Soils polluted with trace elements can become sources of pollution for surrounding soil and water resources as a result of dispersion of contaminated soil particles by water erosion. Vegetative filter strips (VFS) implemented downstream of the source of pollution may be used to trap sediments and their associated contaminants. An experimental flume was designed in order to test the potential of Lolium perenne and Trifolium repens VFS to retain sediment contaminated by As and Pb (Ath) or Zn and Cd (Prayon). T. repens with its creeping posture and large leaves had a higher sediment retention efficiency (SRE = 47%) than L. perenne with an erect posture (SRE = 26%). Sedimentation upstream of and in the strip led to the selective retention of coarse particles and an enrichment in fine soil particles at the outlet of the flume. This effect was most marked for the T. repens VFS compared to L perenne and bare soil. Consequently, for the Prayon soil, the retention efficiency of Cd in T. repens VFS (53%) was higher than the SRE (48%) because Cd concentrations were 3 times higher in the sand fraction than in the clay fraction. On the contrary, the retention of Zn (48%), which was distributed more homogeneously across particle size fractions, was similar to the SRE. For Ath, the retention of both As (26%) and Pb (11%) in the T. repens VFS was lower than the SRE (46%) because both trace elements were preferentially associated with clay size particles. Trace element retention efficiency of VFS may thus be higher or lower than the SRE depending on the distribution of trace elements among particle size classes. This must be taken into account in the proper design of VFS aimed at controlling off-site pollution by trace element-contaminated sediments. (C) 2014 Elsevier B.V. All rights reserved.

The phytostabilization potential of plants is a direct function of their root systems. An experimental design was developed to investigate the impact of Cd and Zn on the root distribution and morphology of Lolium perenne and Trifolium repens.Seedlings were transplanted into columns filled with washed quartz and irrigated daily with Cd- or Zn-containing nutrient solutions during 1 month. Root biomass, root length density (RLD) and diameter were subsequently quantified as a function of depth. Pot experiments were also performed to quantify metal, lignin and structural polysaccharides concentrations as well as cell viability.Lolium perenne accumulated Cd and Zn in the roots whereas T. repens was unable to restrict heavy metal translocation. Cadmium and Zn reduced rooting depth and RLD but induced thick shoot-borne roots in L. perenne. Cd-induced root swelling was related to lignification occurring in the exodermis and parenchyma of central cylinder. Hemicelluloses and lignin did not play a key role in root metal retention. Cadmium slightly reduced mean root cell viability whereas Zn increased this parameter in comparison to Cd.Even though plant species like Lolium perenne and Trifolium repens may appear suitable for a phytostabilization scheme based on their shoot metal tolerance, exposure to toxic heavy metals drastically impairs their root distribution. This could jeopardize the setting up of phytostabilization trials. The metal-induced alterations of root system properties are clearly metal- and species-specific. At sites polluted with multiple metals, it is therefore recommended to first test their impact on the root system of multiple plant species so as to select the most appropriate species for each site.

Vegetative filter strips (VFS) implemented downstream to the source of pollution can trap sediments and thus limit sediment export from agricultural fields. However, their retention efficiencies are determined by many factors, among others the type of plant species and its growth stage. The impact of plant growth and morphology, as well as of incoming sediment concentration, on the efficiency of VFS to trap sediments was assessed by means of an experimental flume. Two different plant species were tested, Lolium perenne and Trifolium repens, after 2 and 4 months of plant growth and for 2 different incoming silty-loam sediment concentrations. Measured retention efficiencies were compared to simulated values using VFSMOD based on goodness-of-fit indicators that take into account uncertainty linked to the measurements.The sediment storage capacity upstream of the VFS was limited in terms of mass, and therefore an increase in sediment concentration led to a decrease in sediment retention efficiency. After 2 months of plant growth, plant morphology affected the VFS potential to trap sediments, as reflected in the higher retention efficiency of T. repens due to its creeping shoot architecture. However, plant growth and development modified the plant morphology and VFS trapping potential. Indeed, L. perenne VFS retention efficiency increased from 35% after 2 months of growth to 50% after 4 months, due to the tillering capacity of grass species. Conversely, the trapping efficiency of T. repens decreased from 49% to 40% after 4 months. This highlights the possible degradation of VFS with time, which in the case of T. repens was due to an increased heterogeneity of plant density within the strips. These modifications of plant characteristics with growth stage, which affected sediment trapping efficiencies, can be effectively integrated into mechanistic models like VFSMOD, mainly through stem spacing and Manning's surface roughness coefficient inputs. Since these parameters were highly conditioned by plant growth and development, modelers should take into account plant dynamics and select plant parameters related to the actual field conditions. (C) 2014 Published by Elsevier B.V.

