Soil redox conditions

Mitsunobu, S., Harada, T. and Takahashi, Y. (2006) Comparison of antimony behavior with that of arsenic under various soil redox conditions. Environmental Science and Technology, 40(23), 7270-76, and supplements. 

P-Glucosidase activity is influenced by soil redox conditions, with higher values under aerobic conditions than nnder anaerobic conditions (Figure 5.19). p-Glucosidase activity is influenced by... 

Note Soil redox conditions were maintained by addition of nonlimiting supply of electron acceptors. DQC = dihydroindole quinone carboxylate ND = not detected SIR = substrate-induced respiration (McLatchey and Reddy, 1998). 

Soil redox conditions or Eh status governs the oxidation and reduction of some trace metals found in wetlands. Trace metals are present in various oxidation states, for example, chromium can exist in several oxidation states from Cr(0), the metallic form, to Cr(Vl). The most stable oxidation states of chromium in the environment are Cr(lll) and Cr(Vl). Besides the elemental metallic form, which is extensively used in alloys, chromium has three important valence forms Cr(ll), Cr(lll), and Cr(Vl). The trivalent Cr(lll) and the hexavalent Cr(Vl) are the most important forms in the environment. 

The chanistry of copper in water and soils is very complex. Potential chemical forms include insoluble organic complexes, sulfide minerals, and solid copper phases. It is clear that changes in soil redox condition (from oxidized to reduced) change the relative distribution of copper in various soil phases. 

In contrast to Cr(III), the soil redox condition strongly influences sorption of Cr(VI). Under oxidized and moderately reduced (+500 to +100 mV) soil conditions, chromium behavior is dominated by Cr(VI) sorption and reduction of Cr(VI) to Cr(III) (DeLaune et al., 1998). Under more reduced soil redox levels (<+100 mV), chromium chemistry and solubility is controlled by the reduction of Cr(VI) by soluble ferrous iron. 

The importance of these gas conduits in aquatic macrophytes of natural wetland areas has been well documented. Methane flux to atmosphere by vascular transport is related to soil redox conditions in which the plant grows (Figure 16.3). 

FIGURE 18.15 Effect of low soil redox conditions on the photosynthesis activity of Spartina alterniflora. Plants were grown in soil suspension in which soil redox level was controlled using techniques described by DeLaune et al. (1984). (Redrawn from DeLaune et al., 1984.)... 

Reddy, K. R., T. C. Feijtel, and W. H. Patrick, Jr. 1986. Effect of soil redox conditions on microbial oxidation of organic matter. In Y. Chen and Y. Avnimelech (eds.) The Role of Organic Matter in Modern Agriculture. Dev. Plant Sci. Martinus Nijhoff, Dordrecht, pp. 117-148. 

Bachmann A, P Walet, P, Wijnen, W de Bruin, JEM Huntjens, W Roelofsen, AJB Zehnder (1988) Biodegradation of alpha- and beta-hexachlorocyclohexane in a soil slurry under different redox conditions. Appl Environ Microbiol 54 143-149. 

Soil redox also strongly affects solubility of the compounds of other trace elements in arid soils. Amrhein et al. (1993) found that Fe, Mn, Ni and V in an evaporation pond soil were more soluble under reducing conditions. Han and Banin (2000) reported that after one year of saturated incubation, the solubility of Fe, Mn, Co, V, Ni, Cu and Zn in two Israeli arid soils with 0.5-23% CaC03 increased, while the solubility of Cd decreased with time. During saturated incubation, soil pH in highly calcareous arid soil containing high content of carbonates decreased. In a loessial soil from Israel, Han and Banin (1996) reported that soil pH decreased from 8.0 to 7.0-7.4 over saturated incubation. With the decrease in Eh over incubation, the parameter pe+pH also decreased from initial values of 12-13.6 to 4 after initial 7-9 days of saturation incubation. 

While not stated explicitly, in this discussion so far, it has been assumed that all the systems were well defined, at equilibrium, and at a constant 25°C. None of these conditions occur in soil in the environment. Soil is not a pure system and, often, all the components affecting redox reactions are not known, defined, or understood, and a host of different redox couples are likely to be present. Unless it is possible to take into account all couples present, it is not possible to describe the exact redox conditions in a soil without measuring it. 

In spite of the limitations, Eh-pH data provide information about the condition of a soil in terms of it being in an oxidizing or reducing condition. Thus, it will indicate the prominent redox conditions of the species present (see also Chapter 6). It will also indicate what changes in Eh or pH may be desirable to effect the desired extraction or analysis for the compound or species of greatest concern [9-11]. 

Effects of Flooding and Redox Conditions onfs. I know of no published data on this. Bnt it is likely that the natnre of particle surfaces in intermittently flooded soils wonld restrict snrface mobility. For ions to diffuse freely on the surface there must be a continuous pathway of water molecules over the surface and uniform cation adsorption sites. But in intermittently flooded soils the surface typically contains discontinuous coatings of amorphous iron oxides on other clay minerals, and on flooding reduced iron is to a large extent re-precipitated as amorphons hydroxides and carbonates as discussed above, resulting in much microheterogeneity with adsorption sites with disparate cation affinities. 

Effects of Flooding and Redox Conditions on OClAIC. Reductive dissolution reactions of the sort indicated in Figures 2.6 and 2.7 will affect the amount of a solute in diffusible forms in the soil and the distribution of the diffusible forms between the soil solid and solution. These processes are discussed in detail in Chapter 3. 1 here exemplify their effects by reference to a study of phosphate diffusion in a soil under different water regimes. 

Transport of solutes and gases through the soil is much slower than through soil-free water because of the restricted cross-sectional area for transport through the soil pore network and because of adsorption and reaction on soil surfaces (Chapter 2). Redox conditions are therefore closely linked to transport processes. 

In this section the redox conditions developing in soils following submergence... 

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