Soil rhizosphere

Khammas, K. M., Ageron, E., Grimont, P A D. and Kaiser, P., 1989. Azospirillum irakense sp. nov., a nitrogen-fixing bacterium associated with rice roots and rhizosphere soil. Res. Microbiol. 140, 679-693. 

Siciliano SD, JJ Germida (1999) Enhanced phytoremediation of chlorobenzoates in rhizosphere soil. Soil Biol Biochem 31 299-305. 

As mentioned before and in Chaps. 4 and 6, the concentration of rhizode-position decreases as the distance from the rhizoplane increases, whereas the opposite generally occurs for the concentration of any plant nutrient in soil. In this context, the role of rhizospheric soil, rather than that of the bulk soil, is crucial for plant nutrition. It has also to be considered that very different situations can occur depending on the type of nutrient (24) and the nutritional status of plants (see Chap. 3) furthermore, different portions of the root system are characterized by differential nutrient-specific rates of uptake (25). All the above statements point to the necessity of reconsidering the concept of plant nutrient availability giving more importance to the situation occurring in the soil surrounding the root. 

Modifiers Modification of the rhizosphere soil with (e.g., protons, reductants)... 

Protection against physical stress Respon.se to high soil strength through inodilication of interface through lubrication and amelioration of rhizosphere soil... 

Protection against competition Modification of rhizosphere. soil with phytoactive compounds (e.g.. allelochemi-cals)... 

W. Cheng, Q. Zhang, D. C. Coleman, C. R. Carroll, and C. A. Hoffman, Is available carbon limiting microbial respiration in the rhizosphere Soil Biol. Biochem. 28 1283 (1996). 

R. Kuchenbuch and A. Jungk, A method for determining concentration profiles at the soil-root interface by thin slicing rhizospheric soil. Plant Soil 68 391 (1982). R. Schonwitz and H. Ziegler, Quantitative and qualitative aspects of a developing rhizosphere microflora of hydroponically grown maize seedlings. Z Pflanzenernahr. Bodenk. 749 623 (1986). 

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... 

G. Cieslinski, K. C. J. Van Rees, A. M. Szmigielska, G. S. R. Krishnamurti, and P. M. Huang. Low-molecular-weight organic acids in rhizosphere soils of durum wheat and their effect on cadmium bioaccumulation. Plant Soil 203 109 (1998). 

D. A. Barber and J. M. Lynch, Microbial growth in the rhizosphere. Soil Biology and Biochemistry 9 305 (1977). 

P. H. Vaast and R. J. Zasoski, Effect of VA-mycorrhizac and nitrogen sources on rhizosphere soil characteristics, growth and nutrient acquisition of coffee seedlings (Coffea arahica L.), Plant and Soil I473. ... 

W. Kolb and P. Martin. Influence of nitrogen on the number of Ni-fixing and total bacteria in the rhizosphere. Soil Biology and Biochemistry 20 221 (1988). 

Fresh organic matter plays a fundamental role in plant nutrition by supplying nutrients released through degradation processes however, humified organic substances also become a source of nutrients when subjected to mineralization processes. The main aspects of the cycle of organic matter at the rhizosphere soil are reported in Chap. 6. 

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. 

Intrinsic difficulties in sampling truly representative rhizospheric soil (Table 4) have hampered workers searching rhizosphere soil for a number of different enzyme activities as high as in the bulk soil. 

In general, one would expect that every enzyme will be released close to the roots either by roots or by microbes and eventually will be immobilized in rhizo.sphere soil. If a given enzyme activity has not yet been measured in rhizosphere soil, this is likely more due to the lack of specifically devoted studies or suitable methods than to a real absence. For example, a recent study has first developed an assay for myrosinase in soil and, second, has hypothesized the... 

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