BIOGEOCHEMICAL CYCLES
Biology 420 Global Change
Introduction
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Remember
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Lithosphere
Hydrosphere
Atmosphere
Biosphere
Earth is exposed to cyclic
phenomena
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Daily rotation/annual revolution
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Variations in orbit – glacial
cycles
Plant photosynthesis/respiration
cycles
Water cycle
Generalized Biogeochemical Cycles
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Major parts of the
biosphere are connected by
the flow of chemical
elements and compounds.
Exchanges of materials
between these different
reservoirs
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Between atmosphere and
biota/oceans can be rapid
Between rocks, soils and
oceans can be more slow.
What is being exchanged?
Major Elements
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Six elements account for 95% of biosphere
C, H, O, N, P, S
 In 1958, Albert Redfield published a paper of great
importance to marine biogeochemistry
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Fairly constant molar ratio of N and P in phytoplankton
 C106N16P (known as the Redfield Ratio) also C106O138N16P
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Major Element Cycles
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There are others – iron, metals, Ca/Si for example
Here we will consider these: C, H, O, N, P, S
 Water Cycle last time (H2O)
 Today
 Carbon
Cycle
 Nitrogen Cycle
 Phosphorus Cycle
 Sulfur Cycle
Let’s Start with Carbon
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More than 1 million known
carbon compounds
Unique ability of carbon atoms
to form long stable chains
makes life possible
Oxidation states ranging from
+IV to –IV
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most common is +IV as in CO2 and
carbonate
CO in trace levels in atmosphere is
+II
Assimilation of carbon by
photosynthesis creates reduced
carbon CH2O
CH4, also trace gas is –IV
More on Carbon
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Seven isotopes of carbon
Carbon Reservoirs
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Reservoir: In geochemistry, a reservoir is the mass of an element (such as carbon) or
a compound (such as water) within a defined “container” (such as the ocean or the
atmosphere or the biosphere).
Atmosphere
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Hydrosphere (oceans)
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CO2 – based on a CO2 concentration of 351.2 ppmv in 1988  corresponds to 747 Pg
of carbon (1 Pg= 1015g)
CH4 – based on CH4 concentration of 1.7 ppmv in 1988  corresponds to 3 Pg of carbon
(most abundant organic trace gas and 2nd most important changing greenhouse gas)
CO –ranging from 0.05 to 0.20 ppmv  0.2 Pg carbon
Dissolved inorganic carbon (DIC)  37,900 Pg C
Dissolved organic carbon (DOC)  1000 Pg C
Particulate organic carbon (POC)  30 Pg C
Marine biota  3 Pg C
Terrestrial Biosphere ranging from 480 – 1080 Pg C
Lithosphere – carbon in rocks, fossil fuels  huge reserves 20 million Pg C in rocks,
104 Pg C in extractable reserves of oil and coal
Carbon Flux
Nitrogen
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Coupled with other elements of living matter (such as
carbon)
Important biological and abiotic processes
Oxidation states from +V to –III
Not found in native rocks, major reservoir is N2 in
atmosphere
Biological Transformation of Nitrogen Compounds
(microbial mediation)
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Nitrogen fixation  enzyme-catalyzed reduction of N2
to NH3, NH4+ or any organic nitrogen
Ammonia assimilation  uptake of NH3, NH4+
Nitrification  oxidation of NH3, NH4+to NO2- or NO3as a means of producing energy
Assimilatory nitrate reduction  reduction of NO3then conversion to biomass
Ammonification  organic nitrogen to NH3 or NH4+
Denitrification  reduction of NO3- to N2 or N2O
(nitrous dioxide, gaseous forms)
Reservoirs and Fluxes
More Nitrogen
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NOx
 NO
(nitric oxide) and NO2 (nitrogen dioxide)
 Formed due to reactions of N and O in air during
combustion
 Air pollution and reactions to form acid rain
