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SummaryThe rhizosphere differs from the bulk soil in a range of biochemical, chemical and physical processes that occur as a consequence of root growth, water and nutrient uptake, respiration and rhizodeposition. These processes also affect microbial ecology and plant physiology to a considerable extent. This review concentrates on two features of this unique environment: rhizosphere geometry and heterogeneity in both space and time. Although it is often depicted as a soil cylinder of a given radius around the root, drawing a boundary between the rhizosphere and bulk soil is an impossible task because rhizosphere processes result in gradients of different sizes. For instance, because of diffusional constraints, root uptake can result in a depletion zone extending <1 mm for phosphate to several centimetres for nitrate, while respiration may affect the bulk of the soil. Rhizosphere processes are responsible for spatial and temporal heterogeneities in the soil, although these are sometimes difficult to distinguish from intrinsic soil heterogeneity. A further complexity is that these processes are regulated by plants, microbial communities and soil constituents, and their many interactions. Novel in situ techniques and modelling will help in providing a holistic view of rhizosphere functioning, which is a prerequisite for its management and manipulation.New Phytologist (2005) 168 : 293-303 © New Phytologist (2005

Phytoextraction makes use of trace element-accumulating plants that concentrate the pollutants in their tissues. Pollutants can be then removed by harvesting plants. The success of phytoextraction depends on trace element availability to the roots and the ability of the plant to intercept, take up, and accumulate trace elements in shoots. Current phytoextraction practises either employ hyperaccumulators or fast-growing high biomass plants; the phytoextraction process may be enhanced by soil amendments that increase trace element availability in the soil. This review will focus on the role of plant-associated bacteria to enhance trace element availability in the rhizosphere. We report on the kind of bacteria typically found in association with trace element – tolerating or – accumulating plants and discuss how they can contribute to improve trace element uptake by plants and thus the efficiency and rate of phytoextraction. This enhanced trace element uptake can be attributed to a microbial modification of the absorptive properties of the roots such as increasing the root length and surface area and numbers of root hairs, or by increasing the plant availability of trace elements in the rhizosphere and the subsequent translocation to shoots via beneficial effects on plant growth, trace element complexation and alleviation of phytotoxicity. An analysis of data from literature shows that effects of bacterial inoculation on phytoextraction efficiency are currently inconsistent. Some key processes in plant–bacteria interactions and colonization by inoculated strains still need to be unravelled more in detail to allow full-scale application of bacteria assisted phytoremediation of trace element contaminated soils.

Plant-assisted bioremediation or phytoremediation holds promise for in situ treatment of polluted soils. Enhancement of phytoremediation processes requires a sound understanding of the complex interactions in the rhizosphere. Evaluation of the current literature suggests that pollutant bioavailability in the rhizosphere of phytoremediation crops is decisive for designing phytoremediation technologies with improved, predictable remedial success. For phytoextraction, emphasis should be put on improved characterisation of the bioavailable metal pools and the kinetics of resupply from less available fractions to support decision making on the applicability of this technology to a given site. Limited pollutant bioavailability may be overcome by the design of plantmicrobial consortia that are capable of mobilising metals/metalloids by modification of rhizosphere pH (e.g. by using Alnus sp. as co-cropping component) and ligand exudation, or enhancing bioavailability of organic pollutants by the release of biosurfactants. Apart from limited pollutant bioavailability, the lack of competitiveness of inoculated microbial strains (in particular degraders) in field conditions appears to be another major obstacle. Selecting/engineering of plant-microbial pairs where the competitiveness of the microbial partner is enhanced through a "nutritional bias" caused by exudates exclusively or primarily available to this partner (as known from the "opine concept") may open new horizons for rhizodegradation of organically polluted soils. The complexity and heterogeneity of multiply polluted "real world" soils will require the design of integrated approaches of rhizosphere management, e.g. by combining co-cropping of phytoextraction and rhizodegradation crops, inoculation of microorganisms and soil management. An improved understanding of the rhizosphere will help to translate the results of simplified bench scale and pot experiments to the full complexity and heterogeneity of field applications.