Less than -200

are often used to predict the most probable products of biological reactions. For example, N2O can be produced from nitrification under aerobic conditions and denitrification under moderately reducing conditions, where the reduction intensity is not strong enough to reduce nitrate completely to N2 gas.

The magnitude of EH depends on E0 and also on the relative activities of the oxidant and the reductant. These quantities are related by the Nernst equation:

EH (V) = E0 - (0.0591/«) log (reduct)/(oxid) + (0.0591m/«) pH.

The EH is the electrode potential of the standard hydrogen electrode, E0 is the standard half-cell potential, n is the number of electrons transferred, m is the number of protons exchanged, reduct is the activity of the reduced species, and oxid is the activity of the oxidized species.

The major redox reactions occurring in soils and the electrode potentials for these transformations are shown in Table 2.3. Typically, dissolved O2 and NO-serve as electron acceptors at EH ~ 350 to 400 mV and above, until their concentrations in the soil solution drop to about 0 at 350 mV. Manganese and Fe serve as electron acceptors starting around 350 mV for Mn and 250 mV for Fe to ~ 100 mV. When Fe3+ in Fe oxide is reduced, the oxide dissolves and Fe2+ goes into solution. Sulfate reduction occurs from EH as high as 350 to ~100mV. Methane production begins when EH is close to ~ 100 mV. Manganese, Fe, and SO4- reduction processes occur over a much wider range compared with O2 and NO- reduction and methane production.

Although the activity of electrons can be described by pE, EH has the advantage of being a standard measurement for investigations of soil redox potential both in the laboratory and in the field. Soil EH can be obtained relatively easily from measurements of the pore water using a platinum (Pt) electrode. It is important to ensure that the electrodes are functioning properly and maintain performance, i.e., cleaned and calibrated before being installed in the soil and checked at regular intervals to ensure that they not become contaminated by surface reactions and lose their efficiency or accuracy.

TABLE 2.4 Temperature Effects on Gaseous Diffusion in (A) Air and (B) Water and (C) Solubility in Water

Temperature (°C)



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