Chemistry of Submerged Soils

MNS University of Agriculture, MultanMNS University of Agriculture, Multan
Sunday, January 19, 2020MNS University of Agriculture, Multan 1
CHEMISTRY OF SUBMERGED
SOILS
GHULAM FARID
PhD Scholar
Department of Soil and Environmental Sciences
MNS-University of Agriculture Multan

CONTENTS
1. Introduction
2. Kinds of submerged soils
3. Characteristics of submerged soils
4. Electro chemical changes in submerged soils
5. Chemical transformations of submerged soils
6. Mineral equilibria in submerged soils
7. References

 72% of the earth crust’s is covered by submerged soils or
sediments.
 Chemical changes in these submerged materials are influence by
a) Character of the sediments or soil that forms,
b) Suitability of wet soils for crops,
c) Distribution of plant species,
d) Quality and quantity of aquatic life and
e) Capacity of lakes and seas to serve as sinks for terrestrial
wastes.
Submerged Soils??

a. WATERLOGGED (GLEY) SOILS
b. MARSH SOILS
c. PADDY SOILS
d. SUBAQUATIC SOILS
Kinds of Submerged Soils??

A. WATERLOGGED (GLEY) SOILS
 Saturated with water for a sufficiently long time annually
 Forms horizons like:
(a) a partially oxidized A horizon high in organic matter
(b) a mottled zone
(c) a permanently reduced zone with bluish green colour

B. MARSH SOILS
Freshwater marsh
 Occur on the fringes of lakes and the networks of streams that feed them
 In this the G horizon is blue or green
 Types,
• Upland (pH 3.5-4.5)
• Lowland (pH 5.0-6.0)
• Transitional
 Marshes are found in estuaries, deltas and tidal flats
 it is green if iron silicates are present and dark grey if pyrites are the
main iron minerals
Saltwater marsh

Fresh water marshes Salt water marshes
B. MARSH SOILS

C. PADDY SOILS
 Developed by cultivation practises of paddy (includes puddling,
levelling and water stagnation)
 When irrigated soil undergoes reduction and turns dark grey.
 Fe, Mn, Si and P become more soluble and diffuse to the surface
 Moves by diffusion and mass flow to the roots and to the subsoil. When Fe2+
and Mn2+ reach the oxygenated surface, the surface of rice roots, or the
oxidized zone below the plough sole they are oxidized and precipitated along
with silica and phosphate

 It is Sandwiched between the oxidized surface layer and the zone of Fe and Mn
illuviation.
 The root zone of rice with reddish-brown streaks along root channels.
 When the land is drained at harvest, almost the entire profile above the water
table is reoxidized, giving it a highly mottled appearance.
 Precipitation in the plough layer is not pedologically of any consequence
because ploughing and puddling redistribute the deposits
C. PADDY SOILS

C. PADDY SOILS
 Downward movement of Fe and Mn causes loss of these elements from the
topsoil. The eluviated Fe and Mn, along with some phosphate, are deposited
below the plow sole to produce an iron-rich B1r horizon overlying a manganese-
rich Bmn horizon.
 Reduction eluviation and oxidative illuviation as the soil forming processes
characteristic of paddy soils and have proposed the new term "Aquorizem" at
the Great Soil Group level to define soils which have the sequence of
reductive eluviation/oxidative illuviation.
 A well developed paddy soil has the horizon sequence Apg,/Birg/B2g/G

D. SUBAQUATIC SOILS
 Formed from river, lake, and ocean sediments.
 Formed by,
 the sediments are formed from soil components
 typical soil-forming processes such as hydrolysis,
oxidation- reduction, precipitation, synthesis, and
exchange of matter
 deep sea sediments contain OM and a living bacterial
flora

CHARACTERISTICS OF SUBMERGED SOILS
A. Absence of Molecular Oxygen
B. Oxidized Mud-Water Interface
C.Exchanges between Mud and Water
D. Presence of Marsh Plants
E. Soil Reduction

A. ABSENCE OF MOLECULAR
OXYGEN Gas exchange between soil and air is drastically reduced
 O2 and other atmospheric gases can enter the soil only by
molecular diffusion in the interstitial water is 10,000 times slower
than diffusion in gas-filled pores
 Within a few hours of soil submergence, microorganisms use up the
oxygen present in the water or trapped in the soil and render a
submerged soil practically devoid of molecular oxygen

