Soil formation, involvement of microorganisms

Microorganisms are essential to soil formation and soil ecology because they control the flux of nutrients to plants (i.e., control of carbon, nitrogen, and sulfur cycles,), promote nitrogen fixation, and promote soil detoxification of inorganic and naturally occurring organic pollutants. Soil microorganisms are also part of several food chains, thus serving as source nutrients to one another, and frequently serve as the primary members of food chains in soil biota.

The roots of plants are also part of soil biota and some fungi . Many bacteria live in symbiotic relation to plant roots, around which there is an area of elevated microbial activity, known as rhizosphere. The Animalia kingdom is also represented in soil biota by Nematodes, Earthworms, Mollusks, Acarina, Collembola, as well as several insects and larvae that feed mostly on decaying organic matter. They all take part in the soil food chain and help to promote the conversion of organic matter into bacterial and fungal biomass. Soil microbiology is a relatively recent discipline and it is estimated that about only one percent of soil microorganisms are so far identified.

The soil ecosystem is composed of inorganic matter (calcium, iron oxide, nitrates, sulfur, phosphates, ash, and stone particles), substrates (fallen leaves, dead organisms, rotten wood, dead roots), organisms (microbes, animals, and plants), air, and water. Bacteria and fungi are mostly heterotrophic organisms that feed on the existing organic matter by decomposing them in order to absorb the resultant micronutrients and minerals. Therefore, they are essential to the recycling process of nutrients that keeps soils in good condition for plant growth. The community of microorganisms in a given type of soil differs from that belonging to another soil type. They are highly dependent on environmental factors such as levels of carbon dioxide, oxygen, hydrogen, soil pH (whether acid, alkaline, or neutral), types of substrates, amounts of available substrates, levels of moisture, and temperature. Each community is highly complex, and so far, little is known about the succession of microorganisms in the food chains and the interconnected food webs they form, or about the sequence of events in the cycling pathways of soil ecosystem.

The arrival of new substrate in the soil increases bacterial populations that feed on them, thus recycling in the process, nutrients important to both plants and other soil organisms. Bacterial expansion leads to a second event, known as succession, which is the growth of protozoa populations that predate bacteria. The expansion of protozoa populations triggers the activity of mites, which feed on protozoans. Substrate arrival triggers as well the activity and expansion of fungi populations, which are also decomposers. Some fungal species compete with other fungal species for the same substrates, such as the Pisolithus and the Fusarium. Nematodes are triggered into action and feed on both fungi and other species of nematodes. Some fungi are able to entrap and feed on nematodes too. In the rhizospheres, these populations are more active than in other parts of the soil and atmospheric factors may influence rhizospheres biota. An American research group is studying the response of soil biota in California grasslands to determine the long-term effect of increased levels of carbon dioxide on soil biota dynamics and on plant growth. They found that in a carbon dioxide enriched atmospheric environment, the colonization of plant roots by fungi is augmented, which facilitates carbon and nutrient exchange between the host plants and the fungi (i.e., symbiosis), thus favoring fungi colonies to expand within the soil, as well favoring the growth of grass. Consequently, the number of soil micro arthropods has also increased, since many of them feed on fungi colonies. However, after six years of experimental carbon dioxide atmospheric enrichment, significant increases on bacterial populations were not recorded. Therefore, the experiment succeeded in illustrating one portion of the food chain in grassland soil, and supplied evidence that the induced enhancement of natural-occurring symbiotic relationships in the rhizosphere may be useful for agricultural productive purposes.

See also Bacterial kingdoms; Composting, microbiological aspects; Microbial symbiosis; Microbial taxonomy; Photosynthetic microorganisms; Protozoa; Slime molds