Soil Aeration and Plant Growth
Oxygen movement in Soil-Plant System
Oxygen diffusion through air is 2.1 x 101 cm2 s1,
whereas oxygen diffusion through waterlogged soil is 1 x 105
cm2 s1, ie ten thousand times slower in wet soil.
Therefore oxygen diffusion in soil is determined primarily by the air-filled
pore space which is determined by total pore space and water content.
Within the soil roots, soil fauna and soil microbes create an oxygen
Oxygen is generally considered to be limiting if the concentration in
the soil air falls below 10%. Except for winter conditions an air-filled
porosity of less than 10% is also considered to indicate inadequate aeration.
Development of anaerobic conditions in soil
Reading: Marschner pp 626-627
Q Where and when do adverse conditions of aeration occur?
Q What is the primary effect of flooding ?
Waterlogging of soils can be the result of climate-soil interaction, ie
too much precipitation for the soil to accept and transmit. Soils with
poorly permeable subsoils can readily become waterlogged. Soil compaction
by vehicular traffic when soil strength is weak can restrict water movement.
The overflowing of rivers and streams is also an obvious occurrence. There
is also the purposeful flooding of soil for production of paddy rice. Much
of our understanding of the impact of waterlogging on soils comes from
studies of this managed flooding.
Q Does inadequate aeration only occur in flooded soils ?
The primary impact of waterlogging is the filling of soil pores with water.
Because the key factor in aeration is the balance between the supply of
oxygen and the demand by the organisms within the soil, anaerobic conditions
can also develop in the centre of aggregates, even when the average O2
concentration is relatively large, if there is intense microbial activity
at the centre or that diffusion of O2 into aggregates is slow.
If the aggregate is surrounded by air-filled macro-pores, but pores
in the aggregate are water-filled, anaerobiosis can develop in aggregates
ranging from 0.4 to 2 cm diameter. The higher the temperature, the smaller
the length of water-filled pores that will be necessary to reduce the flux
In soils containing readily decomposable organic matter microbes will
tend to use oxygen faster than it can diffuse into the soil.
The temperature will be important in developing anoxia:
Poor O2 supply results in reducing conditions (low oxidation
- reduction potential) resulting in reduction of elements or compounds.
This may result in incomplete oxidation of carbon substrates with carboxylic
acids (eg acetic acid -associated with straw breakdown under anaerobic
conditions), alcohols (eg ethanol) and ketones.
These compounds can react with ferric oxides forming chelates as reduction
Further reduction of the chelates can result in CO2,CH4,C2H4
Glucose => (2) pyruvic acid + 4H+ + 4e
Under well aerated conditions oxygen is the terminal acceptor
(O2 + 4e) + 4H+ 2H2O
In the absence of free oxygen other substances take over:
Some anions containing oxygen can accept electrons and release oxygen:
2 NO3- + 12H+ + 10 e- 6
H20 + N2
2NO2 + 6e + 8H+ => N2 + 4H2O
SO42 + 8e + 10H+ => H2S
Some polyvalent cations can accept electrons and reduce their valency
Mn4+ + 2e => Mn2+
The affinity of substances in the soil solution for electrons can be
measured by its redox potential (Eh). The more strongly
reducing is a substance the less is its affinity for electrons so the lower
is the potential.
The oxidation - reduction (redox) potential of the soil is a measure
of the tendency of the soil solution to accept or donate electrons. Oxidation
involves a loss of electrons, reduction results in a gain. The more strongly
oxidizing a solution, the higher its potential; the more reducing, the
lower. In a well-aerated soil, oxygen serves as the electron acceptor and
the potential is high. As 02 is depleted, the potential drops
and other components serve as electron acceptors depending on their tendency
to accept or donate electrons. In the soil, the major components involved
are nitrate ions, manganese, iron compounds, sulfate, and eventually CO2.
