B_i_o_g_e_o_c_h_e_m_i_c_a_l_cycle._p_p_t

Biogeochemical cycles

1. BIOGEOCHEMICAL BIOGEOCHEMICAL
CYCLESCYCLES

2. Definition :
Biogeochemical cycle is defined as a cyclic pathway which brings about
the circulation of chemical elements from the environment to the
organisms & back to the environment.
BIO + GEO + CHEMICAL
This pathway involves living organisms (bio)

A series of chemical reactions (chemical) in abiotic environment (geo).
• Cycling elements:
Macronutrients : Required in large amount. C, H, O, N, P, S, K, Ca, Fe,
Mg.
Micronutrients : Required in small amount but still necessary. B, Cu,
Mo.

4. Pools & Fluxes
•All bioelements reside in compartments or defined spaces in nature.
•A compartment contains a certain quantity, or pool, of bioelements.
•The reservoir portion is called the unavailable pool and cycling portion the
available pool.
•E.g.. The 3 natural pools of Carbon:-
CO
2
stored in atmosphere-: 0.03%
C stored in oceans-: soluble elements like HCO
3
,CO
2
,CO
3
2-
.
C stored in continental biosphere-C in fossil fuels.
•The rate of movement of different elements between pools through the
biosphere is called the flux rate. It is measured as quantity of nutrients passing
from one pools to another per unit of time.
•Flux rate depends upon the physical & chemical properties of each element &the
use to which the living organisms put them. Thus flux refers to the flow of
materials.
•The average length of time a bioelement remains in a compartment is called the
mean residence time

5. Concept of mass-balance
budgets:
Scientist construct budgets to study how elements are gained and
lost in ecosystems.
•Some ecosystems are in steady state: this occurs when fluxes are
balanced. Therefore:
inputs = outputs
•Inputs sometimes exceed outputs, and elements accumulate in
compartments of ecosystems. Therefore:
Inputs > outputs = storage (sink)
(e.g., community development following a disturbance; e.g. volcano)
•Outputs sometimes exceed inputs of nutrients in communities or
ecosystems
Inputs < outputs = loss (source)
(e.g., fire, leaching by acid deposition)
 

6. Types of biogeochemical
cycles
On the basis of material cycled:
1.Hydrological(water) cycle.
2.Gaseous cycles.
3.Sedimentary cycles.
–
Biogeochemical cycles
Hydrological Gaseous Sedimentary

7. Sedimentary cycles
The mineral or the sedimentary cycle is the one in which the
reservoir is in the lithosphere i.e. .earth’s crust.
It consists of 2 phases:-
(i) the salt solution phase
(ii) rock phase.
In sedimentary cycle the element concerned is continually lost
from the biological system thorough erosion & ultimately
deposited in the oceans. Hence these are not perfect cycles.

8. Phosphorous cycle
•P is a component of nucleic acid, ADP, ATP, NADP, phospholipids.
•Occurs in soil as rock phosphate, apatite, Ca phosphate, fluorapatite
[Ca10Fe2(PO4)
6]etc.
•P occurs in soil in 5 forms:
P1(stable organic)
P2(labile organic)
P3(labile inorganic)
P4(soluble)
P5(mineral form)
P3 & P4 are in equilibrium. Entry of P in green plants is through labile
inorganic pool.

9. Phosphorus cycle continued….
CYCLE:
•P is a earth bound element, lacks significant atmospheric component.
•Global resource: geological deposits of 2000 billion tons (but rate of
weathering is slow).
•Phosphates originate through mechanical or chemical weathering of
rocks & from human excreta (2mg of phosphate/person/day).
•Sea is the richest source of available phosphate. Phosphate rich rock
tend to be of marine origin.
•Main rout back to land: through sea birds feeding on phosphate rich
fish & defecating on land. Their dung forms guano (used as manure).
•Natural transfer of P from ocean to land is very small less than 0.03
mmt/year for sea spray & 0.01 mmt/year for guano.

10. Phosphorus cycle continued…..
•Plants and animals concentrate P in their bodies through biological
process as formation of teeth & bones.
•On death it can be recycled into living system OR become immobilized
in the soil. In soil it remain for 1000’s of years, eventually going into
sea through erosion.
•Insolubility is the fate of most free P hence it is main limiting
nutrient in many ecosystems.
•Number of plants have symbiotic association with fungi to secure
additional P. This is VAM.

