Rhizosphere microbial populations

The Effect of Root Exudates on Rhizosphere Microbial Populations... 

In this chapter we review the current literature available on the influence of root exudates on rhizosphere microbial populations and the effects of plant, microbial and soil factors on the processes of rhizodeposition and microbial colonization and activity. We first give a brief overview and definitions of some of the main concepts relating to the rhizosphere and rhizodeposition. 

It has been found that many environmental factors influence the amount and composition of root exudates and hence the activity of rhizosphere microbial populations. Microbial composition and species richness at the soil-plant interface are related either directly or indirectly to root exudates and thus vary according to the same environmental factors that influence exudation. In es.sence, the rhizosphere can be regarded as the interaction between soil, plants and microorganisms. Figure 2 shows some of the factors associated with these interactions, which will be discussed during the course of the chapter. Here we mention briefly the influence of some plant and microbial factors on root exudation and rhizosphere microbial populations, while soil factors are discussed later. 

In addition to the interactions between plants and microorganisms, a third factor, the soil, also plays a role in determining root exudation and the activity and diversity of rhizosphere microbial populations. In this section, physical and structural aspects of the soil are discussed in relation to their effects on root exudation and microbial populations. Consideration is also given to the role of agricultural management practices on rhizosphere processes. In addition, the role of other biotic factors, such as microfaunal predation, is discussed in relation to nutrient cycling in the rhizosphere. 

The effects of physical factors such as temperature, soil moisture content, pH and oxygen availability on microbial survival and activity in soil are well documented (for a review, see Ref. 90). It is also widely acknowledged that these same factors may also influence plant growth and can therefore be presumed to influence both root exudation and rhizosphere microbial populations. 

Soil type and structure also influence the dynamics of rhizosphere microbial populations. Whether nutrients are available for bacteria in the rhizosphere often depends on the sites in the soil where nutrients are present. Organic compounds tightly bound to the soil matrix are often less available for bacteria (226), and those present in smaller pore spaces can be physically protected against mineralization. However, disturbance of the soil often cau.ses these nutrients to become more available to soil microbes (227). 

Numerical techniques are iterative and require considerable computer processing power. With modern desktop computers, this is usually not an issue and solutions of root uptake over days or weeks typically take a few seconds to generate. However, for some strongly nonlinear problems, such as the development of rhizosphere microbial populations (Sect. Ill), where the increase in microbial biomass may be exponential over time, processing time may become important with solutions requiring >60 min to calculate on a modern PC. 

Brimecombe Melissa J, Leij Frans AD, Lynch James M. The effect of root exudates on rhizosphere microbial populations. In The Thizosphere, Pinton Roberto, Varanini Zeno, Nannipieri Paolo, eds. New York, NY Marcel Dekker, Inc., 2001. 

There are many examples of crop rotation sequences that passively suppress nematode populations which will not be reviewed here. Examples of active nematode suppression in crop rotation sequences are typically found with plant species that produce and excrete allelopathic compounds. These compounds then affect plant-parasitic nematodes in the rhizosphere either directly or indirectly by altering rhizosphere microbial populations (Halbrendt, 1996). For the purpose of this chapter, allelopathic... 

In this chapter I describe how microbial activity may be estimated and what data are presently available on a) rates of microbial transformation and utilization, b) phenolic acid effects on soil and rhizosphere microbial populations, and c) the influences of soil and rhizosphere microbial populations on phenolic acid phytotoxicity. The resulting insight is then used to suggest a possible approach by which this hypothesis may be tested experimentally. 

Phenolic acid enrichment of soils containing roots can also lead to an increase in rhizosphere microbial populations which can utilize phenolic acids as a carbon source. Shafer and Blum observed that the addition of up to 0.25... 

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... 

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)... 

In summary soil incorporation of inhibitory concentrations of tissues, such as wheat and sunflower resulted in a simultaneous stimulation of rhizosphere microbial populations that could utilize phenolic acids as a sole carbon source and an inhibition of the growth of cucumber seedlings. However, this relationship was... 

To Determine the Effects of Phenolic Acids on Bulk-Soil and Rhizosphere Microbial Populations (Section 2.4.4)... 

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