The oxidation - reduction potential for reduction of NO3-
to NO2- is 420 mV at pH 7 and 530 mV at pH 5. Nitrate
is readily reduced to N2O or N2 at these oxidation-reduction
As the redox potential drops, these components serve, in turn, as electron
acceptors. As long as 02 is present in significant quantity
the redox potential remains high (800-900 mv). When 02 is depleted,
the potential will drop quickly to point that either Mn+3 or
NO3- ions, depending on the pH, serve as electron
acceptors and are reduced. Nitrate is reduced first to nitrite (NO), then
to nitrous oxide (N2O) or to N2. This process is
known as denitrification.
The overall reaction is as follows:
NO3 + 2e + 2H+ => NO2 + H2O
NO2 + 6e + 8H+ => N2 + 4H2O
2 NO3- + 12H+ + 10 e- =>6
H2O + N2
When NO3- or Mn4+ ions are present, the
redox potential will remain poised, ie will decrease slowly. When all the
NO3- or Mn4+ ions are reduced, the potential
will drop to point where Fe3+ is reduced. This sequence will
continue through the following reactions.
Manganese is reduced as follows:
MnO2 + 4H+ + 2e- =>Mn2+
+ 2 H2O (410 mV at pH 7, 640 mV at pH 5)
The redox potential is given by :
Carbon dioxide will also act as an acceptor:
Fe(OH)3 + 3H+ + e- => Fe+2
+ 3 H2O (-180 to + 170 mV)
SO42- + 10H+ + 8e- => H2S
(hydrogen sulfide) + 4 H2O (-220 to -70)
CO2 + 8H+ + 8e- => CH4
(methane) + 2H2O (-240 to -120)
where Kr is the equilibrium constant for the redox reaction.
n is the number of electrons transferred, F is the Faraday constant, and
values in brackets are the activities of the two species.
||Redox potential (mV)
||Stabilized by oxygen
||stabilized by nitrate
||stabilized by manganese
||stabilized by organic matter
||200 <=> -40
||stabilized by ferric ions
||stabilized by sulphate
||stabilized by carbon dioxide
As a soil becomes more depleted of oxygen the redox potential falls,
but the magnitude of the change depends on the concentration of ions that
take part in redox reactions.
As the redox potential falls, manganese and iron can be reduced to
soluble forms, which may then reach concentrations that are toxic to plants.
In some soils, ethylene can form after waterlogging. The substrate
for formation is methionine, and oxygen is required for the process.
Plants can alter the conditions in the rhizosphere and at the root
meristem. Oxygen can diffuse through aerenchyma tissue in the cortex of
roots to the root tip and even into the soil. The proportion of the root
cortex that consists of aerenchyma varies with plant species which is related
to their sensitivity to poor aeration.
Response of Plants to Anaerobic Conditions
Reading: Marschner: pp 627-641.
Q What causes the detrimental effects of poor aeration on plant growth
Effects are due to one or a combination of the following.
a) O2 deficiency
Root elongation slows dramatically as the flux of oxygen decreases. The
critical flux at 10C appears to be close to 10-9 g cm-2.sec-1,
consistent with that required to maintain unrestricted respiration of cells
in the root apex:
- cessation of root growth
Symptoms shown by crops under anaerobic conditions.
Stomata closure commonly occurs
Sensitive plants wilt within a few hours
Uptake of some ions is reduced, while others (eg iron and manganese)
may become more available. The illustration is of boron deficiency in sunflowers
developed under flooding (Control plant is on LHS, plant grown in waterlogged
soil is on RHS).
Q Why do plants wilt in flooded soil?
Reduction in photosynthesis - leaves will become chlorotic and die
Inhibition of shoot growth - leaves and stems
Leaf, flower, fruit abscission
Enhanced root exudation
Water transport (in the xylem) is decreased.
As K uptake (in some plants at least) is not affected, the stomatal closure
is not necessarily associated with potassium deficiency.
Changes in vapour pressure gradients between the leaf and the atmosphere
Effect of flooding on stomatal conductance in peas
If yL falls the root resisistance
or the xylem resistance must have increased.
Xylem resistance would not be expected to increase markedly in the short
term, so the evidence would suggest that the root resistance was the one
that changed. There are changes in root permeability as seen by the enhanced
exudation. In fact we know that tyloses can develop
which then partly occlude the lumen of xylem vessels. So that there may
be changes in both resistances. Also the increase in the root resistance
may be rather short lived.
