Biogeochemical cycles

2. It is a pathway by which a chemical
substances moves through biotic and
abiotic compartments of earth.

4. contd..
 Nitrogen gas in the atmosphere is the major source
of this element . Nitrogen is transformed from the
gaseous state to organic form ( nitrogen fixation)
to inorganic compouinds ( decomposition and
mineralization ) and back to gas ( denitrification)
 Bacteria and blue green algae reduce nitrogrn gas
to ammonia and use ammonia in amino acid
synthesis.

5. contd..
 Nitrogen fixation
 Nitrification
 Assimilation
 Ammonification
 Denitrification

6. contd..

7. contd...
 Nitrogen enters the water through the precipitation, runoff,
or as N2 from the atmosphere.
 Nitrogen cannot be utilized by phytoplankton as N2 so it
must undergo nitrogen fixation which is performed
predominately by cyanobacteria.
 Ammonia and urea are released into the water by excretion
from aquatic organisms. Bacteria are able to convert
ammonia to nitrite and nitrate .
 Ammonification or Mineralization is performed by
bacteria to convert organic nitrogen to ammonia.
 Nitrification can then occur to convert the ammonium to
nitrite and nitrate. N2 can be returned to the atmosphere
through denitrification.

9.  Phosphates move quickly through plants and animals;
however, the processes that move them through the soil or
ocean are very slow, making the phosphorus cycle overall
one of the slowest biogeochemical cycles.
 The global phosphorus cycle includes
(i) exposure of phosphorus-bearing rocks such as apatite to
surface weathering
(ii) physical erosion, and chemical and biological weathering
of phosphorus-bearing rocks to provide dissolved and
particulate phosphorus to soils
(iii) riverine and subsurface transportation of phosphorus to
various lakes

10.  Phosphorus retention by soil minerals (e.g., adsorption
onto iron and aluminum oxyhydroxides in acidic soils)
 This process can lead to the low level of dissolved
phosphorus concentrations in soil solution. Various
physiological strategies are used by plants and
microorganisms for obtaining phosphorus from this low
level of phosphorus concentration.
 Soil phosphorus is usually transported to rivers and lakes
and can then either be buried in lake sediments or
transported to the ocean via river runoff. Atmospheric
phosphorus deposition is another important marine
phosphorus source to the ocean.

11.  In surface seawater, dissolved inorganic phosphorus,
mainly orthophosphate (PO4
3−), is assimilated by
phytoplankton and transformed into organic phosphorus
compounds.
 Phytoplankton cell lysis releases cellular dissolved
inorganic and organic phosphorus to the surrounding
environment. Some of the organic phosphorus compounds
can be hydrolyzed by enzymes synthesized by bacteria and
phytoplankton and subsequently assimilated.
 The vast majority of phosphorus is remineralized within
the water column, and approximately 1% of associated
phosphorus carried to the deep sea by the falling particles is
removed from the ocean reservoir by burial in sediments.

13. potassium

14. contd..
 Potassium is a macronutrient taken up by plants in
large quantities.
 Potassium does not form gases that could be lost
to the atmosphere.
 Unlike N, P and S, K is present in the soil solution
only as a positively charged cation (K+) .
 On weathering potassium bearing minerals, the
release of potassium takes place in the soil solution
which can be either directly taken up by plant root
or be adsorbed on soil colloids.
 Potassium is necessary to many plant functions,
osmotic regulation, and protein synthesis etc.

15. contd..
 The micas and the feldspar constitute the major
Potassium bearing minerals which on weathering
gradually release the potassium into soil. besides
this secondary minerals like kaolinite , Halloysite
etc releaae potassium into the soil.
 Fertliser application such as Muriate of Potash etc.
 There are four forms of potassium in soil are :
• soil solution
• exchangeable potassium
• non exchangeable potassium
• reserve potassium

16. contd..
 Exchangable and soil solution potassium
equilibrate quickly while difficuilty or fixed form of
potassium equilibrates very slowly with its
exchangeable and soil solution forms.
 Release of potassium from the reserve potassium
to any other form of potassium is extremely slow
and hence it is strictly considered unavailable form
of potassium to plants.

17. Sulphur

18. contd..
 On early Earth, most sulfur was present in
minerals such as pyrite (FeS2). Over Earth history,
the amount of mobile sulfur increased through
volcanic activity as well as weathering of the crust
in an oxygenated atmosphere. Earth's main sulfur
sink is the oceans SO42−.
 When SO42− is assimilated by organisms, it is
reduced and converted to organic sulfur, which is
an essential component of proteins.

