Marie-Claude Turmel

Early rhizosphere microbiome composition is related to the growth and Zn uptake of willows introduced to a former landfill.

Although plants introduced for site restoration are pre-selected for specific traits (e.g. trace element bioaccumulation, rapid growth in poor soils), the in situ success of these plants likely depends on the recruitment of appropriate rhizosphere microorganisms from their new environment. We introduced three willow (Salix spp.) cultivars to a contaminated landfill, and performed soil chemical analyses, plant measurements, and Ion Torrent sequencing of rhizospheric fungal and bacterial communities at 4 and 16 months post-planting. The abundance of certain dominant fungi was linked to willow accumulation of Zn, the most abundant trace element at the site. Interestingly, total Zn accumulation was better explained by fungal community structure 4 months post-planting than 16 months post-planting, suggesting that initial microbial recruitment may be critical. In addition, when the putative ectomycorrhizal fungi Sphaerosporella brunnea and Inocybe sp. dominated the rhizosphere 4 months post-planting, Zn accumulation efficiency was negatively correlated with fungal diversity. Although field studies such as this rely on correlation, these results suggest that the soil microbiome may have the greatest impact on plant function during the early stages of growth, and that plant-fungus specificity may be essential.

Relevance of Rhizosphere Research to the Ecological Risk Assessment of Trace Metals in Soils

ABSTRACT The chemical, mineralogical, and microbial properties of the rhizosphere of a range of forested ecosystems were studied to identify the key processes controlling the distribution and fate of trace metals at the soil–root interface. The results of our research indicate that: (1) the rhizosphere is a soil microenvironment where properties (e.g., pH, organic matter, microbes) and processes (nutrient and water absorption, exudation) differ markedly from those of the adjacent bulk soil; (2) the rhizosphere is a corrosive medium where the weathering and neoformation of soil solid phases are enhanced; (3) the concentrations of solid-phase and water-soluble trace metals like Cd, Cu, Ni, Pb, and Zn are generally higher in the rhizosphere as shown by both macroscopic and microscopic approaches; (4) a larger fraction of water-soluble metals is complexed by dissolved organic substances in the rhizosphere; and (5) soil microorganisms play, either directly or indirectly, a distinct role on metal speciation, in particular Cu and Zn, in the rhizosphere. These results improve our capacity to estimate metal speciation and bioavailability at the soil–root interface. Furthermore, the research emphasizes the crucial physical position occupied by the rhizosphere with respect to the process of elemental uptake by plants and its key functional role in the transfer of trace metals along the food chain. We conclude that the properties and processes of the rhizosphere should be viewed as key components of assessments of the ecological risks associated with the presence of trace metals in soils.

Trace elements in the pollen of Ambrosia artemisiifolia: What is the effect of soil concentrations?

Concentrations of nine trace elements (Ba, Cd, Cr, Cu, Mn, Ni, Pb, Tl and Zn) were measured in a plant bearing allergenic pollens (ragweed) and their transfers from soils to the roots and then to the pollens were investigated. The soil, roots and pollens collected from flowers were sampled at 26 urban sites. Soil pH, soil organic carbon and total-recoverable trace elements (TE) in soil, roots and pollens were measured. The three biogeochemical compartments are well discriminated according to their TE concentrations. The concentrations (in μg g(-1)) in pollens decreased as follow: Zn (59.5-205)>Mn (19.4-117)>Ba≈Cr≈Cu≈Ni≈Pb (0.54-27.7)>Cd (0.06-0.77)>>Tl (0.0015-0.0180). Mean elemental allocation within ragweed always favored roots over pollen but, at site level, inverse pattern is also observed mostly for Zn and slightly for Cu and Ni. Significant predictive models of TE concentrations in pollens were obtained using soil or root properties only for Cd, Ni and Pb. They all involved positive relationships between TE concentrations in pollens and in soil or roots. Estimates of short-term exposure of human to TE carried out by ragweed pollens indicate TE absorption of less than 50 ng, far below thresholds of air quality criteria. Investigating the TE chemistry of pollens is a required first step to validate the impact of TE in pollens on human health and on the prevalence and intensity of allergy symptoms and atopic diseases.

