Bulk-soil and rhizosphere microbial

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). 

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. 

In soil, the chances that any enzyme will retain its activity are very slim indeed, because inactivation can occur by denaturation, microbial degradation, and sorption (61,62), although it is possible that sorption may protect an enzyme from microbial degradation or chemical hydrolysis and retain its activity. The nature of most enzymes, particularly size and charge characteristics, is such that they would have very low mobility in soils, so that if a secreted enzyme is to have any effect, it must operate close to the point of secretion and its substrate must be able to diffuse to the enzyme. Secretory acid phosphatase was found to be produced in response to P-deficiency stress by epidermal cells of the main tap roots of white lupin and in the cell walls and intercellular spaces of lateral roots (63). Such apoplastic phosphatase is safe from soil but can be effective only when presented with soluble organophosphates, which are often present in the soil. solution (64). However, because the phosphatase activity in the rhizo-sphere originates from a number of sources (65), mostly microbial, and is much higher in the rhizosphere than in bulk soil (66), it seems curious that plants would have a need to secrete phosphatase at all. 

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... 

However, relatively few studies have included growing plants in their experimental systems. In order to mechanistically understand the effects of pine roots on microbial N transformations rates under conditions of N limitation, l-year-old pine seedlings were transplanted into Plexiglas microcosms (121) and grown for 10-12 months. Seedlings were labeled continuously for 5 days with ambient CO concentration (350 iL L ) with a specific activity of 15.8 MBq g C. Then, soils at 0-2 mm (operationally defined as rhizosphere soil) and >5 mm from surface of pine roots (bulk soil) of different morphology and functional type (coarse woody roots of >2 mm diameter fine roots of <2 mm diameter ... 

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. 

Skipper HD, Gilmour CM, Furtick WR (1967) Microbial versus chemical degradation of atrazine in soils. Soil Sci Soc Am Proc 31 653-656 Sliwinski MK, Goodman RM (2004) Comparison of cienarchaeal consortia inhabiting the rhizosphere of diverse terrestrial plants with those in bulk soil in native environments. Appl Environ Microbiol 70 1821-1826 Stumm W, Morgan JJ (1996). Aquatic chemistry - chemical equlibrium and rates in Natural Waters (3rd edn). Wiley, New York Vega D, Bastide J (2003) Dimethylphthalate hydrolysis by specific microbial esterase. Chemosphere 51 663-668... 

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... 

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