Phytostabilization is a management strategy aiming at limiting risks of heavy metal dispersion out of a polluted area by means of the use of appropriate plant species and soil amendments. Several aspects of this technique were studied based on laboratory experiments. We first clarified heavy metal mobility and bioavailability terms and showed what different laboratory measurements performed on contaminated soil samples can really indicate about these concepts. An interesting output of this work was the demonstration that heavy metal mobility and plant uptake are not always positively correlated. The impact of 2 contrasted soil amendments on heavy metal mobility, uptake by perennial ryegrass and metal leaching was assessed based on specifically-designed pot experiments. The suitability of steel shots for phytostabilization purposes was shown, but also that the development of plant species into a polluted soil could sometimes lead to an increased risk in metal leaching. Column experiments performed with ryegrass and white clover and with Cd- and Zn-contaminated substrates highlighted a potential risk linked to metal phytostabilization, i.e., a metal-induced impairment of root distribution within the soil profile which could decrease the longevity of the plant cover and limit soil stabilization. The use of an experimental flume allowed us to investigate the mechanisms of metal trapping by vegetative filter strips (VFS) during water erosion events. The effectiveness of a plant species to control soil erosion varied according to its growth and development dynamics. We showed also that the retention of metal contaminants by VFS was affected not only by the sediment retention, but also by the metal concentration in the various particle size classes. This metal distribution between the different particle size fractions varied according to the pollutant and soil physico-chemical properties as well as the nature of the pollution. Lessons from these experiments were combined with feedbacks from field trials in the literature to propose some guidelines for the in situ implementation of phytostabilization strategies. Such field experiments are decisive for the future of phytostabilization.

Urbanization, industrialization and agricultural practices have resulted in soil contamination with heavy metals in many world areas. Phytostabilisation is an emerging solution to limit pollutant dispersion out of the contaminated area without expensive costs. However, because of the heavy metal phytotoxicity, the implementation of a plant cover could be jeopardized. Therefore, we assessed the impact of Cd and Zn, 2 common metal pollutants, on the setting up of root system of 2 plant models, Lolium perenne and Trifolium repens. After 1 month of metal application, we measured the root mass, length and diameter according to the depth thanks to scanner and image analysis. These results were linked to root metal concentrations. We focused also on ultrastructural modifications by means of histochemical analyses and on quantification of structural polysaccharides and lignin through Van Soest global method and lignin staining. Drastic alterations of root system were highlighted, especially for Lolium perenne.

Les metaux lourds presents dans des sols pollues par des activites industrielles peuvent etre disperses dans l’environnement, suite a une erosion hydrique. L’etude presente vise a quantifier l’impact de l’implantation de bandes enherbees pour limiter cette dispersion. Pour ce faire, un dispositif experimental a ete mis au point, qui permet d’evaluer la retention en sediments et en metaux lourds de differents types de bandes enherbees (ray-grass anglais et trefle blanc), et ce pour differentes pentes, debits de ruissellements et concentrations en sediments. 2 sols fortement pollues differents ont ete utilises. Des mesures physico-chimiques sur les sediments collectes nous ont permis d’expliquer les differences d’efficacite de retention en metaux lourds entre les differents types de bandes et de sols utilises.

