This paper reviews major processes that potentially affect the fate of arsenic in the rhizosphere of plants. Rhizosphere interactions are deemed to play a key role in controlling bioavailability to ...crop plants and for a better understanding and improvement of phytoremediation technologies. Substantial progress has been made towards an understanding of As transformation processes in soils. However, virtually no information is available that directly addresses the fate of As in the rhizosphere. We are proposing a conceptual model of the fate of As in the soil–rhizosphere–plant system by integrating the state-of-the art knowledge available in the contributing disciplines. Using this model and recent studies on hyperaccumulation of As, we discuss research needs and the potential application of rhizosphere processes to the development of phytoremediation technologies for As-polluted soils.
Metals are common contaminants worldwide. Long-term deposition of metals in soils can lead to accumulation, transport and biotoxicity/zootoxicity caused by mobility and bioavailability of significant ...fraction of the metals. Contaminant bioavailability is increasingly being used as a key indicator of potential risk that contaminants pose to both environmental and human health. However, the definition of bioavailability and the concepts on which it is based are still unclear, the methods adopted for its measurement vary and as such there is no single standard technique for the assessment of either plant availability of contaminants or their ecotoxicological impacts on soil biota. Moreover, bioavailability is often assumed to be static in nature where most decisions on risk and remediation are based on laboratory estimations of the bioavailable fraction, which may vary with time, nature of species as well as with temporal variation in environmental factors. Because of their immutable nature, strict natural attenuation processes alone may not be sufficient in mitigating the risks from metals. However, accelerating these processes with human interference (i.e., assisted natural remediation) that effectively immobilizes metals might be a viable option. Application to soils of certain amendments that enhance key biogeochemical processes in soils that effectively immobilize metals have already been demonstrated in Europe and North America on a field scale. Case studies using lime, phosphate and biosolid amendments have demonstrated, under field conditions, enhanced natural remediation resulting in substantially improved vegetation growth, invigorated microbial population and diversity, and reduced offsite metal transport. Depending on soil/hydrogeochemical properties, source term and metal form/species, and land use, the immobilization efficacy induced by such assisted natural remediation may be enduring. The use of green plants as a remediation tool in environmental cleanup has also offered some potential. Plants can uptake and bioaccumulate (phytoextraction) as well as immobilize (phytoimmobilization) certain trace elements, in conjunction with their rhizospheric processes. While long-term stability of certain metal complexes, such as metal pyromorphites has been shown in model systems, the influence of plant roots and its microbial and mycorrhizal association on such stability is unknown. A suite of chemical and biological tests are available to monitor the efficacy of assisted natural remediation.
1. Forest ecosystems are critical for the global regulation of carbon (C), a substantial portion of which is stored in above-ground biomass (AGB). While it is well understood that taxonomic and ...functional composition, stand structure and environmental gradients influence spatial variation in AGB, the relative strengths of these drivers at landscape scales have not been investigated in temperate forests. Furthermore, when biodiversity enhances C storage, it is unclear whether it is through mass-ratio effects (i.e. the dominant trait in communities regulates AGB) or through niche complementarity (i.e. increased AGB due to interspecific resource partitioning). 2. To address these mechanisms, we analysed data from a census of 28,262 adult trees sampled across 900 ha of temperate deciduous forest in southwestern Pennsylvania. We used data on four key plant functional traits to determine if (1) there is a positive relationship between species diversity and AGB and (2) whether this is due to mass-ratio effects or niche complementarity. We also sought to (3) identify the physical stand structural attributes and topographic variables that influence AGB across this landscape. 3. We found AGB was positively related to species richness and negatively related to species evenness, albeit weakly, while functional diversity indices had neutral effects. Above-ground biomass was enhanced in communities dominated by traits related to greater maximum tree height, deeper minimum rooting depths and larger seeds. Most importantly, areas with high AGB were dominated by Acer saccharum and Liriodendron tulipifera. Overall, these results support mass-ratio effects, with little evidence for niche complementarity. 4. Synthesis. Stand structure, topography, and species and functional composition, but not taxonomic or functional diversity, were found to be key drivers of above-ground biomass at landscape scales (<900 ha) in this temperate deciduous forest. Our findings suggest that simultaneously managing for both high diversity and for above-ground carbon storage may prove challenging in some forest systems. Our results further indicate that the impact of tree biodiversity loss on above-ground carbon stocks will depend greatly on the identity of the species that are lost.
