Rhizosphere radial

Extraction of rhizosphere soil (22,34,51,52) is an approach that can provide information about long-term accumulation of rhizosphere products (root exudates and microbial metabolites) in the soil. Culture systems, which separate root compartments from adjacent bulk soil compartments by steel or nylon nets (52-54) have been employed to study radial gradients of rhizosphere products in the root environment. The use of different extraction media can account for different adsorption characteristics of rhizosphere products to the soil matrix (22,34). However, even extraction with distilled water for extended periods (>10 min) may... 

As water moves through the soil pores in response to water potential gradients, it moves with it the solutes dissolved in soil solution. In a rhizosphere context, water moves radially toward the root to replace water taken up by the roots for transpiration. The flux of solute due to water movement (7 .) is simply the product of the rate of water flow at that point and the concentration in soil solution ... 

In a cylindrical system, because equal amounts of water are being transported across successive concentric radial increments, the rate of flow of water must vary across the rhizosphere, with the fastest flows at the root surface. It is... 

Armstrong W, Beckett PM. 1987. Internal aeration and the development of stelar anoxia in submerged roots. A multishelled mathematical model combining axial diffusion of oxygen in the cortex with radial losses to the stele, the wall layers and the rhizosphere. New Phytologist 105 221-245. 

One of the primary functions of the root is the uptake of nutrients from the soil solution this activity determines the formation of radial and longitudinal ion gradients in the rhizosphere. 

In water logged soils radial oxygen loss from the root raises the redox potential in the rhizosphere as a consequence of which iron oxide plaques are seen to develop on root surfaces. Bacha and Hossner (1977) removed the coatings on rice roots growing under anaerobic conditions. The coatings were composed primarily of the iron oxide mineral lepidocrocite (y-FeOOH) as the only crystalline component. St-Cyr and Crowder (1990) studied the iron oxide plaque on roots of Phragmites and detected both Fe and Mn. The Fe Mn ratio of the plaque resembled the ratio of Fe Mn in substrate carbonates. The plaque material also contained Cu. 

The longitudinal fluxes that may arise from the preferential flow paths of water and solutes along large root channels are even less well documented than the radial fluxes of trace elements in the rhizosphere. It is well known that roots tend to colonize former biopores, as indicated in Section 4.2. As many of those macropores, especially vertical/subvertical biopores created by earthworms or root activity, are prone to preferential flow during re-saturation events, they may complicate our understanding of (1) the directions of trace elements and water fluxes, and ultimately, (2) the origin of the solutes circulating in the root environment. This process has received virtually no attention so far, however, possibly because of the practical difficulties associated with its quantitative assessment. 

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). 

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