12. Sulphur cycle
Sulphur cycle links air, water & land.
S is essential constituent of certain amino acids.
It occurs -in soil & rocks as sulphides (FeS, ZnS etc.) & crystalline
sulphates.
- in atmosphere in form of SO2 & H2S gas.
CYCLE:
S in form of SO2 is formed during combustion of fossil fuel or
decomposition.
H2S gas is released from H2O logged soil, continental shelf lakes &
springs.
Organic & inorganic S & SO2 are formed through oxidation of H2S
in atmosphere.
Few organisms require S in organic form as amino acid & cystine.

13. Sulphur cycle continued…
Inorganic sulphate major source of biologically significant sulphur.
Biologically incorporated S is produced in the soil from aerobic
breakdown of proteins by bacteria & fungi
2 H2S + O2
Baggiatoa spp
2S + 2H2O

2S + 2H2O + 3O2
Thiobacillus(Thio-oxidation)
2 H2SO
4
Green & purple bacteria use H of H2S as the O2 acceptor reducing
CO2. Green bacteria oxidize sulphide to elemental sulphur. Purple
bacteria can carry oxidation to sulphate stage.
Under anaerobic condition sulphate is reduced to elemental sulphur or
to sulphides (H2S) by heterotrophic bacteria such as Desulfavibrio.
Sulphate (SO
4
2-
) is soluble in H2O , hence it is a source of elemental
sulphur.

14. Sulphur cycle continued
S returns back to the environment through the decay of dead organic
remains.
Sedimentary aspects of S cycling involves precipitation of S in presence of
iron under anaerobic condition sulphides of Fe, Cu, Zn, Cd, Co are insoluble
in water. Hence S is bound to limit the amount of these elements.
Thus, cycle afford excellent e.g. of interaction & biochemical regulation
between different mineral cycles.

16. Nitrogen cycle
N most important for plant growth. It is required for the synthesis of
amino acids, proteins, enzymes, chlorophyll, nucleic acid etc.
Atmospheric N (79% ) is not directly available to the organisms with
exception of some prokaryotes like BGA, N fixing bacteria.
CYCLE:
N fixationN fixation-- Conversion of free N of atmosphere to the biologically
acceptable form or nitrogenous compounds.
It is of 2 types:
a) Physicochemical or non biological.
b) Biological N fixation.

17. •Physicochemical N fixation- Atmospheric N combines with oxygen
during lightening or electrical discharge in the cloud & produce
different nitrogen oxides:
N
2 + O
2 2NO
2NO + O
2
2NO2
Nitrogen oxides dissolve in rain water & on earth they react with mineral
compounds forming nitrates &nitrogenous compounds.
4NO
2 + 2H2O + O
2
4HNO3
2HNO3 + CaO Ca(NO3) 2 + H2O
•Biological N fixation-carried by certain prokaryotes .
- BGA fix N in oceans, lakes, soil.
- symbiotic BGA: species of Nostoc, Anabaena, found in thalli of
Anthoceros, Salvenia, Azolla, coralloid roots of Cycas. ( Mutualistic
relation)
- Free living N fixing bac.: Azotobactor, Clostridium, Rhodospirillium.
- Fungus (actinomycetous): Frankia found in root of Alnus, Casuarina.

18. - - Sym bio tic ba ct eria ( Rhizobiu m) in ro ot n od u les of leg um in ou s plan ts.

19. Nitrogen cycle continued..
•N N assimilation-assimilation- conversion of inorganic nitrogen (nitrates, nitrites,
ammonia) to nitrogenous organic compound by green plants.
NITRATES

AMMONIA +organic acids

AMINOACIDS used in

PLANT PROTEIN SYNTHESIS ( ENZYMES CHLOROPHYLL NUCLEIC
ACID)

TRANSFERRED TO ANIMALS
• Ammonification Ammonification - Release of ammonia.
Dead organic remains are acted upon by microorganisms
actinomycetes & bacilli. (Bacillus vulgaris, B.ramosus )

20. Nitrogen cycle continued….
Nitrification Nitrification - Formation of nitrates
Nitrosomonas, Nitrococcus, Nitrospira, Nitrosogloea bacteria in
oceans &soil convert ammonia into nitrites.
2NH4
+
+ 2O2 2NO2
-
+ 2H2O + Energy

