ZLY 311
Biogeochemical Cycles
Introduction
Living things require various kinds of chemical
elements for their synthetic and metabolic
processes.
Energy, water & many elements cycle through
ecosystems and influence growth & reproduction.
About 10 major nutrients and six trace nutrients
are essential to all animals and plants.
Carbon, hydrogen, nitrogen and oxygen form the
basic constituent components of carbohydrates,
fats and proteins.
Introduction
 All these elements with phosphorus form nucleic
acids.
 Sulphur is essential for the formation of amino acids,
 magnesium for chlorophyll biosynthesis,
 phosphorus for transformation of energy through
cells,
 calcium for strengthening cell walls,
 potassium highly indispensible for growth,
 iron molybdenum and copper for the activity of
certain enzymes and so on.
 The aforementioned elements constitute the basic
building blocks of the protoplasm.
Introduction
The absorption and utilization of these elements
by organisms is compensated by their recycling
and regeneration back into the environment by
the breakdown of these organic compounds.
The more or less cyclic path of these elements in
the biosphere, i.e. from environment to organisms
and back to the environment is described as
biogeochemical cycles.
Biogeochemical Cycles & the Ecosphere
• The global ecosphere is the thin film around the
earth where the biosphere interact with the
atmosphere,
hydrosphere,
cryosphere
and
lithosphere in a complex system involving biological,
geological, and chemical processes and cycles (Next
slide).
• This biogeochemical system of spheres and processes
is powered mainly by energy from the sun.
• The ecosphere is made up of individual ecosystems,
such as tropical forests, grasslands, tundra, coral
reefs, and estuaries.
• Matter and energy flow between and within these
ecosystems occurs in interconnected biogeochemical
cycles.
Biogeochemical Cycles & the Ecosphere
Biogeochemical Cycles & the Ecosphere
In the ecosystem, gaseous chemical compounds
are produced, consumed & exchanged in the air.
In the atmosphere, they may react to form other
compounds before returning to the earth’s
surface.
Some of these chemical compounds are
greenhouse gases viz; CO2, CH4.
They act in the atmosphere to warm the planet.
Others, like dimethyl sulphide gas, react with
other atmospheric chemicals to form minute
airborne particles (aerosols) that directly or
indirectly help to cool the climate.
Biogeochemical Cycles Pitfalls
Certain factors are difficult to describe:1. Irreversibility: The system does not return to its
exact previous state if it goes through a disturbance;
2. Transitional phenomena: The system tends to switch
from one state to another and perhaps back again,
rather than simply moving from “before” to “after”;
3. Evolution: The system progressively changes in a
particular direction;
a)
Positive feedback: Enhance the original
perturbation to the system
b)
Negative feedback: Relieve the
perturbation .
Essentials of Biogeochemical Cycles
• Many elements enter living systems in the
gaseous state from the atmosphere or as water
soluble salts from the soil.
• As the flux of these elements through an
ecosystem gives some measure of its continuity
and productivity, the analysis of exchange of
various components of the biosphere is essential.
• Furthermore society depends upon this lifesupport system of the earth for sustained and
increased production of food, fodder, fibre and
fuel.
Key Points in Biogeochemical Cycles
Major reservoirs: part of a system that can store
or accumulate and be a source of one of the
substances that compose the system.
Fluxes: Movement of a variable or a substance
into or out of a reservoir.
Oxidation states of elements: Biogeochemical
cycles primarily represent changes in oxidation
states.
Chemical properties of various molecular species,
Magnitude of human impacts,
Biogeochemical Cycles & the Ecosphere
• The global ecosphere is the thin film around the
earth where the biosphere interact with the
atmosphere,
hydrosphere,
cryosphere
and
lithosphere in a complex system involving biological,
geological, and chemical processes and cycles (Next
slide).
• This biogeochemical system of spheres and processes
is powered mainly by energy from the sun.
• The ecosphere is made up of individual ecosystems,
such as tropical forests, grasslands, tundra, coral
reefs, and estuaries.
• Matter and energy flow between and within these
ecosystems occurs in interconnected biogeochemical
cycles.
Hydrologic Cycle
 Water Cycle is described as the journey water
takes as it circulates from the land to the sky and
back again.
