Bulk soil

Fig. 11. Bulk soil water content and root ABA content in consecutive 10 cm soil layers on days 3, 9,12 and 18 after withholding water (A) from maize plants growing in 1 m deep soil columns. Well-watered plants (A) received water daily throughout the experimental period. Points are means of five measurements. Modified from Zhang Davies (1989).

Fig. 12. Relationships between root ABA content and bulk soil water content for maize plants growing in drying soil columns. Data are from Fig. 11, but do not include soil water contents less than 0.1 g cm in which many roots were non-living. Modified from Zhang Davies (1989).

Plant survival and crop productivity are strictly dependent on the capability of plants to adapt to different environments. This adaptation is the result of the interaction among roots and biotic and abiotic components of soil. Processes at the basis of the root-soil interaction concern a very limited area surrounding the root tissue. In this particular environment, exchanges of energy, nutrients, and molecular signals take place, rendering the chemistry, biochemistry, and biology of this environment different from the bulk soil. 

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. 

Both H-thymidine incorporation and radiolabeled leucine incorporation techniques have been recently used to determine bacterial activity and growth in the rhizosphere of barley seedling (28), Bacteria were initially released from the rhizosphere using homogenization and centrifugation before adding the labeled substrates. The cell incorporation rate was twice as high in the rhizosphere than in bulk soil. In addition, both the leucine and thymidine incorporation rates increased with the distances from the root tip (28). 

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. 

M. Young, Variation in moisture contents between bulk soil and the rhizosheath of wheat (Tritieum aestivum L. cv. Wembley). New Phytol. 730 135 (1995). 

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... 

Figure 3 Model of proposed interactions in the rhizosphere and in the bulk soil.

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. 

Many more papers deal with rhizosphere phosphatase activity (63-83) in the presence of a number of different plant species this will partly be due to the simplicity of the enzyme activity assay (85,86) and the generally reported, well-correlated variation trends among organic and inorganic phosphorus content and phosphatase activity. More precisely, closer to the roots, the inorganic P depletion zone in comparison with bulk soil is more pronounced in addition, organic and inorganic P contents are inversely correlated, and the mineralization rate of or-... 

Plexiglas box divided by nylon gauze into various vertical compartments differently proximate to roots Rhizosphere and bulk soil initially compartmentalized. Soil-root interface scarcely resolved. Apparatus time expensive and difficult to build up. No particular constraints for root growth. 94, 95, 130-132... 

Enhanced nutrient cycling in both the rhizosphere and bulk soil may depend on the bacterial grazing by protozoa or nematodes with release of inorganic N. Nematodes appear to be the primary consumers of bacteria in the rhizosphere, whereas protozoa are equally prevalent in rhizosphere and bulk soil (41,97). Estimated C-to-N ratios of bacterial-feeding nematodes range from 5 1 to 10 1 (98,99) and are generally higher than those of their bacterial food source thus the excess N is excreted as ammonia (100,101) by nematodes. The estimated... 

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 ... 

There are indications that the variety of C substrates in the rhizosphere soil is basically too wide to be significantly affected by changes in quality of plant residues from the previous crop. For example, legumes as preceding crop were shown to increase significantly microbial diversity in the bulk soil, as estimated by Biolog assay, whereas in the rhizosphere soil this effect of legumes could not be detected (145). 

In concurrent research involving a longer-term study, the P. fluorescens Pf-5 strain was inoculated into soil used to grow lupine (Lupinus aihus) and barley Hordeum vulgare L.) over a several-week, period (17). It was shown that iron stress was greatest in the zone behind the root tips as compared to older root zones and in the bulk soil. Calibration of the ice-nucleation reporter activity... 

J. E. Loper and M. D. Henkels, Availability of iron to Pseudomonas fliiorescens in rhizosphere and bulk soil evaluated with an ice nucleation reporter gene. Appl. Environ. Microbiol. 63 99 (1997). 

R. K. Reid, C. P. P. Reid, P. E. Powell, and P. J. Szaniszlo, Comparison of. siderophore concentrations in aqueous extracts of rhizosphere and adjacent bulk soils. Pedohiology 26 263 (1984). 

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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