Phytoremediation is a promising and cost-effective strategy to manage heavy metal polluted sites. In this experiment, we compare phytoextraction and phytostabilisation technologies on a soil contaminated by Cd and Zn fallouts from a former Zn smelter. Lolium perenne plants were cultivated for 2 months under controlled conditions in a 27.6dm³-pot experiment filled with the contaminated soil. A leachate tank below the soil compartment allowed the collect of leachates. The heavy metals phytoextraction was promoted by adding Na-EDTA (0.5 g.kg-1) in watering solution 4 days after germination. Phytostabilisation was assessed by mixing soil with steel shots (1%) before Lolium perenne sowing. Plants, soils and leachates were regularly monitored during 2 months for plant growth, leachate volume, elemental concentrations (Ca, Cd, Fe, K, Mg, Na and Zn), pH and electrical conductivity. Shoot biomass was low, because of high heavy metal toxicity. Presence of plants exacerbated heavy metal leaching, by improving soil hydraulic conductivity. Use of EDTA for phytoextraction increased heavy metal mobility in soil, leading to higher concentration of heavy metal in shoots. Moreover, biomass production was also increased despite higher Cd and Zn concentration in shoots, because EDTA was able to reduce metal toxicity in plant tissues. However, this higher heavy metal extraction was insufficient to significantly reduce the heavy metal content in soil, and did not justify the important leaching induced by EDTA. On the other hand, addition of steel shots decreased efficiently both CaCl2 (0.01M) extraction of Cd and Zn, and therefore heavy metal mobility in soil, and their leaching. This input improved also shoot biomass production. However, improvement of growth conditions by steel shots led to higher heavy metal mass in shoot tissues. Therefore, soil heavy metal mobility and plant uptake are not always linked, as reduction of metal mobility enhanced plant metal uptake in our experiment.

A rhizospheric biotest, consisting of a thin layer of substratum in close contact with roots of Lolium multiflorum, was used on two contrasting contaminated soils (Cabezo and Brunita) issued from a former mining area in La Union (Spain). On top of this biotest, soil characterisation, including CaCl2 selective extractions, was performed. Total heavy metal concentrations were the highest in the soil from Cabezo, but CaCl2 extractions indicated higher heavy metal mobilities in Brunita soil. On the base of heavy metal concentrations and biomass production in L. multiflorum seedlings, availability assessed by the rhizospheric biotest was higher than the values obtained from CaCl2 extraction, except for Mn and Pb. Rhizospheric biotest also revealed higher heavy metal bioavailability for Cabezo. The low pH of Brunita (3.47) could explain the high CaCl2-extractable heavy metal concentrations as well as the high transfer factor found for Cu, Mn and Zn in this substrate. Cu, Mn and Zn toxicities were also detected for shoot tissues. Transpiration rates were clearly lower for seedlings exposed to Brunita than for those exposed to Cabezo, while water use efficiency was higher for the former (4.8 mg DW ml−1) than for the latter (3.8 mg DW ml−1). Iron nutrition was found to interfere with heavy metal root absorption, mainly through negative interactions during root absorption. It is concluded that rhizospheric test offers the advantage to consider the root–soil interactions in a dynamic perspective and constitutes a useful tool for the assessment of heavy metal availability on contaminated soils. Heavy metal bioavailability assessment should not be based on only one measure alone, but on different and complementary approaches.

Phytoremediation is a promising and cost-effective strategy to manage heavy metal polluted sites. In this experiment, we compared simultaneously phytoextraction and phytostabilisation techniques on a Cd and Zn contaminated soil, through monitoring of plant accumulation and leaching. Lolium perenne plants were cultivated for 2 months under controlled environmental conditions in a 27.6 dm(3)-pot experiment allowing the collect of leachates. The heavy metal phytoextraction was promoted by adding Na-EDTA (0.5 g kg(-1) of soil) in watering solution. Phytostabilisation was assessed by mixing soil with steel shots (1%) before L. perenne sowing. Presence of plants exacerbated heavy metal leaching, by improving soil hydraulic conductivity. Use of EDTA for phytoextraction led to higher concentration of heavy metal in shoots. However, this higher heavy metal extraction was insufficient to satisfactory reduce the heavy metal content in soil, and led to important heavy metal leaching induced by EDTA. On the other hand, addition of steel shots efficiently decreased both Cd and Zn mobility, according to 0.01 M CaCl(2) extraction, and leaching. However, improvement of growth conditions by steel shots led to higher heavy metal mass in shoot tissues. Therefore, soil heavy metal mobility and plant metal uptake are not systematically positively correlated.