Rhizosphere gradients

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... 

Polyvinyl chloride cylinders -1- nylon gauze -1- device for continuous water supply Soil slices at measurable distance from soil-root interface. High bulk density of soil sampled. Nutrient uptake through an induced root hairs surface. Study of rhizosphere effect over a time and distance gradient from the soil-root interface. 47, 67, 127-129... 

Reciprocally, the growth on single C source significantly decreases the bacterial diversity. For example, in the rhizosphere soil of potato, a dramatic reduction in the number of ribotypes was found by temperature gradient gel electrophoresis (TGGE) after 48 h of incubation with single C source substrate in Biolog microplate wells (I46). 

To describe the diffusion of solutes in the rhizosphere, where concentration gradients change with time, /, as well as space, mass conservation is invoked with the spatial geometry appropriate for the cylindrical root (8) ... 

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 ... 

Duineveld BM, Kowalchuk GA, Keijzer A, van Elsas JD, van Veen JA (2001) Analysis of Bacterial Communities in the Rhizosphere of Chrysanthemum via Denaturing Gradient Gel Electrophoresis of PCR-Amplified 16S rRNA as Well as DNA Fragments Coding for 16S rRNA. Appl Environ Micrbiol 67 172-178... 

The calculations in Section 6.2 indicate that the root system as a whole can sustain considerable rates of O2 loss to the rhizosphere without compromising their internal O2 requirements. The standard O2 flux in the calculations in Section 6.2 was 0.5 nmol dm (root) s for the parts releasing O2. For rice roots grown in soil, Begg etal. (1994) obtained values of 0.1-1.2nmol dm (root) s from rates of Fe + oxidation and Fe(III) accumulation near planar layers of rice roots in anaerobic soil, and Kirk and Bajita (1995) obtained 0.1-0.2 nmol dm (root) s with the same experimental system but a soil with a smaller ferrous iron content. These values probably underestimate the total O2 release because they did not allow for O2 consumed by soil microbes. Revsbech et al. (1999) obtained values of 1-3 nmol dm (root) s from measurements of O2 gradients made with a microelectrode near rice roots in the soil used by Kirk and Bajita (1995). These values are in the middle of the range described above. 

A schematic presentation of the fluxes and gradients occurring in the rhizosphere is reported in Figure 9.1. 

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. 

One of the most important features in the soil-plant relationship is the rhizosphere extent. This factor is highly variable, ranging from <1 mm to several millimeters and strongly dependent on the gradients that develop in the rhizosphere as a consequence of different processes. In these processes a crucial role is played by the root hairs, tubular outgrowths of root epidermal cells the development of root hairs is dependent on the genotype and is affected by the environmental conditions (e.g. nutrient availability, abiotic stresses and hormones). 

On the other hand, at a pH value of 5.0 there were no large differences in the strength of bonds between humic acids and metals such as Ca, Mg, Mn, Co, Ni, and Zn, whereas bonds with Pb, Cu, and Fe were stronger than with other metals (Schnitzer and Kahn, 1972) this behavior indicates that at different pH values, metal humic substance complexes of different stability are formed in the soil. This aspect is of particular relevance in an environment such as the rhizosphere, where dynamic pH gradients are present mainly due to the availability of nutrients and to their selective uptake by roots. With regard to plant availability, great importance lies in the molecular dimension and solubility of humic substances (Briimmer and Herms,... 

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