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Soil microorganisms can be classified as bacteria, actinomycetes, fungi, algae and protozoa. Each of these groups has characteristics that define them and their functions in soil. [6] [7] Up to 10 billion bacterial cells inhabit each gram of soil in and around plant roots, a region known as the rhizosphere.
Bacteria live in soil water, including the film of moisture surrounding soil particles, and some are able to swim by means of flagella. The majority of the beneficial soil-dwelling bacteria need oxygen (and are thus termed aerobic bacteria), whilst those that do not require air are referred to as anaerobic , and tend to cause putrefaction of ...
Some nitrogen originates from rain as dilute nitric acid and ammonia, [44] but most of the nitrogen is available in soils as a result of nitrogen fixation by bacteria. Once in the soil-plant system, most nutrients are recycled through living organisms, plant and microbial residues (soil organic matter), mineral-bound forms, and the soil solution.
Bacteria function and reproduce as individual cells, but they can often aggregate in multicellular colonies. [54] Some species such as myxobacteria can aggregate into complex swarming structures, operating as multicellular groups as part of their life cycle , [ 55 ] or form clusters in bacterial colonies such as E.coli .
Some soil actinomycetota (such as Frankia) live symbiotically with the plants whose roots pervade the soil, fixing nitrogen for the plants in exchange for access to some of the plant's saccharides. Other species, such as many members of the genus Mycobacterium , are important pathogens .
Without soil fauna, microbes that directly prey upon competitors of plants, and plant mutualists, interactions within the rhizosphere would be antagonistic toward the plants. Soil fauna provides the rhizosphere's top-down component while allowing for the bottom-up increase in nutrients from rhizodeposition and inorganic nitrogen.
[10] [11] Soil temperature influences biological and biochemical processes in soil, playing an important role in microbial and enzymatic activities, mineralization and organic matter decomposition. [12] Air is vital for respiration in soil organisms and in plant growth. [13] Both wind and atmospheric pressure play critical roles in soil ...
Water, in turn, has a strong effect on soil structure, directly via the dissolution and precipitation of minerals, the mechanical destruction of aggregates [27] and indirectly by promoting plant, animal and microbial growth. Soil structure often gives clues to its texture, organic matter content, biological activity, past soil evolution, human ...