International audience | Phytomanagement manipulates the soil-plant system to lower the risk posed by contaminated soils. In this process, the addition of amendments, such as biochar, and bacteria can improve the fertility of poor contaminated soils and consequently ameliorate plant growth. A number of studies based on the inoculation of soil with microorganisms of the genus Bacillus, previously isolated from contaminated sites, revealed positive effects on soil properties and plant growth. Furthermore, when the Bacillus isolates were used in association with biochar, better results were obtained, as biochar can ameliorate soil properties and serve as habitat for microorganisms. Accordingly, a mesocosm study was set-up using a mining technosol amended with biochar and inoculated with an endogenous Bacillus isolate, to evaluate the effect of inoculation on soil properties, metal(loid) immobilization, and Salix viminalis growth. Two inoculation methods were compared: (1) direct inoculation of bacteria (Bacillus sp.) and (2) inoculation using biochar as a carrier. Results showed that the Bacillus isolate modified soil properties and ameliorated plant growth, while having a reduced effect on metal(loid) accumulation. The microbial activity was also stimulated, and the community composition was shifted, more importantly when biochar was used as a carrier. In conclusion, this research revealed an improvement of the plant growth and microbial activity after the addition of the endogenous bacterium to the analyzed former mining soil, with better results recorded when a carrier was used.

Copper (Cu) induced phytotoxicity has become a serious environmental problem as a consequence of significant metal release through anthropogenic activity. Understanding the spatial distribution of Cu in plants such as willow is essential to elucidate the mechanisms of metal accumulation and transport in woody plants, particularly as affected by variable environment conditions such as soil flooding. Using synchrotron-based X-ray fluorescence (μ-XRF) techniques, the spatial distribution of Cu and other nutrient elements were investigated in roots and stems of Salix (S.) integra exposed to 450 mg kg⁻¹ Cu under non-flooded (NF)/flooding (F) conditions for 90 d. S. integra grown in the F condition exhibited significant higher tolerance index (TI, determined by the ratio of total biomass in Cu treatments to control) (p < 0.05) than that in the NF condition, indicating soil flooding alleviated Cu toxicity to willow plants. The μ-XRF revealed that Cu was preferentially located in the root cap and meristematic zone of the root tips. Under the NF condition, the Cu intensity in the root epidermis was more highly concentrated than that of the F condition, suggesting the soil flooding significantly inhibited Cu uptake by S. integra. The pattern of the Cu spatial distribution in the S. integra stem indicated that the F condition severely reduced Cu transport via the xylem vessels as a consequence of decreasing the transpiration rate of leaves. To our knowledge, this is the first study to report the in vivo Cu distribution in S. integra in a scenario of co-exposure to the Cu and the soil flooding over a long period. The finding that Cu uptake varies significantly with flooding condition is relevant to the development of strategies for plants to detoxify the metals and to maintain the nutrient homeostasis.

Energy crops are an important renewable source for energy production in future. To ensure high yields of crops, N fertilization is a common practice. However, knowledge on environmental impacts of bioenergy plantations, particularly in systems involving trees, and the effects of N fertilization is scarce. We studied the emission of volatile organic compounds (VOC), which negatively affect the environment by contributing to tropospheric ozone and aerosols formation, from Miscanthus and willow plantations. Particularly, we aimed at quantifying the effect of N fertilization on VOC emission. For this purpose, we determined plant traits, photosynthetic gas exchange and VOC emission rates of the two systems as affected by N fertilization (0 and 80 kg ha−1 yr−1). Additionally, we used a modelling approach to simulate (i) the annual VOC emission rates as well as (ii) the OH. reactivity resulting from individual VOC emitted. Total VOC emissions from Salix was 1.5- and 2.5-fold higher compared to Miscanthus in non-fertilized and fertilized plantations, respectively. Isoprene was the dominating VOC in Salix (80–130 μg g−1 DW h−1), whereas it was negligible in Miscanthus. We identified twenty-eight VOC compounds, which were released by Miscanthus with the green leaf volatile hexanal as well as dimethyl benzene, dihydrofuranone, phenol, and decanal as the dominant volatiles. The pattern of VOC released from this species clearly differed to the pattern emitted by Salix. OH. reactivity from VOC released by Salix was ca. 8-times higher than that of Miscanthus. N fertilization enhanced stand level VOC emissions, mainly by promoting the leaf area index and only marginally by enhancing the basal emission capacity of leaves. Considering the higher productivity of fertilized Miscanthus compared to Salix together with the considerably lower OH. reactivity per weight unit of biomass produced, qualified the C4-perennial grass Miscanthus as a superior source of future bioenergy production.

There is currently a requirement for studies focusing on the long-term sustainability of phytoremediation technologies. Trace element uptake by Salix, Populus and Alnus species planted in dredged contaminated canal sediment and concentrations in sediment and pore waters were investigated, eight years after a phytoremediation trial was initiated in NW England. Soil biological activity was also measured using invertebrate and microbial assays to determine soil quality improvements. Zinc was the dominant trace metal in foliage and woody stems, and the most mobile trace element in sediment pore water (∼14 mg l⁻¹). Biological activity had improved; earthworm numbers had increased from 5 to 24, and the QBS index (an index of microarthropod groups in soil) had increased from 70 to 88. It is concluded that biological conditions had improved and natural processes appear to be enhancing soil quality, but there remains a potential risk of trace element transfer to the wider environment.