Plants may adapt to poor aeration by
||Stomatal Conductance (cm sec1)
|Time after flooding
Since the transfer of electrons from cytochrome oxidase to oxygen cannot
take place in anoxic conditions, ATP formation by the terminal oxidase
system is prevented. NADH and other flavoproteins produced by the glycolytic
pathway and the TCA (Krebs) cycle cannot be re-oxidised so the cycle stops.
This in turn can lead to the cessation of H+ efflux pumps and
acidification of the cytosol.
Effecting anaerobic metabolism which leads to ethanol production.
One way of generating ATP and using NADH is the formation of ethanol.
Alcohol dehydrogenase production in marsh plants was seen to be produced
in proportion to the sensitivity to flooding.
One suggestion is that flood sensitive plants form ethanol which is
toxic. However, the intermediate is acetaldehyde which is much more toxic
than ethanol. Some tolerant species produce ethanol in the same concentrations
as in sensitive species.
Lactate is another compound that is produced, that is also toxic.
An alternative explanation of flood tolerance is the formation of malate
by reduction of oxaloacetate with NADH. It is assumed that the oxidation
of malate to pyruvate is either blocked by the inhibition of enzyme formation
or activity. However, malate does not appear to accumulate to any great
extent in tolerant species.
Some plants will accumulate malate or lactate in their vacuoles. This
would help to maintain the pH of the cytosol.
Some plants produce superoxide dismutase (SOD) which is thought to
provide protection from damage when aerobic conditions return.
In many marsh plants flooding promotes extension of stem internodes or
petioles. In deep-water rice, coleoptiles and internodes increase in length
Promotion of extension of internodes and petioles.
Swelling of the stem base or hypocotyl is common in herbaceous plants subject
Flooding can induce root formation at or just below the water level.
Promotion of adventitious roots
This is largely mediated by ethylene, which may be transferred from the
root system as its precursor AAC (1-aminocyclopropane-1-carboxlate).
The precursor is converted to ethylene in the presence of oxygen in the
shoot. In the shoot ethylene can disturb the balance of phytohormones leading
to changes in water and nutrient status.
Aerenchyma formation in the roots
This is mediated by ethylene formation in the root. The formation of aerenchyma
takes place in the cortex, and becomes well developed 3-4 cm behind the
root tip. The lacunae result from the lysis of cells (see micrograph supplied
by M.C. Drew). Plant roots respond very variably to exogenously applied
ethylene. There appears to be stimulation of elongation when the external
concentration is small, though the magnitude of the affect is much greater
for rice than for other crops that have been studied. As the concentration
increases so the stimulation turns to inhibition of extension. The rate
of endogenous ethylene production may be important in this aspect of the
response of plants to waterlogging.
The contribution of aerenchyma is important, and if the lacunae represent
about 15% of the cross-sectional area of the root, a root 1mm in diameter
can penetrate some 10 cm below the water surface.
The oxygen supply to roots can be very effective, and oxygen may leak
out into the surrounding soil. This can be sufficient to precipitate Fe3+
around the root.
Additional physiological responses mediated by root signalling induce
changes in shoots.
Tolerance to toxins is also important, especially to manganese, iron and
Stomatal closure may well be induced by ABA from the root which is
rapidly released into the xylem at the onset of waterlogging. ABA
levels in leaves certainly increase due to export from the roots.
Cytokinin export from leaves declines. Gibberellin activity declines.
All the consequences of flooding on plant growth is summed up in the
following diagram based on Drew (1983) Pl. Soil 75,179-199.
Significance of land drainage
Much of the agricultural land in Ontario has some under drainage to prevent
waterlogging of soils in spring. The effectiveness of the drainage system
in removing excess water in spring has to be balanced by the potential
water shortage in summer. The drainage requirements for crops has received
little attention. For winter wheat, work at Letcombe Laboratory some 20
years ago established that lowering the water table below 0.5 m had no
significant benefit on crop yield.