Oxygen Movement into Submerged Soils

B. OXIDIZED MUD-WATER INTERFACE
 Concentration of O2 may be high in the surface layer which is a few
millimeters thick and in contact with oxygenated water
 Below the surface layer, the O2 concentration drops abruptly to
practically zero
 The chemical and microbiological regimes in the surface layer resemble
those in aerobic soils

C. EXCHANGES BETWEEN MUD AND WATER
 The presence of this oxygenated surface layer in lake and ocean muds is of
the most ecological importance because it acts as a sink for phosphate and
other plant nutrients and as a chemical barrier to the passage of certain plant
nutrients from the mud to the water
 The surface may use up oxygen faster than it receives it, undergo reduction
and release large amounts of nutrients from the lake mud into the water
 In summer, some lakes undergo thermal differentiation into three layers:
• Epilimnion
• Thermocline
• Hypolimnion

The epilimnion is the surface layer of warm water 10-20 m
deep which because of mixing by wind action, is uniform in
temperature and is saturated with atmospheric O2 from top to
bottom.
Immediately below this is the thermocline, a layer in which
there is a rapid fall in temperature with depth. In this, the
concentration of O2 is relatively constant in lakes poor in plant
nutrients (oligotrophic lakes), but it decreases with depth in lakes
rich in plant nutrients (eutrophic lakes)
The hypolimnion is the layer of cold stagnant water practically
isolated from the epilimnion, except for solids, both organic and
inorganic, that sink through it and accumulate on the mud
surface. Bacteria in the surface layer use the O2 in it to oxidize
the organic matter.
THERMAL DIFFERENTIATION LAYERS

D. PRESENCE OF MARSH PLANTS
 Plants growing in submerged soils have two adaptations that enable the roots
to ward off toxic reduction products, accumulate nutrients, and grow in an O2
-free medium: O2 transport from the aerial parts and anaerobic respiration
 It has been known for quite some time that the roots of marsh plants receive
their oxygen from the aerial parts (shoot, air roots or stilt roots) through gas
spaces connecting these organs

E. SOIL REDUCTION
 The most important chemical difference submerged soil is in a
reduced state.
 Except for the thin, brown, oxidized layer at the surface (and sometimes
an oxidized zone in the subsoil), a submerged soil is grey or greenish,
has a low oxidation-reduction potential, and contains the reduced
counterparts of NO2-, SO4
2-, Mn4+, Fe3+, and CO2, NH4
+, H2S, Mn2+,
Fe2+, and CH4

OXIDATION AND REDUCTION IN AN AEROBIC
SOIL
 Organic matter in soil gives up
4 electrons (e-) which are
received by O2. As a result, O2
is reduced.
 Hydrogen ions (H+) react
with the reduced O2 to form
water (H2O).
4 e- + O2 + 4 H+→ 2 H2O

OXIDATION AND REDUCTION IN AN ANAEROBIC SOIL
10 e- + 2 NO3
- + 12 H+→ 1 N2 + 6 H2O
 Electrons (e-) from organic matter in soil are
accepted by nitrate (NO3
-) instead of O2.
 Nitrogen (N) in NO3
- is reduced; the N
compound becomes nitrogen gas (N2)
 Hydrogen ions (H+) react with oxygen from
NO3
- to produce H2O.

A change in chemistry results in a change of soil color
 bright colors indicate a well-drained
soil
 submerged soils change to a gray
or blue-green color (often referred
to as gley)
 Reddish-yellowish brown colors are
an indication of iron oxides in a well-
drained environment
 Submergence causes iron to be
reduced resulting in a different iron
form and the gley color
Well-drained soil
profile
Reduced soilprofile

1. OXIDATION-REDUCTION POTENTIAL
 Oxidation-reduction is a chemical reaction in which electrons are
transferred from a donor to an acceptor.
 The source of electrons for biological reductions is organic matter.
 Redox potential (Eh) is a quantitative measure of the tendency of a given
system to oxidize or reduce susceptible substances.
Eh is positive and high in strongly oxidizing systems; Negative in negative
and low in strongly reducing systems

