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 demand. 

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? 
 

  • 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 What is the primary effect of flooding ?  
  • The primary impact of waterlogging is the filling of soil pores with water. 
Q Does inadequate aeration only occur in flooded soils ? 
  • 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 of oxygen. 
    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 products. 
    Further reduction of the chelates can result in CO2,CH4,C2H4 and H2 

    In glycolysis 

    Glucose => (2) pyruvic acid + 4H+ + 4e 

    Under well aerated conditions oxygen is the terminal acceptor 

    (O2 + 4e) + 4H+ 2H2

    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 + 4H2

    SO42 + 8e + 10H+ => H2S + 4H2

    Some polyvalent cations can accept electrons and reduce their valency state: 

    Mn4+ + 2e => Mn2+ 

Redox Potential 

    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 potentials. 
    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. 

                NO3 + 2e + 2H+ => NO2 + H2
                NO2 + 6e + 8H+ => N2 + 4H2
          The overall reaction is as follows: 
                2 NO3- + 12H+ + 10 e- =>6 H2O + N2 
          Manganese is reduced as follows: 
                MnO2 + 4H+ + 2e- =>Mn2+ + 2 H2O  (410 mV at pH 7, 640 mV at pH 5) 
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. 
                  Fe3+ + e => Fe2+ 
              Fe(OH)3 + 3H+ + e- => Fe+2 + 3 H2O (-180 to + 170 mV) 
              SO42- + 10H+ + 8e- => H2S (hydrogen sulfide) + 4 H2O (-220 to -70) 
          Carbon dioxide will also act as an acceptor: 
              CO2 + 8H+ + 8e- => CH4 (methane) + 2H2O (-240 to -120) 
The redox potential is given by : 
 
 
    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. 
 
 
Soil state Redox potential (mV)
Aerobic
Stabilized by oxygen (800)600<=>400(300)
stabilized by nitrate  530<=>420
stabilized by manganese 640<=>410
Anaerobic
stabilized by organic matter 200 <=> -40
stabilized by ferric ions 170 <=>-180
stabilized by sulphate -70 <=>-220
stabilized by carbon dioxide -120<=>-240
 
    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 
      - cessation of root growth 
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: 
       
       

      - reduction of nutrient absorption 
      This is very pronounced for nitrogen uptake. The example shown is for reduced boron uptake. 
       

    b) Toxic compounds - ethylene, ethanol, Mn2+, Fe2+, H2
    c) Nitrogen deficiency due to denitrification or leaching 

Symptoms shown by crops under anaerobic conditions. 

Observations: 
      Sensitive plants wilt within a few hours 
                    Stomata closure commonly occurs 
      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).
      Reduction in photosynthesis - leaves will become chlorotic and die 

      Inhibition of shoot growth - leaves and stems 

      Premature senescence 

      Leaf, flower, fruit abscission 

      Enhanced root exudation

Q Why do plants wilt in flooded soil? 
  • 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 are unlikely. 
Hence as 
 
 
  • 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. 
Effect of flooding on stomatal conductance in peas  
 
 
  Stomatal Conductance (cm sec1)
Time after flooding 

(days)

Control Flooded
0 0.39  
3 0.43 0.02
5 0.48 0.03
7 0.28 0.02
Plants may adapt to poor aeration by 
      • Effecting anaerobic metabolism which leads to ethanol production. 
    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. 
    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. 
 
      • Promotion of extension of internodes and petioles. 
    In many marsh plants flooding promotes extension of stem internodes or petioles. In deep-water rice, coleoptiles and internodes increase in length 
     
      • Hypertrophy 
    Swelling of the stem base or hypocotyl is common in herbaceous plants subject to flooding. 
     
      • Promotion of adventitious roots 
    Flooding can induce root formation at or just below the water level. 
      • Epinasty of petioles 
    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. 

      • Tolerance to toxins is also important, especially to manganese, iron and phophorus. 
 Additional physiological responses mediated by root signalling induce changes in shoots. 
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. 
 
 
 
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