 This biogeochemical system of spheres and
processes is powered mainly by energy from the
sun.
 Water is by far the most important substance
necessary for all life forms.
 It determines the structure and function of the
ecosystem and regulates the plant environment to
a large extent.
Hydrologic Cycle
Importance of Water
1. The cycling of all other elements is also dependent
on water, because it provides the solvent medium
for their uptake.
2. It provides H+ for reduction of CO2 during
photosynthesis.
3. It has moderating effect on temperature of the
environment by virtue of its heat absorbing ability.
4. The protoplasm described as the basis of life is
primarily composed of 85 to 95% water.
5. Water content from organism to organism varies
and it is often marked by their needs e.g. human
blood is 90% water.
Hydrologic Cycle
Water cycle involves an exchange of water
between the earth’s surface and the atmosphere
via precipitation and evapo-transpiration.
It covers about 75% of the earth surface,
occurring in lakes, ponds, rivers, seas, oceans etc.
The ocean occupies 70% of the surface and
contains approximately 97% of all the water on
the earth.
The remainder is left as frozen in ice caps and
glaciers.
Hydrologic Cycle
The water in rivers and lakes is comparatively
small. Less than 1% is in the form of ice-free
freshwaters in rivers, lakes and aquifers.
This negligible volume of the planet’s water is
crucially important to all forms of terrestrial and
aquatic life.
Water is also trapped underground, and soils
near the surface also serve as reservoirs for
enormous quantity of water.
Hydrologic Cycle
Hydrologic Cycle
• Heat from the sun warms the ocean and land
surfaces and causes water to evaporate.
• Water vapour enters circulation of the air.
• On a global scale, warm air rises in the atmosphere
and cooler air descends.
• The water vapour rises with the warm air.
• The farther from the warm planetary surface the air
travels, the cooler it becomes.
• Cooling causes water vapour to condense on small
particles (cloud condensation nuclei) in the
atmosphere and to precipitate as rain, snow, or ice
and fall back to the earth’s surface.
Hydrologic Cycle
• When the precipitation reaches the land surface,
it is evaporated directly back into the atmosphere,
runs off or is absorbed into the ground, or is
frozen in snow or ice. Some of the precipitation
soaks into the ground.
• Some of the underground water is trapped
between rock or clay layers (groundwater).
• Most of the water flows downhill as runoff (above
ground or underground), eventually returning to
the seas as slightly salty water.
Hydrologic Cycle
Hydrologic Cycle
• Water cycle is being significantly affected by
water usage and contamination of water stocks.
• Currently, humans use an amount of water
equivalent to about 25% of total terrestrial
evapotranspiration and 55%, or 6,800 cubic
kilometers per year, of the runoff of water from
the continents that is accessible, only about 20%
of the world’s drainage basins have pristine water
quality.
The Nitrogen Cycle
• Nitrogen forms part of the molecules that make
up living things, such as amino acids (building
blocks of proteins) and DNA.
• The nitrogen in proteins bonds together with
various amino acids to form the protein structure.
• Amount of nitrogen in the atmosphere is very
large compared to that in the oceans or rocks.
• Of the elements C, N, P, S, and O, only nitrogen is
found in more abundance in the atmosphere than
in rocks.
The Nitrogen Cycle
6 major forms of atmospheric nitrogen, viz;
1. the gaseous forms of diatomic nitrogen (N2),
2. Ammonia (NH3),
3. Dinitrogen oxide (N2O),
4. NOx (NO and NO2),
5. The aerosols of ammonium (NH4+) and
6. Tri-oxo-nitrate (V) (NO3-).
The Nitrogen Cycle
• Nitrogen and phosphorus are two of the most
essential mineral nutrients for all types of
ecosystems and often limit growth if they are not
available in sufficient quantities.
• This is why the basic ingredients in plant
fertilizer are nitrogen, phosphorus, and
potassium, commonly abbreviated as NPK.
• A slightly expanded version of the basic equation
for photosynthesis shows how plants use energy
from the sun to turn nutrients and carbon into
organic compounds:
The Nitrogen Cycle
• The basic equation for photosynthesis shows how
plants use energy from the sun to turn nutrients
and carbon into organic compounds:
CO2 + PO4 + NO3 + H2O → CH2O, P, N (organic tissue) + O2
• Atmospheric nitrogen (N2) is inert and cannot be
used directly by most organisms, therefore,
microorganisms that convert it into usable forms
of nitrogen play central roles in the nitrogen
cycle.