Chemical Reduction Sequence of Submergence
MnO2
O2
N2
Mn2
+
Fe2+
NO3
-
Fe3+
CO2
CH4
SO4
-
H2S
H2O
MnO2
Reaction sequence following submergence
Reaction sequence after draining
Slightly
Reduced
Moderately
Reduced
Strongly
Reduced
Oxidized

ELECTROCHEMICAL CHANGES IN SUBMERGED
SOILS
Submerging a soil brings about a variety of electrochemical changes in soils.
These include,
 (a) a decrease in redox potential,
 (b) an increase in pH of acid soils and a decrease in pH of alkaline soils,
 (c) changes in specific conductance and ionic strength,
 (d) drastic shifts in mineral equilibria,
 (e) cation and anion exchange reactions,
 (f) sorption and desorption of ions.

A. REDOX POTENTIAL
 The low potentials (0.2 to -0.4 V) of submerged soils and sediments reflect this
reduced state.
 The high potentials (0.8 to 0.3 V) of aerobic media, their oxidized condition.
 When an aerobic soil is submerged, its Eh decreases during the first few days and reaches a
minimum (-0.42 V ).
 Then it increases, attains a maximum, and decreases again asymptotically to a value characteristic
of the soil, after 8-12 weeks of submergence
 The presence of native or added organic matter sharpens and hastens the first minimum, nitrate
abolishes it (0.2 V). The rapid initial decrease of Eh is apparently due to the release of reducing
substances accompanying oxygen depletion before Mn(IV) and Fe(III) oxide hydrates can mobilize
their buffer capacity.
 The course, rate, and magnitude of the Eh decrease depend on:
 kind and amount of organic matter
 nature, and content of electron acceptors
 temperature, and the duration of submergence

B. pH
Decrease of pH in first few days of submergence, then it
reaches minimum and increases to a stable value (6.7 – 7.2)
pH of soils

C. SPECIFIC CONDUCTANCE
 The specific conductance of depends on the kind
and concentration of ions present.
 Ionic strength (I) = ½ ∑ CiZi
Where, Ci= concentration of ions
(mol/lit) Zi = valence of ions
 Under reduced condition ionic strength
was equal to 16 times the specific
conductance (k) in mhos/cm at 25°C

CHEMICAL TRANSFORMATIONS IN SUBMERGED
SOILS
A. Carbon
B. Nitrogen
C. Iron
D. Manganese
E. Sulfur
F. Phosphorus
G. Silicon
H. Trace Elements

FORM OF COMPOUNDS IN AERATED AND
SUBMERGED SOIL
Element Aerated soil
(Oxidized)
Submerged soil
(Reduced)
Oxygen (O) Oxygen gas (O2) Water (H2O)
Nitrogen (N) Nitrate ion (NO3
-) Nitrogen gas (N2)
Manganese (Mn) Manganese IV ion (Mn4+) Manganese II ion (Mn2+)
Iron (Fe) Iron III ion (Fe3+) Iron II ion (Fe2+)
Sulfur (S) Sulfate ion (SO4
2-) Hydrogen sulfide (H2S)
Carbon (C) Carbon dioxide (CO2) Methane (CH4)

A. CARBON
 The two main transformations of carbon in nature are photosynthesis and
respiration. On the balance between these two processes depend
(a) the amount of organic matter that accumulates in soils and
sediments
(b) the quality of streams, lakes, and estuaries.
 In submerged soils, respiration (decomposition of organic matter) is the
main transformation.

