FEDERAL UNIVERSITY LAFIA
FACULTY OF SCIENCE
COURSE TITLE: SOIL MICROBIOLOGY
COURSE CODE: MCB 314
COURSE LECTURER: Mr. UYI GERARD
A TERM PAPER ON MECHANISMS OF MINERAL
TRANSFORMATION IN SOIL
is a complex mixture of minerals, organic matter, gases, liquids, and organisms
that together support life. The Earth’s body of soil is the pedosphere, which
has four important functions: it is a medium for plant growth; it is a means of
water 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,
Soil acts as an engineering medium, a habitat for soil organisms, a recycling
system for minerals and organic wastes, a regulator of water quality, a
modifier of atmospheric composition, and a medium for plant growth, making it a
critically important provider of ecosystem services (Richard, et al, 2002 ). Soil
microorganisms play key geoactive roles in the biosphere particularly in the
areas of element biotransformations and biogeochemical cycling, metal and
mineral transformations, decomposition, bioweathering, soil and sediment
formation. All kinds of microbes, including prokaryotes and eukaryotes and
their symbiotic associations with each other and “higher organisms”, can
contribute actively to geological phenomena, and central to many such
geomicrobial processes are metal and mineral transformations (Gadd, 2013). Minerals in the soil are taken up by the plant through its
roots. To be taken up by a plant, a nutrient element must be located near the
root surface; however, the supply of minerals in contact with the root is
rapidly depleted. There are three basic mechanisms whereby nutrient ions
dissolved in the soil solution are brought into contact with plant roots:
Mass flow of waterDiffusion within waterInterception by root growth
three mechanisms operate simultaneously, but one mechanism or another may be
most important for a particular nutrient (Luther,
et al, 1977).
SOIL MINERALS AND THEIR TRANSFORMATION
MECHANISMS IN SOIL
obtain their carbon from atmospheric carbon dioxide. About 45% of a plant’s dry
mass is carbon. The respiration of CO2 by soil micro-organisms
decomposing soil organic matter contributes an important amount of CO2
to the photosynthesizing plants (Luther, et al,
2013). Atmospheric CO2 is fixed into organic compounds by plants, together with
phototrophic and chemoautotrophic microorganisms. The organic compounds thus
synthesized undergo cellular respiration and CO2 is returned to the
atmosphere. The carbon may have been passed along a food chain to consumers
before this occurs. Carbon dioxide is also produced by the decomposition of
dead plant, animal and microbial material by heterotrophic bacteria and fungi. Methanogenic
bacteria produce methane from organic carbon or CO2.
This in turn is oxidised by methanotrophic
bacteria; carbon may be incorporated into organic material or lost as CO2(Hogg,
Nitrogen is the
most critical element obtained by plants from the soil and nitrogen deficiency
often limits plant growth (Roy, 2006).
Plants can use the nitrogen as either the ammonium cation (NH4+)
or the anion nitrate (NO3?). Usually, most of the
nitrogen in soil is bound within organic compounds that make up the soil
organic matter, and must be mineralized to the ammonium or nitrate form before
it can be taken up by most plants (Luther, et al, 1977).
microorganisms are able to metabolize organic matter and release ammonium in a
process called mineralization. Others take free ammonium and oxidize it
to nitrate. Nitrogen-fixing bacteria are capable of metabolizing N2
into the form of ammonia in a process called nitrogen fixation. The nitrogenase enzyme
complex responsible for the reaction is very sensitive to oxygen. Many
nitrogen-fixing bacteria are anaerobes; those that are not have devised ways of
keeping 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 nitrogen
fixation in thick-walled heterocysts which help maintain anoxic conditions.
Nitrate may also be lost from the soil when bacteria
metabolize it to the gases N2 and N2O. The loss of
gaseous forms of nitrogen to the atmosphere due to microbial action is called denitrification.
A final pathway of nitrogen cycling has only been
discovered in recent years. It is known as anammox (anaerobic ammonia
oxidation), and is carried out by members of a group of Gram-negative bacteria
called the Planctomycetes. The reaction, which can be represented thus:
NH4+ + NO2? = N2 + 2H2O has considerable potential
in the removal of nitrogen from wastewater (Hogg,
The soil mineral
apatite is the most common mineral source of phosphorus. While there is on
average 1000 lb of phosphorus per acre in the soil, it is generally
unavailable 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 and
aluminum. Phosphorus is largely immobile in the soil and is not leached but
actually builds up in the surface layer if not cropped.
Sulphur is found in living
organisms 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 its
elemental form, sulphur is unavailable to most organisms; however, certain bacteria
such as Acidithiobacillus are able to oxidize it to sulphate, a form
that can be utilized by a much broader range of organisms: 2S + 3O2 + 2H2O????????H2SO4.
Powdered sulphur is often
added to alkaline soils in order to encourage this reaction and thereby reduce
the pH. Sulphate-reducing bacteria convert the sulphate to hydrogen sulphide
gas using either an organic compound or hydrogen gas as electron donor:
8H+ + SO42?????????H2S + 2H2O + 2OH? (Hogg,
These bacteria are obligate
anaerobes, and the process is termed dissimilatory sulphate reduction.
Plants are also able to
utilise sulphate, incorporating it into cellular constituents such as the amino
acids methionine and cysteine (assimilatory sulphate reduction). When
the plants die, these compounds are broken down, again with the release of
Green and purple
photosynthetic bacteria and some chemoautotrophs use hydrogen sulphide as an
electron donor in the reduction of carbon dioxide, producing elemental sulphur
and thus completing the cycle:
H2S + CO2????????(CH2O)n + S0
In soil mineral transformation, soil
microorganisms play a very important role in the conversion of certain minerals
forms to substances that can be used by other life forms in the soil. Key soil
minerals in the soil such as Carbon, Sulphur, Nitrogen and Phosphorus are most
often in forms that are unusable by plants. In light of this, mechanisms have
been developed over time by soil microorganisms in order to utilize the
minerals in the soil and this process often results into redox reactions which
births substances which may become usuable by other life forms in the soil.
Each mineral transformation has its own mechanisms, therefore, requires
microorganisms that can act on the mineral to either kick-start or facilitate
the transformation process.
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