Rhizosphere continued

Barber and Lynch (9) used this equation to recalculate data from previous studies on microbial growth in soil, using a constant maintenance coefficient (///). They found no case where energy input exceeded the requirement for maintenance, and suggested, therefore, that apart from zones immediately around recently incorporated plant and animal residues, appreciable and continuous activity in soils can be expected only in the rhizosphere. 

Table 1 Characteristics of Rhizosphere and Nonrhizosphere Soil for Three Grass Species Grown in a Continuous C-C02 Labelled Atmosphere After 8 Weeks ...

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... 

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 ... 

Nevertheless, cereal plants can interact with endosymbionts, capable of nitrogen fixation in other species, and be stimulated in their productivity. The odds of soil life are balanced for some bacteria by their interactivity at rhizosphere level, and a realm of exchanged signals dictates entry into hormonally reprogrammed root sites. Specificity for partner plant species is part of a fine speciation process that actively involves the bacterial nodulation genes, and continues to drive their variation dynamics. 

Eive zones can be distinguished the floodwater standing on the soil per se, the floodwater-soil interface, the anaerobic bulk soil, the rhizosphere, and the subsoil. These are to some extent continuous with each other, and they are certainly linked so that the function of the system as a whole is greater than the sum of its parts. But they provide convenient boundaries for discussion. 

Organic compounds released from sloughed-off root cells and tissues are a major carbon source for rhizosphere microorganisms but may indirectly have an impact as microbial metabolites on nutrient availability and on exclusion of toxic elements in the rhizosphere (Brimecombe et al., 2007). Continuous root turnover is a general feature of plant development, and insoluble root debris may comprise 50-90% of total rhizodeposition (Darrah, 1991). 

As summarized by Jacobson (1994), biological Fe(III) reduction will be more important than chemical reduction when amorphous Fe(IIl) oxides are plentiful and continually regenerated, or H2S production is low relative to the Fe(III) concentration. This first condition is likely to be met in the rhizosphere where radial O2 loss drives Fe oxide formation. The second condition will be met in low-salinity wetlands, or in saline systems with mineral (i.e., iron-rich) sediments. However, even chemical reduction of Fe(III) is ultimately due to microbes since the H2S that reduces the Fe is the result of a biological process, SO4" reduction (Megonigal et al., 2004). 

Phytotoxins synthesized in the rhizosphere by microorganisms may play a greater role in plant growth than those derived from the breakdown of crop residues. This is chiefly because the rhizosphere flora feeds on a continuous supply of exuded food that is especially suitable for antibiotic-producing microorganisms. The antibiotics produced ... 

Both intensity and capacity of soil reduction appear to influence plant functioning in wetland ecosystems. In wetland soils, plants are faced with a substantial demand for oxygen in the rhizosphere and the potential for loss of oxygen to soil, and thus the plants must deal with additional root stress. As soil reduction continues and intensifies, a progressively greater demand is imposed on roots for... 

In the rhizosphere, the presence of hiomolecules continuously released hy plants as root exudates or microorganisms play a crucial role in the dynamics and bioavailability of metals and metalloids for plants and biota. The influence of biomolecules as well as of biotic and abiotic components on trace elements transformation and mobility deserves to be studied with particular attention. 

Among the first to consider root hairs in their model were Bhat et al. (1976) and Itoh and Barber (1983), while attempting to explain experimentally obtained P uptake values exceeding those calculated by one of the previously available models. Three principal approaches to integrate root hairs into a rhizosphere model are found in the literature (1) The boundary where exudation and uptake occurs is extended by the length of the root hairs (e.g. Kirk, 1999) (2) The continuity equation for root uptake is extended with a separate sink term (e.g. Geelhoed et al., 1997) and (3) The transport equation is solved in a three-dimensional model with cylindrical coordinates (Geelhoed et al., 1997). 

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