1. DECOMPOSITION OF ORGANIC
MATTER
 In well drained soils aerobic microbes will decompose OM to form CO2,
NO3
-, SO4
2-.
 Under submerged condition anaerobic microbes will decompose
OM to produce CO2, H2, CH4, NH4
+, amines, mercaptans, H2S, and
partially humified residues

2. PYRUVIC ACID METABOLISM
 This will occur in both aerobic and submerged conditions.
 The precursor is sugars like glucose
C6H12O6 + 2ATP + 2NAD+ 2CH3COCOOH + 4ATP + 2NADH + 8H+
(Pyruvic acid)
 Under submerged condition Pyruvic acid will transforms,
(a) reduction to lactic acid,
(b) decarboxylation to CO2 and CH3CHO
(c) dissimilation to lactic, butyric and acetic acids and CO2,
(d) cleavage to acetic, formic acids, H2, and CO2,
(c) carboxylation to oxaloacetic acid
(f) condensation with itself or acetaldehyde to give acetylmethylcarbinol

3. KINETICS OF
CO2
 1 to 3 tons of CO2 are produced in the ploughed layer of 1 ha of a soil
during the first few weeks of submergence.
 Being chemically active, it forms HCOO-, HCO3
- and insoluble CO3
2-.
 The excess accumulates as gas.
 The partial pressure of CO2 in a soil increases after submergence,
reaches a peak of 0.2-0.8 atm 1-3 weeks later and declines to a fairly
stable value of 0.05-0.2 atm
 The decline in Pco2 after 1-4 weeks of submergence is due to escape,
leaching, removal as insoluble CO3
2-, dilution by CH4 produced during
the decomposition of organic acids, and bacterial reduction of CO2to
CH4

4. KINETICS OF VOLATILE ORGANIC
ACIDS
 The main organic acids found in anaerobic soils and sewage are formic, acetic,
propionic, and butyric acids.
 When a soil is submerged, the concentration of volatile organic acids
increases, reaches a peak value of 10-40 mmol/lit in 1-2 weeks and then
declines to less than 1 mmol/lit a few weeks later.
 Soils high in native or added organic matter produce high concentrations
of acids.
 Low temperature retards acid formation slightly, but acid destruction
markedly.
 Thus organic acids persist longer in cold soils than in warm soils.
 Ammonium sulphate appears to increase acetic acid formation but suppresses
the formation of propionic and butyric acids

5. METHANE FERMENTATION
 Methane is the typical end product of the anaerobic
decomposition of organic matter.
 Some of the methane is oxidized bacterially at the surface of paddy
soils and in the oxygenated strata of lakes.
 Methane formation is ecologically important because it helps the disposal
of large amounts of organic matter sedimented in lakes.

 Methane is produced by a small group of obligate anaerobes
(like Methansarcina inethanica).
 Methane bacteria function best at temperatures above 30°C, but most
abundant in natural anaerobic waters, produces methane even at 50°C.
 Methane bacteria are highly substrate specific and can metabolize only a
small number of simple organic and inorganic substances, usually the
products of fermentation.
Conti…

B.
NITROGEN
In submerged soils, the main transformations are
1. Accumulation of ammonia,
2. Denitrification,
3. Nitrogen fixation.
1. ACCUMULATION OF AMMONIA
Ammonia production in submerged soils follows a roughly asymptotic course
and the kinetics of ammonia release can be described by
log (A-y) = log A – ct
Where, A = mean maximum NH4-N concentration
y = actual concentration ‘t’ days after submergence
c = parameter depending on the soil.

2. DENITRIFICATION
 Nitrate undergoes two transformations in submerged soils:
a. assimilation or reduction of NO3
- with incorporation ofthe
products into cell substance
a. dissimilation or nitrate respiration in which NO3
- functions as an
alternative to O2 as an electron acceptor
 Rate of denitrification increases with temperature up to 60°C.
 Denitrification will occurs at below the redox potential of 350 mv
 Denitrification is slow in high OM soils (OM provides C, H and O2 to
microbes )
 Alternate wetting and drying increases denitrification loss

3. N2 FIXATION
 BNF is reduction of N2 to NH3.
 It requires high electron activity or low pE
 pE = - log ae Where ae = activity of e-
 Microbes help in BNF are Nostoc, Anabaena, Ocillatoria, Tolypothrix,
Calothrix, Phormidium and some algae species
 Slight alkaline and high P will increase the N- fixation
 They fix as much as 22 kg /ha of N2