Thlaspi goesingense is able to hyperaccumulate extremely high concentrations of Ni when grown in ultramafic soils. Recently it has been shown that rhizosphere bacteria may increase the heavy metal ...concentrations in hyperaccumulator plants significantly, whereas the role of endophytes has not been investigated yet. In this study the rhizosphere and shoot-associated (endophytic) bacteria colonizing T. goesingense were characterized in detail by using both cultivation and cultivation-independent techniques. Bacteria were identified by 16S rRNA sequence analysis, and isolates were further characterized regarding characteristics that may be relevant for a beneficial plant-microbe interaction--Ni tolerance, 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase and siderophore production. In the rhizosphere a high percentage of bacteria belonging to the Holophaga/Acidobacterium division and alpha-Proteobacteria were found. In addition, high-G+C gram-positive bacteria, Verrucomicrobia, and microbes of the Cytophaga/Flexibacter/Bacteroides division colonized the rhizosphere. The community structure of shoot-associated bacteria was highly different. The majority of clones affiliated with the Proteobacteria, but also bacteria belonging to the Cytophaga/Flexibacter/Bacteroides division, the Holophaga/Acidobacterium division, and the low-G+C gram-positive bacteria, were frequently found. A high number of highly related Sphingomonas 16S rRNA gene sequences were detected, which were also obtained by the cultivation of endophytes. Rhizosphere isolates belonged mainly to the genera Methylobacterium, Rhodococcus, and Okibacterium, whereas the majority of endophytes showed high levels of similarity to Methylobacterium mesophilicum. Additionally, Sphingomonas spp. were abundant. Isolates were resistant to Ni concentrations between 5 and 12 mM; however, endophytes generally tolerated higher Ni levels than rhizosphere bacteria. Almost all bacteria were able to produce siderophores. Various strains, particularly endophytes, were able to grow on ACC as the sole nitrogen source.
Aims We imaged the sub-mm distribution of labile P and pH in the rhizosphere of three plant species to localize zones and hot spots of P depletion and accumulation along individual root axes and to ...relate our findings to nutrient acquisition / root exudation strategies in P-limited conditions at different soil pH, and to mobilization pattern of other elements (Al, Fe, Ca, Mg, Mn) in the rhizosphere. Methods Sub-mm distributions of labile elemental patterns were sampled using diffusive gradients in thin films and analysed using laser ablation inductively coupled plasma mass spectrometry. pH images were taken using planar optodes. Results We found distinct patterns of highly localized labile P depletion and accumulation reflecting the complex interaction of plant P acquisition strategies with soil pH, fertilizer treatment, root age, and elements (Al, Fe, Ca) that are involved in P biogeochemistry in soil. We show that the plants respond to P deficiency either by acidification or alkalization, depending on initial bulk soil pH and other factors of P solubility. Conclusions P solubilization activities of roots are highly localized, typically around root apices, but may also extend towards the extension / root hair zone.
Metal-accumulating woody species have been considered for phytoextraction of metal-contaminated sites. We investigated Zn and Cd accumulation in tissues of adult trees and associated herbaceous ...species collected from contaminated areas in Central Europe. We found considerable Cd and Zn accumulation in various willow, poplar and birch species with up to 116
mg
Cd
kg
−1 and 4680
mg
Zn
kg
−1 in leaves of
Salix caprea. Annual variation of Cd and Zn concentrations in leaves of
Salix caprea were small, indicating that data obtained in different years can be compared. Metal concentrations in leaves were not related to total (
aqua regia) or labile (1
M NH
4NO
3 extract) concentrations in soil but the accumulation factors (leaf concentration: soil concentration) for Cd and Zn followed an inverse log type function. Metal partitioning between tissues showed a minimum in the wood, with increasing concentrations of Cd and Zn towards the leaves and fine roots.
Adult field-grown Salix caprea, Populus tremula and other tree species accumulate up to 4680
mg
Zn
kg
−1 and 116
mg
Cd
kg
−1 in their leaves.
