Reduction sequence soils

Reduction of a submerged soil proceeds roughly in the sequence predicted by thermodynamics ... 

Arsenic in soilds has been fractionated by Jackson s T28) procedure for soil phosphorus (15. 27). In this laboratory, a modification of Jackson s procedure is being used on sediment solids. A series of 1 molar solutions of NH4CI, NH4OH, acid ammonium oxalate (29) and HCl are used in sequence. The chloride fraction, or exchangeable fraction, contains weakly adsorbed, coulombically bound arsenic. The hydroxide fraction, contains iron and aluminum arsenate precipitates and surface precipitates (i.e. adsorbed arsenic species having both chemical and coulombic bonding to oxide surfaces). The oxalate, or reductant soluble fraction, contains arsenic occluded in amorphous weathering products. The acid, or calcium, fraction contains arseno-apatites. 

The order of competing terminal electron accepting processes can vary with any number of factors that influence the thermodynamics of the system. One factor that must be considered in ecosystems with mineral sediments or soils is the composition of Fe(III) and Mn(IV) minerals. The typical sequence of Fe(III) reduction before SOl reduction can be reversed with a change in the abundance of labile Fe(III) minerals such as ferrihydrite (Postma and Jakobsen, 1996). This is one explanation for the common observation that the zones of Fe(III) reduction and SOl reduction overlap in marine sediments (Boesen and Postma, 1988 Canfield, 1989 Canfield et al., 1993b Goldhaber et al., 1977 Jakobsen and Postma, 1994). Postma and Jakobsen (1996) predicted that the overlap between Fe(III) reduction and SOl reduction should increase as Fe(III) oxide stability (or surface area) increases. 

The viability of the various soil consortia was dramatically affected by multiple freeze-thaw cycles, with CFU/ ml typically decreasing by five orders of magnitude after 48 cycles (Figure 2). This was then repeated for a second 48 cycles after reculturing the survivors (not shown). The overall decrease in viability did not simply result from a reduction in numbers since the appearance of the colonies indicated that there was a shift in the complexity of the populations. Evidence for this differential susceptibility to freeze-thaw treatments is shown by the lack of E. coli colonies after 24 cycles and the absence of P. chloroaphis after 48 cycles (Figure 2). After isolating DNA from 16 colonies that survived this rigorous freeze-thaw selection in one series of experiments, five distinct sequences were identified. The bacteria from which these five sequences were isolated were then used... 

The importance of pH as a master variable controlling chemical reactions in soils has been stressed in previous chapters. However, soils subjected to fluctuations in water content come under the influence of another master variable the reduction-oxidation (or redox) potential Under conditions of water saturation, the lack of molecular oxygen can result in a sequence of redox reactions that changes the soil pH. In this sense the redox state of the soil exerts control over the pH. The nature of redox reactions will be discussed in this chapter, as these reactions profoundly influence metal ion solubility and the chemical form of ions and molecules dissolved in soil solution. The reader is referred to section 1.2f in Chapter 1 for a review of the basic chemical principles necessary for the understanding of redox reactions. 

Figure 7.5. The reduction and oxidation sequence in soil solutions at pH 7. Theoretical potentials are indicated by solid lines, assuming equal activities of reduced and oxidized species unless otherwise noted (the pressure of H2 is arbitrarily set at 10 atmosphere). Measured ranges of soil potentials over which the indicated species react (change concentration) during soil reduction and oxidation are specified by boxes (shaded for reduction, open for oxidation, black for initial appearance of the reduced form during reduction). (Data, in part, from W. H. Patrick and A. Jugsujinda. 1992. Sequential reduction and oxidation of inorganic nitrogen, manganese, and iron in flooded soil. Soil Sci. Soc. Am. j. 56 1071-1073.)...

In the sequence of emission-transport/con-version-deposition, the last phase has been dealt with in detail (Guderian 2000, 2001). Less emission leads to less deposition. Air pollutants exist as dust particles and in gaseous form, and for the effect of air pollutants their concentration in the air (immission) and their quantity deposited on receptors (deposition) is important. In ecosystems, both nutrients and harmful substances are deposited, and air pollutants may act as either acidic or alkaline, and as reductive or oxidative. The impact of immissions on plants may occur either directly (on plant leaves) or indirectly (through the soil), visible or invisible, latent or acute and chronic. Since they serve as sensitive receptors, plants and ecosystems (e.g., crops, forests, natural vegetation), soils and waters are of major interest. In order to characterize the atmospheric pollutant load of a special site in the landscape, it is first necessary to know the concentration situation and the deposited quantity per time and surface unit - the surface load. Concentrations may be measured over shorter or longer (> 24 h) periods. The concentration of gaseous air pollutants is measured in ppm, ppb or jg m. ... 

List the inorganic redox system found in flooded soils and sediment. What is the order of sequence of reduction of the inorganic redox system ... 

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