Rhizosphere dimension

As already noted by Campbell and Greaves (16), the rhizosphere lacks physically precise delimitations and its boundary is hard to demarcate. Dimensions may vary with plant species and cullivar, stage of development, and type of soil. Soil moisture may affect the measurable size of the rhizosphere as well wetter soils may stick better to roots than drier soils (Fig, 1). This will change the volume of soil regarded as rhizosphere soil upon separation of rhizosphere from bulk soil and thus alter the measured concentration in rhizosphere and non-rhizosphere soil of a response variable in exudate concentration or microbial production. 

Architectural models explicitly specify the di.stribution of roots in space. An alternative approach, which is also useful for rhizosphere studies, is the continuum approach where only the amount of roots per unit soil volume is specified. Rules are defined that specify how roots propagate in the vertical and horizontal dimensions, and root propagation is u.sually viewed as a diffusive phenomenon (i.e., root proliferation favors unexploited soil). This defines the exploitation intensity per unit volume of soil and, under the assumption of even di.stribution, provides the necessary information for the integration step above. Acock and Pachepsky (68) provide an excellent review of the different assumptions made in the various continuum models formulated and show how such models can explain root distribution data relating to chrysanthemum. 

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,... 

What is needed at this point are quantitative data on phenolic acid utilizing microbes (e.g., bacteria, fungi, actinomycetes) in field soils + phenolic acid enrichment during the spring, summer, and fall for various crop and forest systems. In addition, we need quantitative data describing the relationships between bulk-soil and rhizosphere phenolic acid utilizing microbes and the observed phytotoxicity of phenolic acids for sensitive species, i.e., a form of dimension analysis utilizing equations to predict useful and/or consistent relationships. 

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