19. contd..
 Atmospheric sulpur originates from volcanicn activity, and
through combustion of sulphur contaninig fossil fuels.
 Sulphur can be absorbed directly from the atmosphere by
plants and incorporated into organic matter.
 It also can be oxidized to sulphuric acid in the atmosphere
and reach the land surface through rain. Sulphuric acid in
rainfall is the major cause of acid rain.
 Organic matter in the soil decomposes as sulphur is
released ( mineralized) as sulphide, which in the presence of
oxygen is oxidized to sulphate. the oxidation oflphide to
sulphhate. sulphate in soil is highly soluble. it is adsorbed
by plants or microorganism and it may be leached from the
soil.

20.  Dimethylsulfide is produced by the decomposition of
dimethylsulfoniopropionate (DMSP) from dying
phytoplankton cells in the ocean's photic zone, and is the
major biogenic gas emitted from the sea, where it is
responsible for the distinctive “smell of the sea” along
coastlines.
 DMS is the largest natural source of sulfur gas, but still
only has a residence time of about one day in the
atmosphere and a majority of it is redeposited in the oceans
rather than making it to land.

21.  The calcium cycle links ionic and non ionic calcium
together in both marine and terrestrial environments and is
essential for the functioning of all living organisms.
 In animals, calcium enables neurons to transmit signals by
opening voltage gated channels that allow
neurotransmitters to reach the next cell, bone formation
etc.
 In plants, calcium promotes enzyme activity and ensures
cell wall function, providing stability to plants.
 It also enables crustaceans to form shells and corals to
exist, as calcium provides structure, rigidity and strength to
structures when complexed (combined) to other atoms.

22. Calcium

23.  When lime is leached into soil, calcium levels inevitably
increase, both stabilising pH and enabling calcium to mix
with water to form a Ca 2+ ions, thus making it soluble and
accessible to plants to be absorbed and utilised by the root
system.
 Plant and animal death results in the return of calcium
contained within the organism back into the soil to be
utilised by other plants.
 Decomposing organisms break them down, returning the
calcium back into the soil and enabling the cycling of
calcium to continue.
 Additionally, these animals and plants are eaten by other
animals, similarly continuing the cycle.

24. Calcium when deposited in the form of lime, it
can't be used by plants. To combat this, carbon
dioxide produced by plants reacts with water in
the environment to produce carbonic acid.
Carbonic acid is then able to dissolve limestone,
enabling the release of calcium ions. This
reaction is more readily available with smaller
particles of limestone than it is with large pieces
of rock due to the increased surface area.

26. Magnesium

27.  Magnesium in the environment is usually found in its ionic
form, Mg2+, which is found dissolved in the soil solution. It
is a very important nutrient for plants.
 The rocks and clay will, over time, weather away and
minerals like magnesium are released, but the process can
be very slow.
 Humans impact the amount of magnesium that is delivered
through the crops and soil. Many humans put fertilizers or
manure on the soil around the plants to make them grow
healthier.
 Things like crop removal can make magnesium levels
decline.

28.  The Mg2+ is then transfered to below ground.
 Mg is absorbed by plant from the soil solution as Mg2+ ion.
 Mg is also present in soil as its various form like water
soluble, exchangeable and non exchangeable. All these
forms are in equilibrium.
 Exchangeable and water soluble are important in plant
nutrition. in some soils, non exchnageable Mg may play
great role in plant growth.

29.  Silicon commonly occurs in nature as silicon
dioxide (SiO2) also callede as silica.
 The silica cycle is the biogeochemical cycle in
which silica is transported between the Earth's
systems. Opal silica (SiO2) is a chemical compound
of silicon, and is also called silicon dioxide. Silicon
is considered a bioessential element and is one of
the most abundant elements on Earth.
 The primary source of silicate to the terrestrial
biosphere is weathering.
 Wollastonite (CaSiO3) and enstatite (MgSiO3) are
examples of silicate-based minerals.

30.  Silicate is transported by rivers and can be deposited in soils
in the form of various siliceous polymorphs.
 Plants can readily uptake silicate in the form of Silicic acid
(H4SiO4) it is a very important nutrient in ocean, for the
formation of phytoliths.
 Phytoliths are tiny rigid structures found within plant cells
that aid in the structural integrity of the plant.
 Silica release from phytolith degradation or dissolution is
estimated to occur at a rate double that of global silicate
mineral weathering.
 Considering biogeochemical cycling within ecosystems, the
import and export of silica to and from terrestrial ecosystems
is small.

31.  A small no. reach the ocean floor, where they remain or
forming a siliaceous ooze, and return to the photic zone by
upwelling.
 Silicate is an essential chemical requirement only for
certain biota diatom, radiolaria, silicoflagellate.
 These organism extract dissolve silicate from open surface
water for build of skeletal structure.
 When they died part of siliceous skeletal material dissolves
and settle through the water column.
 This help in enriching the deep water with dissolved silica.

33. • Brady.N.C., 2001, The Nature and Properties
of Soils, pp 315-380
• Das.D.K., 1996, Introductory Soil Science, pp
320-399