FEDERAL UNIVERSITY LAFIAFACULTY OF SCIENCEDEPARTMENT OFMICROBIOLOGY COURSE TITLE: SOIL MICROBIOLOGYCOURSE CODE: MCB 314COURSE LECTURER: Mr. UYI GERARD A TERM PAPER ON MECHANISMS OF MINERALTRANSFORMATION IN SOIL BYGROUP 7 Group members:20512000082051500039205150001120515000142051500016 Brief IntroductionSoilis a complex mixture of minerals, organic matter, gases, liquids, and organismsthat together support life. The Earth’s body of soil is the pedosphere, whichhas four important functions: it is a medium for plant growth; it is a means ofwater storage, supply and purification; it is a modifier of Earth’s atmosphere;it is a habitat for organisms; all of which, in turn, modify the soil (Simonson, 1957).Soil acts as an engineering medium, a habitat for soil organisms, a recyclingsystem for minerals and organic wastes, a regulator of water quality, amodifier of atmospheric composition, and a medium for plant growth, making it acritically important provider of ecosystem services (Richard, et al, 2002 ).
Soilmicroorganisms play key geoactive roles in the biosphere particularly in theareas of element biotransformations and biogeochemical cycling, metal andmineral transformations, decomposition, bioweathering, soil and sedimentformation. All kinds of microbes, including prokaryotes and eukaryotes andtheir symbiotic associations with each other and “higher organisms”, cancontribute actively to geological phenomena, and central to many suchgeomicrobial processes are metal and mineral transformations (Gadd, 2013). Minerals in the soil are taken up by the plant through itsroots. To be taken up by a plant, a nutrient element must be located near theroot surface; however, the supply of minerals in contact with the root israpidly depleted. There are three basic mechanisms whereby nutrient ionsdissolved in the soil solution are brought into contact with plant roots:Mass flow of waterDiffusion within waterInterception by root growthAllthree mechanisms operate simultaneously, but one mechanism or another may bemost important for a particular nutrient (Luther,et al, 1977). SOIL MINERALS AND THEIR TRANSFORMATIONMECHANISMS IN SOIL i. CarbonPlantsobtain their carbon from atmospheric carbon dioxide. About 45% of a plant’s drymass is carbon.
The respiration of CO2 by soil micro-organismsdecomposing soil organic matter contributes an important amount of CO2to the photosynthesizing plants (Luther, et al,2013). Atmospheric CO2 is fixed into organic compounds by plants, together withphototrophic and chemoautotrophic microorganisms. The organic compounds thussynthesized undergo cellular respiration and CO2 is returned to theatmosphere. The carbon may have been passed along a food chain to consumersbefore this occurs. Carbon dioxide is also produced by the decomposition ofdead plant, animal and microbial material by heterotrophic bacteria and fungi.
Methanogenicbacteria produce methane from organic carbon or CO2. This in turn is oxidised by methanotrophicbacteria; carbon may be incorporated into organic material or lost as CO2(Hogg,2005). ii. NitrogenNitrogen is themost critical element obtained by plants from the soil and nitrogen deficiencyoften limits plant growth (Roy, 2006).Plants can use the nitrogen as either the ammonium cation (NH4+)or the anion nitrate (NO3?).
Usually, most of thenitrogen in soil is bound within organic compounds that make up the soilorganic matter, and must be mineralized to the ammonium or nitrate form beforeit can be taken up by most plants (Luther, et al, 1977). Somemicroorganisms are able to metabolize organic matter and release ammonium in aprocess called mineralization. Others take free ammonium and oxidize itto nitrate.
Nitrogen-fixing bacteria are capable of metabolizing N2into the form of ammonia in a process called nitrogen fixation. The nitrogenase enzymecomplex responsible for the reaction is very sensitive to oxygen. Manynitrogen-fixing bacteria are anaerobes; those that are not have devised ways ofkeeping the cell interior anoxic. Azotobacter species, for example,utilise oxygen at a high rate, so that it never accumulates in the cell,inactivating the nitrogenase. Many cyanophytes (blue-greens) carry out nitrogenfixation in thick-walled heterocysts which help maintain anoxic conditions.Nitrate may also be lost from the soil when bacteriametabolize it to the gases N2 and N2O. The loss ofgaseous forms of nitrogen to the atmosphere due to microbial action is called denitrification.
A final pathway of nitrogen cycling has only beendiscovered in recent years. It is known as anammox (anaerobic ammoniaoxidation), and is carried out by members of a group of Gram-negative bacteriacalled the Planctomycetes. The reaction, which can be represented thus:NH4+ + NO2? = N2 + 2H2O has considerable potentialin the removal of nitrogen from wastewater (Hogg,2005). iii.
PhosphorusThe soil mineralapatite is the most common mineral source of phosphorus. While there is onaverage 1000 lb of phosphorus per acre in the soil, it is generallyunavailable in the form of phosphates of low solubility. When phosphorus does form solubilized ions of H2PO4?,they rapidly form insoluble phosphates of calcium or hydrous oxides of iron andaluminum. Phosphorus is largely immobile in the soil and is not leached butactually builds up in the surface layer if not cropped. iv. SulphurSulphur is found in livingorganisms in the form of compounds such as amino acids, coenzymes and vitamins.It can be utilized by different types of organisms in several forms. In itselemental form, sulphur is unavailable to most organisms; however, certain bacteriasuch as Acidithiobacillus are able to oxidize it to sulphate, a formthat can be utilized by a much broader range of organisms: 2S + 3O2 + 2H2O????????H2SO4.
Powdered sulphur is oftenadded to alkaline soils in order to encourage this reaction and thereby reducethe pH. Sulphate-reducing bacteria convert the sulphate to hydrogen sulphidegas using either an organic compound or hydrogen gas as electron donor:8H+ + SO42?????????H2S + 2H2O + 2OH? (Hogg,2005).These bacteria are obligateanaerobes, and the process is termed dissimilatory sulphate reduction.Plants are also able toutilise sulphate, incorporating it into cellular constituents such as the aminoacids methionine and cysteine (assimilatory sulphate reduction).
Whenthe plants die, these compounds are broken down, again with the release ofhydrogen sulphide. Green and purplephotosynthetic bacteria and some chemoautotrophs use hydrogen sulphide as anelectron donor in the reduction of carbon dioxide, producing elemental sulphurand thus completing the cycle:H2S + CO2????????(CH2O)n + S0 CONCLUSIONIn soil mineral transformation, soilmicroorganisms play a very important role in the conversion of certain mineralsforms to substances that can be used by other life forms in the soil. Key soilminerals in the soil such as Carbon, Sulphur, Nitrogen and Phosphorus are mostoften in forms that are unusable by plants. In light of this, mechanisms havebeen developed over time by soil microorganisms in order to utilize theminerals in the soil and this process often results into redox reactions whichbirths substances which may become usuable by other life forms in the soil.Each mineral transformation has its own mechanisms, therefore, requiresmicroorganisms that can act on the mineral to either kick-start or facilitatethe transformation process.
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