• Crucial to the use of nitrogen by most living
things is the ability of breaking the N-N triple
bond, which is known as “nitrogen fixation”.
The Nitrogen Cycle
• Nitrogen fixation is accomplished by nitrogen-fixing
bacteria that contain a metallo-enzyme known as
nitrogenase, by lightening, and by human industry.
• Most organisms can only assimilate “fixed” nitrogen
in the form of ammonia or nitrate.
• These microorganisms take inert nitrogen (N2) from
the atmosphere and convert it to ammonia nitrate
(NH4NO3) and another nitrogen compound, which
plants utilize.
• Some of these bacteria live in mutualistic relationships in
the roots of plants, mainly legumes (peas and beans), and
provide nitrogen directly to the plants.
The Nitrogen Cycle
• At the back end of the cycle, decomposers break
down dead organisms and wastes, converting
organic materials to inorganic nutrients.
• Other bacteria carry out denitrification, breaking
down nitrate to gain oxygen and returning gaseous
nitrogen to the atmosphere.
The Nitrogen Cycle
The Nitrogen Cycle
• Part of the modern global biogeochemical cycle of
nitrogen, emphasizing interactions among the land,
atmosphere, and ocean (Next Slide).
• Fluxes between the ocean, land, and groundwater
are shown as arrows; with quantities given in
million tons of nitrogen per year (Mt N/yr.).
• Fluxes within reservoirs are shown as circling
arrows. “Ind. fix” is industrially fixed N (for the
manufacture of fertilizers),
• “Bio. fix” is biologically fixed N, DN is dissolved N,
PON is particulate organic nitrogen, and “pollutant”
is the excess nitrogen that has resulted from human
activities.
The Nitrogen Cycle
The Nitrogen Cycle
Major Processes leading to Exchange of Nitrogen:
1. Biological fixation is the process whereby N2 is
withdrawn from the atmosphere and converted to
N compounds that plants can use (e.g., NH3 and
subsequently NO3-).
2. Denitrification is the process by which nitrogen as
N2 or as N2O is returned to the atmosphere.
3. On a global scale, rivers may already carry more
nitrogen from human activities than was
transported in the natural state.
4.
The Nitrogen Cycle
• This increased nitrogen flux to lakes, rivers, and coastal
marine environments are one cause of increased
regional and global eutrophication.
• Rivers supply only a small percentage of nitrogen to the
coastal zone (Next Slide). Most of the nitrogen there,
other than that recycled in the zone, upwelled from the
deep ocean to the surface.
The Nitrogen Cycle
The Nitrogen Cycle
• River input of N to the ocean compared to the fluxes
involved with the internal recycling of N in the
ocean is due to biological productivity and decay.
• Besides ocean fluxes, almost 90% of N involved in
biological production is simply recycled in the
shallow surface waters of the coastal and open
oceans.
• Some nitrogen escapes from the surface ocean as
organic matter and settles at the benthos of the
ocean, where they decay and nitrogen is released.
The Dinitrogen Oxide Flux
The Dinitrogen Oxide Flux
• N2O is a natural product of biological denitrification
in soils and in the ocean.
• The N2O produced by denitrification is only about
15% of all N returned to the atmosphere.
• N2O is an important greenhouse gas, accounting for
about 9% of the enhanced greenhouse effect.
• Presently, atmospheric concentration is 312 parts
per million volume (ppmv) and a residence time of
about 130 years.
• This concentration is about 8% greater than in preindustrial time and is increasing at a rate of 0.2–
0.3% per year because of anthropogenic activities,
The Dinitrogen Oxide Flux
• These activities includes combustion of fossil fuels,
burning of biomass and emissions from urea and
ammonium nitrate applied to crop lands. These
emissions amount to 0.13 to 2.8 million tons of
nitrogen annually.
• N2O is chemically inert in the troposphere.
• In
the
stratosphere,
N2O
is
converted
photochemically to nitric oxide (NO), which acts as a
catalyst in the destruction of stratospheric ozone
• The series of reactions by which this is accomplished
has been one of the regulators of stratospheric ozone
concentration through geologic time.
