Soil Biology Part 2

The role of earthworms: Worms represent the largest biomass of any species on earth, attaining between 1-4 tons per hectare of land depending on conditions. It is estimated 250 000 worms pass between 300-600 tons of soil through their digestive tubes. Their numbers can dramatically drop by 80% in heavily compacted and polluted soils or double in healthy conditions, especially in the presence of nitrogen fixing plants. The soils they live in are more stable and less prone to erosion as they ingest soil and reject it in the form of agregates,the chemical properties of which are modified. The more neutral ph that results from their activity is important in nutrient uptake, the ideal being around 6.4, a level at which the uptake by plants of most essential minerals is maximimised. Worms dig galleries which accelerate the infiltration of water thus limiting run off and erosion as well as improving aeration and the circulation of liquid and gases which reach roots more easily. Furthermore, these galeries permit roots to spread more easily and increase the nutrient exchange between the soil and the plant. Along with other microorganisms, worms also play a role in the recyclage of organic material and the enrichment of soils. Particularly active close to root systems, they fragment and bury organic residuals as well as participating in their decomposition through ingestion and digestion. Thus, they mix mineral particles from the deeper soils and spread them throughout higher levels during their movemements. Their dejections on the walls of their galleries also induces a modification of the environment and stimulates the activity of microorganisms.

‍Root exudates : The rhizosphere is the volume of soil around living roots which is influenced by root activity. Plants have the ability to secrete a wide range of compounds into the rhizosphere such as amino acids,organic acids, sugars, polysaccarides, and protein compounds. These then regulate soil microbial activity to foster beneficial symbiosis and change the the chemical and physical properties of the soil, as well as secreting compounds that may mediate natural disease resistance by secreting proteins which defend the plant against potential soil-borne pathogens. For example, a recent study showed that plants that do not produce rhizathaline were found to be more susceptible to herbivorous insects. These exudates are also a high level source of carbon which acts as chemo-attractants for bacteria thus increasing bacterial colonisation of the rhizosphere. Root exhudation can also play a major role in acquiring other micronutrients such as iron and they may enhance the activity of phosphate-solubilising bacteria thus increasing the plants supply of phosphates.

The role of mycorrhizal fungi: These fungi are incapable of completing their life cycle in the absence of roots, so they penetrate and colonise them. They then differentiate into highly branched structures known as arbuscules which are thought to be the principle sites of nutrient exchange between the 2 organisms. The mycorrhizes actually break down organic material and exchange the minerals for sugars produced by photosynthesis of the plant.

The role of bacteria: Certain types of bacteria are also found around root systems where they feed off dead root cells and root exhudates. These microbes desagregate and depolymerise organic matter, providing another source of minerals which can then be absorbed by the plant.  

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