregates where soil mineral particles are bound with organic materials), (4) the detritusphere (larger concentrations of leaf litter and organic matter), and (5) the porosphere (the spaces between soil aggregates). It is within these hot spots that habitats and resources are suitable for soil organisms, and where most decomposition and nutrient cycling happens in soils. Because of their effect on the spatial distribution of these hot spots, the distribution, composition, and physiology of the plant community thus can have very strong controls on organism activities in soils. Through their controls on nutrient cycling, the availability of nutrients in the soil, and soil physical characteristics, the activity and composition of the soil microbes and fauna, in turn, has a strong influence on the plant community. The nature and degree of such above- and belowground linkages is a major research question in contemporary soil ecology.

While specific soil organisms or functional types are linked to decomposition and nutrient-cycling processes, the explicit link between levels of biodiversity and soil ecosystem function is still in question. While this is likely the result of the high functional redundancy in the community of soil organism, this trend is also partly due to problems of taxonomic resolution mentioned above. However, recent advances in molecular techniques hold the promise of being able to move past problems of in situ identification of soil microbes. Techniques can now identify the presence of genes that code for specific biogeochemical processes (such as, lignin decomposition, nitrification, and phosphorus utilization), and some techniques are roughly quantifiable, so it is possible to see how much of a particular gene is in the soil, actively working on an enzymatic process. The integration of these molecular techniques into studies of biogeochemistry is in its infancy, but holds strong promise for allowing researchers to link specific organisms and physiological processes at the level of microbes to a specific nutrient cycling pathway in ways that were previously unthinkable.

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  • charley davidson
    What are roles of macrofauna in soil?
    1 year ago

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