Destruction of Stratospheric Ozone Layer
The Dinitrogen Oxide Flux
• The flux of N2O from the earth to the atmosphere has
been increasing because of anthropogenic activities and
increases in organic carbon in coastal waters.
• Increase in organic carbon in coastal waters has formed
an important link between the carbon and nitrogen
cycles.
• The rate of denitrification and N2O emissions from
coastal waters may have increased because rivers are
bringing more organic carbon to these systems anda re
undergoing eutrophication as they receive increased
inputs of nutrients from fertilizer, sewage, and the
atmosphere.
Effect of N2O 0n Ecosphere
• With warming, the most important biotic feedbacks
involving N2O has lead to changes in the denitrification
& nitrification rates in soils, sediments, and ocean water.
• N2O fluxes from nitrogen-bearing fertilizers applied to
the land surface and subsequent sewage discharges into
aquatic systems will be affected by warming.
• The reactions involving N2O are bacterially mediated,
therefore an increase in temperature lead to enhanced
evasion rates of N2O from the earth’s surface.
• This is a positive biotic feedback on accumulation of N2O
in the atmosphere ending with global warming.
• It could also lead to a small enhanced destruction of
stratospheric ozone.
The Ammonia Flux
• Ammonia is released to the atmosphere by organic
decomposition and volatilization.
• There, it reacts with water droplets to form ammonium
ion (NH4+) and hydroxyl ion (OH-).
• NH4+ appears to be removed from the atmosphere
mainly by being deposited back on the earth in the
aerosols of ammonium tetra-oxo-sulphate (VI)
[(NH4)2SO4] and ammonium tri-oxo-nitrate (V)
(NH4NO3).
• Incidentally, (NH4)2SO4 links the nitrogen and sulphur
biogeochemical cycles, since its deposition on the earth is
also one of the ways oxidized sulphur is removed from
the atmosphere; the other is by deposition of tetra-oxosulphate (VI) acid (H2SO4).
The Ammonia Flux
The Ammonia Flux
•
•
•
•
•
Interactions involved in NH3 Cycle affecting Global
Warming
NH3 interacts with OH- to produce NOx.
In a warmer world; the decomposition that releases NH3
is enhanced; this would slightly increase the stress on the
OH- concentration of the atmosphere and enhance
production of NOx.
NH3’s reaction with NO3 and SO42- to produce aerosols
containing ammonium.
Aerosols are known to cool the planet, although the
amount of the effect is unclear.
An increase in atmospheric NH3 could lead to a small
negative feedback on potential warming.
Summary
The most dramatic change in the nitrogen cycle is the
amount of nitrogen humans now fix through industrial
processes.
It is estimated that human activities have probably
doubled the amount of nitrogen that is fixed per year.
Humans have also altered the concentration of nitrogencontaining compounds in the atmosphere.
Dinitrogen oxide (N2O) concentrations are increasing at
a rate of about 0.2-0.3% per year.
Dinitrogen oxide is both a greenhouse gas and a cause of
stratospheric ozone depletion.
Nitric oxide (NO) concentrations are also increasing.
About 80% of all NO emissions are caused by humans.
Summary
 Nitric oxide contributes to photochemical smog and acid
rain.
 An increase in the amount of fixed nitrogen in the
environment changes the composition of plants. Many
ecosystems have historically been nitrogen-limited.
 In some areas, nitrogen may no longer be limiting. Many
aquatic ecosystems are visibly burdened by the high
growth of plants and bacteria.
 Nitrate in ground water can cause problems for infants
who drink the water because inside their bodies they
convert the nitrate to nitrite and nitrite binds to iron in
hemoglobin, preventing oxygen from binding, which leads
to suffocation.
Summary
Nitric acid is contributing to the acidification of aquatic
ecosystems.
Anthropogenic activities add roughly as much nitrogen
to terrestrial ecosystems each year as the amount fixed
by natural processes
Excess nitrogen promotes algal blooms, which then
deplete oxygen from the water when the algae die and
decompose.
Additionally, airborne nitrogen emissions from fossil fuel
combustion promote the formation of ground-level
ozone, particulate emissions, and acid rain.
End of Part One
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Lecture Slides for Biogeochemical Cycles 311