The following are the effects of redox reactions in anaerobic soils on nutrient availability and plant growth;
Reduction Reactions
Set in under anaerobic conditions when oxygen diffusion within the soil is reduced mainly by excess water to a level below 2 to 4 x 10-6M concentration. The anaerobic condition (low O2 concentration) gives rise to a series of microbial reactions that always take place in the same sequence.
Although reduction reactions are associated mainly with waterlogged or wet soils, they sometimes occur in all soils at some time during the year.
Immediately after flooding, the exchange of gases between the soil and the atmosphere is cut down as the diffusion of gases in water is approximately 10-4 x that in the atmosphere. Also, the reduction reaction induces an abrupt drop in the redox potential (Eh) to values between 0.2 and 0.2 volts.
Thereafter, the soil layers below the soil-water interface become depleted of oxygen which has rapidly been consumed by the aerobic micro-organisms and cannot be replenished.
Within one or two days virtually all 02 disappears giving way to the increased activity of facultative and obligate anaerobic microorganisms.
As a result of the metabolic activity of these organisms, many compounds with high oxidation states can be reduced, either directly by acting as electron acceptors in the anaerobic dissimilation process, or indirectly by forming organic decomposition products with properties.
The most important process taking place upon flooding include:
Accumulation of gases such as CO2, H2, and CH4 reduction (Denitrification) of nitrate, NO2 to e.g. N2
Reduction of manganese and iron oxides to Mn2+ and Fe2+
Reduction of sulphates to sulfides considerable increases in the concentrations of Ca2+, K+, Na+, H2PO4– and CHO3– in the interstitial water.
Production of ammonia and soluble organic compounds (fatty acids, amino acids, and mercaptans) as a result of the anaerobic decomposition of fresh organic material.
Stabilization of the pH between 0.5 and 7.0 and decrease of redox potential.
Effect of Reduction Reaction on Plant Growth
Several products of anaerobic metabolism (such as CH4, N2, NH4+, S, S2-, Fe2+ (ferrous oxides), and Mn2+ (manganous oxides) are known to be toxic to root cells; some of these products also upset the balance of growth factors e.g. auxins and abscisic acid resulting in epinasty of the leaves or abscission of the reproductive system (i.e. flower abscission).
Plants may also be predisposed to diseases being weakened enough by this production of anaerobic metabolism to become more susceptible to pests and diseases.
Root cells convert glucose into ethyl alcohol (C2H5 OH) which can badly upset the plant development, especially at a flowering time for plants such as cowpea and cereals causing a serious loss in yields.
In plants adapted to anaerobic conditions, malic acid accumulates from sugars and it is not toxic to the cells. For plant root metabolism to remain aerobic, the oxygen gradient between the root cell and outside the cell must be high enough to permit diffusion of oxygen from outside into the plant root.
Root growth in oxygen-deficient soils is stunted by the reduced rate of water and nutrient transfer from the soil to the xylem tissue.
Even on the same soil type, roots grow faster in areas of good aeration than in poor aeration. Poor aeration causes denitrification and loss of nitrogen and crops in such soil become pale and yellow in coloration.
Some crops are more tolerant to poor aeration than others; for example, sorghum yields are higher than those of maize grown in poorly drained soils.
Read Also: Effects of Redox Reaction in Aerated Soils on Nutrient Availability and Plant Growth
