Rhizosphere Soil
II. RHIZOSPHERE
A. Biological, Chemical, and Physical Complexity
The soil-root interface is constitutively very variable, as plant roots are extremely dynamic in space and time. Soil niches corresponding to root tip, mucilage, meri- stematic zone. elongation/differentiation zone, or root hairs are not comparable for most aspects (4). Particularly favorable conditions in soil can led to unusual branching patterns in root systems. Thus roots vary enormously in their morphol- ogy, longevity, activity, and influence on soil as a result of physiological, environ- mental, and genetic differences (5). Moreover, movement of water, nutrients, and microbes is more convoluted around the roots than in the bulk soil. Water poten- tial regime in the rhizosphere soil is generally lower than in the bulk soil, thus causing a net mass flow toward the roots and a nutrient gradient across the rhizo- sphere environment (6). Concentrations of available C are usually low around the root despite the rhizodepositions due to generally rapid microbial assimilation and depletion of the resource (7). Selective root uptake of ions causes some ions to be depleted, whereas those not assimilated tend to accumulate if not leached.
Soil pH changes depend also on the root cation:anion ratio uptake (8,9). Redox potential in rhizosphere soil is generally much more negative than in bulk soil
(IO), also due to the higher oxygen consumption during respiration of both roots and microorganisms. The rhizosphere generally experiences higher mineral
weathering rates than bulk soil ( 1 I , 12). For all of these reasons, it is very difficult to obtain a representative sample of rhizosphere soil; consequently the reliability of any finding regarding soil rhizosphere processes depends strictly on the reli- ability of the model system adopted for the study.
On the other hand, to study the soil and the plant as separate and indepen- dent entities is “nonsense” at many levels: pedogenetic, ecological (in the field plants cannot grow in the presence of particularly prohibitive climatic or environ- mental conditions), and methodological (any model system for studying soil-
plant relationships not involving both single components may result strongly mis- leading from a holistic viewpoint). As the soil as a whole is itself a complex body consisting of organic and inorganic components, also the use of simplified artificial soil-plant systems-for instance, plants grown either hydroponically or on mineral or organic support-may generate partial information that is totally unreliable, since interactions between components might be even more important than the mere sum of single contributions.
From an operational viewpoint and depending on the investigation’s scale of resolution, the detailed description of the spatial and temporal variation of soil microhabitats, especially at the soil-root interface due to the enormous number of microbial species and underlying processes, it would require a huge amount of soil samples with, in any case, actually insurmountable difficulties in integrating results. Thus, at the moment, the only ongoing approaches for studying mineral- ization-immobilization in the rhizosphere are process-oriented and based on man- ageable microcosms or mesocosms simplified at various extents in comparison with field conditions. More precisely, the spatial-temporal discrimination of the rhizosphere soil is lost when pot experiments are used. The level of completeness at an immediately higher, the microcosm level, usually permits study of the rhizo- sphere effect both temporally and spatially, despite the addition of some constric- tions for root growth and/or soil physical properties. Mesocosm level studies usually allow the same completeness of field conditions but in thermodynamically closed systems-that is, controlling the exchange of energy and matter with the exterior.
B. Rhizosphere Characterization-
Theoretical Considerations
By definition, all carbon (except above-ground littering) enters the soil via the rhizosphere, which is a highly dynamic and complex environment both in time and in space. As discussed in the Chaps. 2 and 4, the root excretions as well as root debris consist of a wide array of chemical compounds, most of which can be utilized by soil microorganisms ( 1 3). These compounds can be arbitrarily di-
vided into two groups with distinct physical behaviors-i.e., soluble and struc- tural particulate organic matter (14). Moreover, these groups relate to three dis- tinctive environments: (1) growing root tip (exudation of solubles diffusing away from the source into the soil, characterized by a gradient of substrate concentra- tions), (2) aging root (cortical senescence with release of solubles and cell mate- rial, diffusion, and hot spot), and (3) dying roots (structural compounds that are nonsoluble particulate characterized by a locally high concentration of substrates) (15).
As already noted by Campbell and Greaves (16), the rhizosphere lacks physically precise delimitations and its boundary is hard to demarcate. Dimen- sions may vary with plant species and cultivar, 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.
response variable
\
rhizosphere
L
non-rhizosphere
Figure 1 Schematic representation of the dynamics of a response variable, e.g., concen-
tration of rhizodeposited C, in the rhizosphere (dashed line) and the measured concentra- tions in rhizosphere and nonrhizosphere samples (solid lines). The vertical arrow indicates the separation of rhizosphere and nonrhizosphere soil; the effect of soil moisture is indi- cated by horizontal arrows.