Radial O2 loss

The presence of iron oxyhydroxide coatings (i.e., Fe plaque, often dominated by ferrihydrite) on the surface of wetland plant roots is visual evidence that subsurface iron oxidation is occurring in otherwise anoxic wetland soils and sediments. Oxygen delivered via radial O2 loss may react with reduced iron in soil pore spaces to form oxidized iron that can be deposited on the plant roots as Fe plaque. Despite a long history of observing Fe plaque on wetland plant roots and understanding the basics of plaque formation [i.e., reaction of plant-transported O2 with Fe(II) in soils and sediments], it was largely assumed that plaque formation is predominately an abiotic (i.e., chemical) process because the kinetics of chemical oxidation can be extremely rapid (Mendelssohn et al., 1995). However, recent evidence has demonstrated that populations of lithotrophic FeOB are associated with Fe plaque and may play a role in plaque deposition. 

As summarized by Jacobson (1994), biological Fe(III) reduction will be more important than chemical reduction when amorphous Fe(IIl) oxides are plentiful and continually regenerated, or H2S production is low relative to the Fe(III) concentration. This first condition is likely to be met in the rhizosphere where radial O2 loss drives Fe oxide formation. The second condition will be met in low-salinity wetlands, or in saline systems with mineral (i.e., iron-rich) sediments. However, even chemical reduction of Fe(III) is ultimately due to microbes since the H2S that reduces the Fe is the result of a biological process, SO4" reduction (Megonigal et al., 2004). 

In the model, the internal structure of the root is described as three concentric cylinders corresponding to the central stele, the cortex and the wall layers. Diffu-sivities and respiration rates differ in the different tissues. The model allows for the axial diffusion of O2 through the cortical gas spaces, radial diffusion into the root tissues, and simultaneous consumption in respiration and loss to the soil. A steady state is assumed, in which the flux of O2 across the root base equals the net consumption in root respiration and loss to the soil. This is realistic because root elongation is in general slow compared with gas transport. The basic equation is... 

A sufficiently high inlet velocity will cause the flame to be extinguished [270]. There are two reasons for the extinction. One is heat loss to the wall, which reduces the flame temperature and hence the chemical reaction rates. The second, and perhaps less obvious, is strain extinction. As the inlet velocity increases and the boundary layer thins, the radial velocity increases (the general shape of the radial velocity profiles are shown in Fig.6.6). As the radial velocity increases, the residence time in the flame zone also decreases. The reduced residence time, in turn, limits the time available for the relatively slow radical-recombination reactions to keep the flame temperature high. Reduced temperature and residence time limit the relatively slow the chain-branching reaction H + O2 OH + O, which is needed to sustain a flame. Ultimately a flame cannot be sustained [214],... 

As the radial distance Increases, the external radiation field dominates cosmic-ray ionisation and the radical OH becomes the dominant reactive species. In addition to loss through photoprocesses, OH can react with a large number of species and, in particular, with 0 and N atoms, the latter formed in the photodissociation of N2, to produce O2 and NO. The reaction 0 + OH —> O2 + H... 

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