Nitrites are converted into nitrates by several microbes like
Penicillium sps. Nitrobacter, Nitrocystis (marine autotroph).
2NO2
-
+ O2 NO3
-
+ Energy
Some nitrates are formed by weathering of nitrate containing rocks.
DenitrificationDenitrification- Conversion of ammonia & free nitrites into free
nitrogen.
Includes those dissimilatory reductive reactions which result in the
production of any or all of the following gases: NO, N2O, N2.
2NO3
-
2NO2
-
2NO N2O N2
Denitrifying bac.: Pseudomonas,Thiobacillus denitrificans.
The N2O (nitrous oxide) released, diffuses from troposphere to
stratosphere where it changes to NO

21. NO reacts with O3 to form NO2
-
+

O2

. NO2
-
formed changes to NO.
The nitrogen oxides are slowly converted to HNO3 which returns to
earth.
Thus ,increased use of combined nitrogen inputs from biological or
industrial N fixation will increase the rate of denitrification thus may
cause O3 depletion.
CYCLE
Source of N for plants & animals- Biologically fixed N
-Inorganic N fixed by lightening
-N released from dead organic
matter.
Plants utilize &convert nitrates & ammonia into amino acid.
Consumers convert them to different type of proteins.
Animals release N by excretion .
Breakdown of dead plants & animals by bacteria &fungi release
ammonia.
Ammonia utilized by plants or find its way to atmosphere.
Nitrates & nitrites carried by river to lakes, seas. The denitrifying
bacteria use them as nutrient &convert them in molecular N.

23. Carbon cycle
“Without COWithout CO
22 earth would be as cold as moon earth would be as cold as moon”
Carbon one of the primary element forming human tissues .
Necessary to plants , basis of human food.
Forms of carbon-
CO2 –free state in atmosphere=0.03 %
CH
4
–in atmosphere 0.0002%
Organic carbon compounds
Dissolved state in oceans as bicarbonates etc.
CO2 enters living system through photosynthesis by green plants &
phytoplankton in presence of sunlight & chlorophyll.
Carbon is taken from the atmosphere at the surface of the oceans
near the poles, where the water becomes cooler and is able to dissolve
more carbon dioxide .

24. Carbon cycle continued…
Net gain of C in ecosystem by- forest plantation, accumulation of humus
and litter in forest, in grasslands & swamps in boreal zones, peat
accumulation in peat lands.
C is released to atmosphere - as CO2 in respiration by plants, animals,
- by bacteria & fungi attack on dead
remains.
- burning of forest (jhum cultivation),
fossil fuels.

- volcanic eruptions.

Source of C - fossil fuel, deforestation, oxidation of humus. organic C
incorporate in earth’s crust as coal, gas, petroleum, limestone, coral
reef.
Forests have vast reservoir of fixed but readily oxidisable C in form of
wood, humus.
Most C involved in cycle is in ocean.

25. Carbon cycle continued…
Ocean acts to buffer or keep constant CO2 concentration .Excess of
CO2 in atmosphere can dissolve in ocean as bicarbonates or carbonate
ions. The oceans can also release CO2 to the atmosphere.
Overview of fluxes of CO2 per year (in billion tons of CO2)
Source:- (i) Emission by use of fossil fuel- 20
(ii) Emission by deforestation & changes in land use- 5.5
Sink:- (i) Uptake in oceans -5.5
(ii) Uptake by CO2 fertilization -7.3
Disturbances in carbon cycle: Rate of release of CO2 in atmosphere is
increased up to about 50% of the expected magnitude. It has been
estimated that its effect will bring about a 3
0
C rise in surface
temperature .

28. Difference between gaseous & sedimentary
cycle
Gaseous cycles
1.Reservoir –hydrosphere or
atmosphere
2.Perfect & complete as
amount of nutrient in any
one phase tend to remain
fairly constant.
Sedimentary cycle
1.Reservoir –lithosphere
2.Imperfect as some of the
element may get stuck in a
certain phase of the cycle.

29. References
•Odum, E.P. – Fundamental of Ecology
•Kormody, E.J. – Concepts of Ecology
•Sharma,P.D. – Ecology and Environment
•Sharma, B.K. – Environmental Chemistry
•www.google.com
•www.ecologicalsocieties.com