The document explains the concept of biogeochemical cycles, highlighting their role in the circulation of chemical elements between the environment and organisms. It details various cycles, including the nitrogen, carbon, and oxygen cycles, emphasizing processes such as nitrogen fixation, assimilation, and denitrification. It also discusses the importance of these cycles in supporting life and maintaining ecological balance.

1. BIOGEOCHEMICAL
BIOGEOCHEMICAL
CYCLES
CYCLES

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.

3. 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

4. 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 fixation
N 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.

5. • Physicochemical N fixation- Atmospheric N combines with oxygen
during lightening or electrical discharge in the cloud & produce
different nitrogen oxides:
N2 + O2 2NO
2NO + O2 2NO2
Nitrogen oxides dissolve in rain water & on earth they react with mineral
compounds forming nitrates &nitrogenous compounds.
4NO2 + 2H2O + O2 4HNO3
2HNO3 + CaO Ca(NO3) 2 + H2O
• Biological N fixation-carried by certain prokaryotes .
- blue-green algae (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.

6. -
- Symbiotic bacteria ( Rhizobium) in root nodules of leguminous plants.

7. 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 )

8. 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.
Denitrification
Denitrification- 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

9. 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.

11. Carbon cycle
“Without CO
Without CO2
2 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 %
CH4 –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 .

12. 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.

13. 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 30
C rise in surface
temperature .

16. Step One of Oxygen Cycle
• Plant release oxygen into the atmosphere as
a by-product of photosynthesis.
oxygen

17. Step Two of Oxygen Cycle
• Animals take in oxygen through the
process of respiration.
• Animals then break down sugars and food.

18. Step Three in Oxygen Cycle
• Carbon dioxide is released by animals and
used in plants in photosynthesis.
• Oxygen is balanced between the atmosphere
and the ocean.