Oxidation and reduction in
soils
• By far the most prevalent electron donor is
organic carbon. Carbohydrate oxidation
serves as an example of organic carbon
oxidation to carbon dioxide.
•
CH2O + H2O = CO2 + 4e-
• This half reaction is what supplies energy
to microorganisms within soils (we will
speak of the electron acceptor they use in
a moment).
• The most prevalent electron acceptor is O2
(g), and is the sole electron acceptor in
aerated systems (aerobic systems).
•
O2 + 4 e- + 4H+ = 2H2O
• Living compounds create energy by
oxidizing carbohydrates and reducing
oxygen. That is, organisms take electrons
from the organic carbon, run it through their
metabolic cycles, and then dump the
electron to an electron acceptor. When
oxygen is not present, microorganisms must
seek alternate electron acceptors. The
energy gain for the organisms is the energy
difference between reduced carbon and the
electron acceptor. In order of favorability,
electron acceptors are:
• O2 > NO3- > Mn(VI) > Fe(III) > AsO43>SO42-
• A lack of oxygen leads to an anaerobic condition
and results in the build up of reduced species:
Mn(II), N2, Fe(II), As(III), and S2-. When
reduced species are build up in the system, it is
termed "REDUCED". When O2 is present with
reduced species such as S2-, Fe(II), Mn(II), or
As(III) it is energetically favorable for oxygen to
oxidizing the previous species and thus become
reduced to water. Thus, if oxygen is introduced
to a ‘reduced’ system, the reduced species are
oxidizes in opposite order:
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SO42- > AsO43- > Fe(III) > Mn(VI) > NO3-
• The redox potential is governed by two
primary factors:
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1. Microbial activity
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2. Availability of electron acceptors.
• Microorganisms are probably the most important factor
controlling the redox status of soils:
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-they reduce carbon to store energy
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-they oxidize it to release energy
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-they use oxygen or other molecules as electron
acceptors, thus forming reduced species under oxygen
limiting conditions.
• Micro-organisms control the redox potential along with
the redox couple of the electron acceptor. Oxygen is the
preferred acceptor because it is most easily reduced to
water of the available acceptors. This leaves the
greatest energy from respiration
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photosynthesis stores energy by reducing C
Red. of C
CO2 + 4 e- + 4 H+ => CH2O + H2O 0.21
V
balancer
2 H2O = 2O2 + 4e- + 4 H+ -1.21 V
overall CO2 + H2O = CH2O + O2
-1.00 V
To acquire the stored energy the C source is oxidized
Oxid of C
CH2O + H2O => CO2 + 4 e- + 4 H+
Red of O2
2O2 + 4e- + 4 H+ => 2 H2O
Overall
CH2O + O2 => CO2 + H2O +1.00
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Alternate electron acceptors, in order of preference (based on E°
values):
NO3 -> N2 (or other reduced N forms)
Mn(IV) --> Mn(II)
Fe(III) --> Fe(II)
SO42- --> S2- (or H2S)
The order of their preference is due to their redox potentials for the
half-reactions. Note that O2 has the highest redox potential,
followed by NO3>Mn(IV)>Fe(III)>SO42-. The reverse order will
apply upon oxidation. Sulfide will go first, followed by Fe(II), and
then Mn(II). Also note that if a reduced species with a low redox
potential is present with a species in the oxidized form which has a
higher redox potential, then there is a high probability for oxidation of
the reduced species and reduction of the oxidized species
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pH Changes
As soils or sediments become reduced, their pH tends to move
toward neutrality. That is, if the pH of the system is acidic then the
pH will increase while soils with pH values above 7 tend to have
decreasing pH values.
Reasons:
• When the pH is initially low, H+ consumption in the reduction
reactions increases the pH.
For example: MnO2 (s) + 4 H+ + 2e- = Mn2+ +
2H2O
• If the pH is initially basic, then the liberation and
reprecipitation of metal ions such as Fe and Mn as hydroxides,
carbonates, or sulfides tends to lower the pH.
For example: Fe2+ + 2 H2O = Fe(OH)2 + 2 H+
Fe2+ + HCO3 = FeCO3 + H+
O2
Oxic
zone
Fe 2+
precipitation
Fe3+ + 3H 2 O = Fe(OH) 3 + 3H +
oxidation
diffusion
Suboxic
zone
Anoxic
zone
Fe 2+
Fe(OH)3 = Fe 2+ + 3OH reductive dissolution
Fe 2+ + S 2- = FeS
pryite precipitation
Fe 2+ + CO3 2- = FeCO 3
Fe-carbonate precipitation
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Nitrogen Cycles
-Oxidation of N2 does not occur except in a few
organisms
-Reduction of NO3- to reduced-N forms accounts for
denitrification in soils
-NO3- to NH4+ is termed mineralization and only occurs
in living organisms
Gaseous Nitrogen Evolution
Suboxic
zone
NO3 NO2 NO
N2 O
N-mineralization
Organic-N
N-fixation
NH3
N2
DENITRIFICATION
Oxic
zone
NITRIFICATION
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