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Atmospheric deposition: elements of
biogeochemical interest deposited on Earth as
 rainfall
 dry
deposition (sedimentation)
 direct adsorption of gases
Processes of Nitrogen Gas Emissions
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Rapid conversion of NH4+ to NH3 at high pH and
low soil moisture  results in gas loss to atmosphere
High organic waste loads (from feedlots) promote
NH3 loss
NO, N2O are byproducts of nitrification
NO, N2O and N2 are products of denitrification
Atmospheric N Deposition
 Acidic
wet and dry deposition due to combustion
 NH4+ from livestock organic waste
Wet Deposition NO3/NH4 (2009)
Phosphorus
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Second most abundant mineral in human body (surpassed only by Ca)
This cycle has no atmospheric component (gaseous P3 is negligible)
Restricted to solid and liquid phases (many mineral reactions)
Unlike nitrogen, not really involved in microbial reactions
Oxidation-reduction reactions play a minor role in reactivity and
distribution of phosphorus
Only 10% of phosphorus from rivers to oceans is available to marine biota
It is suggested that terrestrial net primary productivity is determined by
level of available phosphorus in soil
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P in low concentrations in rocks
N abundant in atmosphere
Other essential plant nutrients are more abundant than P (S, K, Ca, Mg)
Bacteria involved in N cycle require P also
More on Phosphorus Forms
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Dissolved Inorganic Phosphorus  PO43Organic Forms phosphate in DNA, RNA, ATP,
phospholipid
Minerals  apatite [Ca(PO4)3OH]
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Distribution
Sediments 4 million Pg P
 Land 200 Pg P
 Deep Ocean 87 Pg P
 Terrestrial Biota 3 Pg P
 Surface Ocean 2.7 Pg P
 Atmosphere 0.000028 Pg P
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Phosphorus Cycle
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A “sedimentary” cycle with
Earth’s crust as reservoir 
erosion processes they are
washed into rivers and oceans
Plant and animals 
adsorption up the food
chain… small role in
comparison to 1st point
Agriculture  a limiting
nutrient
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Mined for fertilizer
Form of fertilizer is phosphate
Also contain nitrogen
Sulfur Cycle
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Essential to life, also relatively abundant and thus not
limiting
Like phosphorus, has important geochemical cycling
Like nitrogen
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Important gas phases
Oxidation-reduction reactions and oxidation state from -II to +VI
Sulfur Cycle
Sulfur Reservoirs
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The crust  as gypsum (CaSO4) and pyrite (FeS2)
Distribution
2 x 1010 Tg S
 Ocean: 1.3 x 109 Tg S
 Ocean Sediments: 3 x 109 Tg S
 Marine Biota: 30 Tg S
 Soils and Land Biota: 3 x 105 Tg S
 Lakes: 300 Tg S
 Continental Atmosphere: 1.6 Tg S
 Marine Atmosphere: 3.2 Tg S
 Lithosphere:
Sources of Sulfur in Atmosphere
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Volcanic eruptions
 12-30
Tg S averaged over many years
 Tambora, Indonesia in 1815, 1816 – year without
summer ~50 Tg S
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Soil dust
Biogenic gases
Anthropogenic emission
Marine Sulfur Cycle
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Ocean is large source of aerosols (sea salts) that
contain SO42- (mostly re-deposited onto ocean)
DMS
dimethyl-sulfide (CH3)2S is a major biogenic gas emitted
from sea
 Produced during decomposition of dimethylsulfonpropionate (DMSP) from dying phytoplankton
 Small fraction is lost to atmosphere
 Oxidation of DMS to sulfate aerosols  greater cloud
condensation nuclei  more clouds
 Layer of sulfate aerosols (Junge layers) 20-25 km altitude
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Microbial Action
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Assimilative reduction of SO4- to –SH groups in
proteins
Release of –SH to form H2S during excretion,
decomposition and desulfurylation
Oxidation of H2S by chemolithotrophs to form
elemental sulfur or SO4Dissimilative reduction of SO4- by anoxygenic
phototrophic bacteria