Exudation of organic acid anions by plants as well as root-induced changes in rhizosphere pH can potentially improve phosphate (Pi) availability in the rhizosphere and are frequently found to occur ...simultaneously. In non-calcareous soils, a major proportion of Pi is strongly sorbed to metal oxi(hydr)oxides of mainly iron (Fe) and aluminium (Al) and organic anions are known to compete with Pi for the same sorption sites (ligand exchange) or solubilize Pi via ligand-promoted mineral dissolution. Root-induced co-acidification may also further promote Pi release from soil. The relative efficiency of these different solubilization mechanisms, however, is poorly understood. The aims of this study were to gain a better mechanistic understanding of the solubilizing mechanisms of four carboxylates (citrate, malate, oxalate, malonate) in five soils with high and low P surface site saturation. Results indicate that at a lower P saturation of solid phase sorption sites, ligand-promoted mineral dissolution was the main Pi solubilization mechanism, while ligand exchange became more important at higher soil P concentrations. Co-acidification generally increased Pi solubility in the presence of carboxylates; however the relative solubilizing effect of carboxylates compared to the background electrolyte (KCl) control decreased by 20-50%. In soils with high amounts of exchangeable calcium (Ca), the proton-induced Ca solubilization reduced soluble Pi, presumably due to ionic-strength-driven changes in the electric surface potential favoring a higher Pi retention. Across a wider soil pH range (pH 3-8), Pi solubility increased with increasing alkalinity, as a result of both, more negatively charged sorption sites, as well as DOC-driven changes in Fe and Al solubility, which were further enhanced by the presence of citrate. Overall, the relative efficiency of carboxylates in solubilizing Pi was greatest in soils with medium to high amounts of anionic binding sites (mainly Fe- and Al-oxy(hydr)oxides) and a medium P sorption site coverage, with citrate being most effective in solubilizing Pi.
The 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.
We screened 20 different clones of willow and poplar species in hydroponic experiments for their metal resistance and accumulation properties. Plants were exposed for 4 weeks either to single ...additions of (μM) 4.45 Cd or 76.5 Zn, or a metal cocktail containing the same amounts of Cd and Zn along with 7.87 Cu and 24.1 Pb. Plant biomass, metal tolerance and metal accumulation pattern in roots and leaves varied greatly between clones. The leaf:root ratio of metal concentrations was clearly underestimated compared to soil experiments. The largest metal concentrations in leaves were detected in
Salix dasyclados (315
mg Cd
kg
−1 d.m.) and a
Salix smithiana clone (3180
mg Zn
kg
−1 d.m.) but these species showed low metal tolerance. In spite of smaller Cd and Zn concentrations, the metal-tolerant clones
Salix matsudana,
Salix fragilis-1, and
Salix purpurea-1 hold promise for phytoextraction as they produced large biomass and metal contents in leaves.
Hydroponically grown willows and poplar clones accumulate up to (mg
kg
−1 d.w.) 315 Cd and 3180 Zn in leaves.
For the first time, phytosiderophore (PS) release of wheat (Triticum aestivum cv Tamaro) grown on a calcareous soil was repeatedly and nondestructively sampled using rhizoboxes combined with a ...recently developed root exudate collecting tool. As in nutrient solution culture, we observed a distinct diurnal release rhythm; however, the measured PS efflux was c. 50 times lower than PS exudation from the same cultivar grown in zero iron (Fe)‐hydroponic culture. Phytosiderophore rhizosphere soil solution concentrations and PS release of the Tamaro cultivar were soil‐dependent, suggesting complex interactions of soil characteristics (salinity, trace metal availability) and the physiological status of the plant and the related regulation (amount and timing) of PS release. Our results demonstrate that carbon and energy investment into Fe acquisition under natural growth conditions is significantly smaller than previously derived from zero Fe‐hydroponic studies. Based on experimental data, we calculated that during the investigated period (21–47 d after germination), PS release initially exceeded Fe plant uptake 10‐fold, but significantly declined after c. 5 wk after germination. Phytosiderophore exudation observed under natural growth conditions is a prerequisite for a more accurate and realistic assessment of Fe mobilization processes in the rhizosphere using both experimental and modeling approaches.
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