As phytoextraction implementation may be limited by metal toxicity and leaching, we investigated the idea of in situ metal immobilization in bulk soil, while increasing metal bioavailability in the rhizosphere. Salix smithiana was grown in a pot experiment on two Cd/Zn polluted soils. Treatments with or without willows included: no additives; gravel sludge + red mud kg⁻¹; acidification with S to pH 3.5; and metal immobilization combined with soil acidification. Salix smithiana removed up to 0.78 ± 0.06% total Cd and 0.34% (±0.02) total Zn from the non-treated soils. The phytoextraction efficiency in the S treatments was enhanced by up to ∼50% in response to metal solubility that was magnified by reductive co-dissolution from Mn (IV) and Fe(III) (oxy)hydroxides during microbial S oxidation in the willow rhizosphere. The proposed technique proved to enhance phytoextraction efficiency while controlling the risk of metal leaching from the root zone and phytotoxicity.

A mining area affected by the abandoned exploitation of an arsenical tungsten deposit was studied in order to assess its arsenic pollution level and the feasibility of native plants for being used in phytoremediation approaches. Soil and plant samples were collected at different distances from the polluting sources and analysed for their As content and distribution. Critical soil total concentrations of As were found, with values in the range 70–5330mgkg⁻¹ in the uppermost layer. The plant community develops As tolerance by exclusion strategies. Of the plant species growing in the most polluted site, the shrubs Salix atrocinerea Brot. and Genista scorpius (L.) DC. exhibit the lowest bioaccumulation factor (BF) values for their aerial parts, suggesting their suitability to be used with revegetation purposes. The species Scirpus holoschoenus L. highlights for its important potential to stabilise As at root level, accumulating As contents up to 3164mgkg⁻¹.

The distribution of total mercury (THg) within common deciduous trees and the applicability of tree cores as biomonitors of historical environmental THg trends were assessed for both contaminated and reference sites around Kingston, Ontario. Samples were collected from Acer spp., Quercus spp. Populus spp. and Salix spp. Bark and wood THg concentrations were found to be highly correlated whereas soil and wood THg concentrations were not. There were no temporal relationships for THg in dated tree rings corresponding with any other known environmental Hg trends. The shoreline speciess, Populus and Salix spp., had the greatest bark and wood Hg concentrations reaching 18 ng/g, significantly higher than for inland trees Quercus and Acer spp. with maximum values of 7 and 1.2 ng/g for bark and wood respectively. While tree cores cannot be reliably used as temporal THg biomonitors, there is promise for tree species such as Populus spp and Salix spp as spatial indicators of local long-term Hg contamination.

A three-year experiment was conducted to investigate willows of Salix × smithiana Willd. (S. smithiana) phytoremediation of soil contaminated by polycyclic aromatic hydrocarbons (PAHs) derived by fly ash from biomass combustion. The total removal of ash PAHs in phytoremediation treatment was 50.9% after three consecutive years while the ash PAHs were decreased in natural attenuated soil by 9.9% in the end of the experiment. The ash and spiked PAHs with low and medium molecular weight were susceptible to be removed in higher rates than high molecular weight PAHs. Lower bioconcentration factors of individual PAHs were observed in willow shoots than in roots. The estimated relative direct removal of PAHs by S. smithiana in phytoremediation was significantly lower than 1% suggesting that the contribution of S. smithiana to take up PAHs from soil was negligible and the degradation of PAHs occurred mainly in soil. Phytoremediation using S. smithiana could be seen as a feasible and environmentally friendly approach of arable soils impacted by a PAH contaminated biomass fly ash.

Salix caprea is well suited for phytoextraction strategies. In a previous survey we showed that genetically distinct S. caprea plants isolated from metal-polluted and unpolluted sites differed in their zinc (Zn) and cadmium (Cd) tolerance and accumulation abilities. To determine the molecular basis of this difference we examined putative homologues of genes involved in heavy metal responses and identified over 200 new candidates with a suppression subtractive hybridization (SSH) screen. Quantitative expression analyses of 20 genes in leaves revealed that some metallothioneins and cell wall modifying genes were induced irrespective of the genotype's origin and metal uptake capacity while a cysteine biosynthesis gene was expressed constitutively higher in the metallicolous genotype. The third and largest group of genes was only induced in the metallicolous genotype. These data demonstrate that naturally adapted woody non-model species can help to discover potential novel molecular mechanisms for metal accumulation and tolerance.

Landfill leachates usually need to be treated before discharged, and using soil–plant systems for this has gained substantial interest in Sweden and in the UK. A three-year field study was conducted in central Sweden to quantify plant response, treatment efficiency and impact on groundwater quality of landfill leachate irrigation of short-rotation willow coppice (Salix). Two willow varieties were tested and four irrigation regimes in sixteen 400-m2 plots. The willow plants did not react negatively, despite very high annual loads of nitrogen (≤2160 kg N/ha), chloride (≤8600 kg Cl/ha) and other elements. Mean annual growth was 1.5, 9.8 and 12.6 tonnes DM/ha during years 1–3. For one of two willow varieties tested, relative leaf length accurately predicted growth rate. Irrigation resulted in elevated groundwater concentrations of all elements applied. Treatment efficiency varied considerably for different elements, but was adequate when moderate loads were applied. Short-rotation willow coppice was successfully used for treating a strong landfill leachate in central Sweden over three years.