Bulk-soil and rhizosphere microbial populations

Effects of Phenolic Acids on Bulk-Soil and Rhizosphere-Microbial Populations... 

Determine under controlled conditions how phenolic acid-containing plant tissues/residues mixed into soil modify phenolic acid-utilizing bulk-soil and rhizosphere microbial populations. 

Determine Under Controlled Conditions How Phenolic Acids-Containing Plant Tissues/Residues Mixed into Soil Modify Phenolic Acid-Utilizing Bulk-Soil and Rhizosphere Microbial Populations (Staman et al. (2001) Plenum Publishing Corporation, Excerpts Used with Permission of Springer Science and Business Media)... 

Containing Plant Tissues/Residues Mixed into Soil Modify Phenolic Acid-Utilizing Bulk-Soil and Rhizosphere Microbial Populations (Section 3.4.7)... 

Detectable population changes in laboratory systems of bulk-soil and rhizosphere phenolic-acid utilizing microorganisms to phenolic acid enrichment are a function of a variety of soil physicochemical and biotic factors including type of phenolic acid, phenolic acid enrichment concentrations, presence of other available organic compounds, nuttition, soil type, and initial microbial populations. Responses of microbes to phenolic acids or phenolic acid mixtures also varied with the type of microbe (e.g bacteria, actinomycetes, or fungi). 

Conditions in the rhizosphere, the cylinder of soil that surrounds the plant root at a distance of up to 2 - 5 mm (Curl and Truelove, 1986) can be very different from those in the bulk soil. This is the local environment from which the root takes up nutrients, excretes inorganic and organic species, and in which there is shedding and decomposition of parts of the root surface. The pH and the microbial population can both deviate from those in the bulk soil, the latter showing a population density 2-20 times higher. The roots of many plant families are associated with particular fungi, mycor-rhizae, which are very important for the mineral nutrition of plants (Tinker and Gilden, 1983 Marschner et al., 1986, Streit and Stumm, 1993). 

That microorganisms can reduce the observed phytotoxic effects of phenolic acids has been observed by a number of researchers.3,7 8 33 37 38 39 41,45 I am, however, not aware of any study that has attempted to quantify how changes in bulk-soil bacteria might influence the phytotoxicity of phenolic acids. I am aware of only one study that has attempted to quantify how changes in rhizosphere microbial populations may influence the phytotoxicity of phenolic acids. Blum et al.9 observed that a 500% increase of phenolic acid utilizing bacteria in the rhizosphere of cucumber seedlings growing in Cecil A-horizon soil enriched with an equimolar mixture of 0.6 pmol/g p-coumaric acid, ferulic acid, p-hydroxybenzoic acid, and... 

Loss of carbon compounds from roo(s, or rhizodeposition, is the driving force for the development of enhanced microbial populations in the rhizosphere in comparison with the bulk soil. Although rhizodeposition is a general phenomenon of plant roots, the compounds lost from different species or even cultivars can vary markedly in quality and quantity over time and space. 

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