N Transformation in Aerobic Soil

N Transformations in Aerobic vs Anerobic

C.
PHOSPHORUS
 Phosphorus in valence states from +5 to -3
 The forms are phosphite, hypophosphite, phosphine and
phosphate in anaerobic media.
 Soils having forms like,
a. Iron(III) and aluminum phosphates (in acid soils)
b. Phosphates adsorbed or co-precipitated with Fe(IlI) and Mn(IV)
hydrous oxides
c. Phosphates held by anion exchange on clay and hydrous oxides,
d. Calcium phosphates (in neutral & alkali soils)
e. Organic phosphates.
 The increase in concentration of water-soluble P on soil submergence

1. Sandy clay (pH= 7.6)
14. Clay(pH= 4.6)
25.Sandy loam(pH= 4.8)
26.Clay loam(pH= 7.6)
27.Clay(pH= 6.6)
Kinetics of Water Soluble P in Submerged Soils

D. IRON
The reduction of iron has important chemical consequences:
(a) the concentration of water-soluble iron (Fe2+) increases
(b) pH increases
(c) cations are displaced from exchange sites
(d) the solubility of phosphorus and silica increases and
(e) new minerals are formed.

 In acid soils high in OM and Fe will increases to 600 ppm
within 1-6 weeks after submergence
Fe3+Fe2O4.nH2O Fe2+ (Clay)
 Fe2+ diffuses and mass flow to the surface of soil and also to plant
roots where oxidise and forms precipitates under the plough sole
 Grey colour mottles due to FeS2
 Paddy soils contains hydrated magnetite (Fe2O4.nH2O) along with
some hydrtrolilite (FeS.nH2O)
Conti…

E. MANGANESE
 In submerged soils Mn2+ availability is increased by conversion of Mn(IV)
oxides into Mn(II) ions or carbonates
 These Mn2+ ions moves to the oxygenated interfaces in soils by mass flow
and diffusion
 When co2 concentrations in soil increases Mn2+ precipitated as MnCO3

F. SULPHUR
In aerated soils:
1. Elemental S is converted into SO4
2-, sulphides and organic sulphur
compounds
2. Reduction of SO4
2- and incorporation into plant tissues as elemental
S.
In submerged soils
1. SO4
2- to sulphide
2. Other S containing compounds into H2S (forms bad ordous )
3. And used by S reducing microbes like Desulfovibrio

G. SILICON
 In soils occurs as crystalline and amorphous silica
 Also as silicates, adsorbed or co-precipitated with hydrous oxides of Al,
Fe(III) and Mn(IV) and also dissolved in the soil solution.
 Dissolved silica is present as monomeric Si(OH)4.
 The concentration of Si(OH)4, in equilibrium with amorphous silica at 25°C is
120-140 ppm as SiO2, and is independent of pH 2 to 9.
 Submergence will slightly increases (due to release by Fe3+ ions and higher
CO2 concentration) and then decreases the Si concentrations
(decrease in Pco2).

H. TRACE ELEMENTS
 Submergence will increase availability of Co, Cu and Zn.
 Increase in pH of acid soils lower the solubility of nutrients due to release of
Sulphide which forms precipitates
 The elements in reduced layer will moves towards to the oxidized layer

MINERAL EQUILIBRIA IN SUBMERGED
SOILS
A.Redox Systems
B.Carbonate Systems

A. REDOX SYSTEMS
 Reduction sequences as follows under submerged condition of soil
O2, NO3
-, Mn4+, Fe3+, SO4
2-, CO2, N2 and H+.
 These each are associated with H+ ions and Electrons.
 They includes systems like,
1. The O2 – H2O system
2. The N2 system
3. The Mn system
4. The Fe system
5. The sulphur system

B. CARBONATE
SYSTEMS
It includes,
(a) High concentrations of CO2
(b) The presence of the divalent cations, Fe2+, Mn2+, Ca2+ and Mg2+
in most soils, CaCO3 in calcareous soils and NaHCO3 in
sodic soils
(c) Intimate contact between solid, solution, and gas phases
(d) Virtual isolation of the system from the surroundings.
Thus sodic soils behave like NaHCO3, calcareous soils like CaCO3,
ferruginous soils like Fe3O4nH2O, and manganiferrous soils like MnCO3
when submerged and equilibrated with CO2.

The Chemistry of Submerged Soils, F.N. Ponnamperuma,
Advances in Agronomy, Vol. 24.
Reference

Chemistry of Submerged Soils

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