Speciation and bioavailability of trace metals (Cd, Cu, Ni, Pb, Zn) in the rhizosphere of contaminated soils

ABSTRACT The unique characteristics of the rhizosphere, combined with the relevance of its function in the soil:plant system, greatly contribut to the marked interest in this microenvironment. Specific changes occur in the rhizosphere as a consequence of root activity, which in turn impact on the speciation and bioavailability of nutrients and trace metals. In order to better understand the processes involved in the fractionation of trace metals in the rhizosphere, the objectives of this study are (i) to contrast the solid phase fractionation of trace metals (Cd, Cu, Ni, Pb and Zn) between the inner rhizosphere, the outer rhizosphere and the bulk components of forest mineral soils along a soil contamination gradient and, (ii) to determine Cu2+ activity in the liquid phase and establish the relationships with pH, dissolved organic carbon (DOC) and total dissolved Cu. Three sites located at distances of 2.5, 15 and 43 km from a Cu–Ni smelter in the Sudbury area of Ontario, Canada, were selected on the basis of their level of soil contamination. The fine roots from a number of white birch trees (Betula papyrifera Marsh.) were sampled in the B horizon along with the bulk soil (n = 6, 4 and 3 at sites 1, 2 and 3, respectively). The partition of the rhizosphere material from the roots was performed in the laboratory where the rhizosphere was separated into two components: the outer and the inner rhizosphere. The five trace metals were extracted using H2O, 0.1 M BaCl2, 0.1 M Na4P2O7 and an HNO3–HCl digestion. The Cu2+ activity in the water extract was measured with an ion-selective electrode (Cu-ISE). The pH and DOC concentrations were also measured on all samples. Results indicate that the potentially available pools of Cd, Cu and Ni, measured in the H2O and BaCl2 extracts, reflect the level of soil contamination and follow the gradient site 1 > site 2 > site 3. The extent of the difference between the bulk soil, the outer and the inner rhizosphere with respect to metal concentrations varies according to the trace metal, the metal fraction and the sampling site. In most cases, either the outer or the inner rhizosphere contains significantly higher metal concentrations than the bulk soil. In addition, the rhizosphere enrichment ratio (inner rhizosphere/bulk soil metal concentrations) is greater for the water-soluble and BaCl2-exchangeable fractions. This potentially available metal pool represents a small fraction of the total recoverable (HNO3–HCl digestion) metal content both in the bulk soil (3%) and the inner rhizosphere (7%), and is systematically higher in the rhizosphere. The Cu2+ activities in the water extract decrease with distance from the smelter, but are very similar between the three soil components for a given site. The inner rhizosphere contains the highest DOC concentrations and the most acidic pH values. The water-soluble Cu/DOC molar ratios are similar for the bulk soil, the outer and the inner rhizosphere of any given site, suggesting that the complexation power of organic matter is of a relatively similar magnitude in the three soil components. The pCu2+ activities are best predicted using DOC in the bulk soil (r2 =0.707, p = 0.0003), with pH and water-soluble Cu in the outer rhizosphere (r2 = 0.731, p = 0.001), and with pH and DOC in the inner rhizosphere (r2 = 0.903, p = 0.00001).

Phytoextraction of soil trace elements by willow during a phytoremediation trial in Southern Québec, Canada

ABSTRACT The phytoextraction of the trace elements (TEs) As, Cd, Cu, Ni, Pb, and Zn by willow cultivars (Fish Creek, SV1 and SX67) was measured during a 3-year field trial in a mildly contaminated soil. Biomass ranged from 2.8 to 4.4 Mg/ha/year at 30,000 plants/ha. Shoots (62%) were the main component followed by leaves (23%) and roots (15%). Biomass was positively linked to soluble soil dissolved organic carbon, K, and Mg, while TEs, not Cd and Zn, had a negative effect. The TE concentration ranking was: Zn > Cu > Cd > Ni, Pb > As, and distribution patterns were: (i) minima in shoots (As, Ni), (ii) maxima in leaves (Cd, Zn), or (iii) maxima in roots (Cu, Pb). Correlations between soil and plant TE were significant for the six TEs in roots. The amounts extracted were at a maximum for Zn, whereas Fish Creek and SV1 extracted more TE than SX67. More than 60% (91–94% for Cd and Zn) of the total TE was in the aboveground parts. Uptake increased with time because of higher biomass. Fertilization, the selection of cultivars, and the use of complementary plants are required to improve productivity and Cd and Zn uptake.

Soil trace element changes during a phytoremediation trial with willows in southern Québec, Canada.

ABSTRACT This study determined the changes in trace elements (TE) (As, Cd, Cu, Ni, Pb, Zn) chemistry in the soils of a willow (“Fish Creek” – Salix purpurea, SV1 – Salix x dasyclados and SX67 – Salix miyabeana) plantation growing under a cold climate during a three-year trial. The soil HNO3-extractable and H2O-soluble TE concentrations and pools significantly decreased under most cultivars (Fish, SX67). Yet, TE changes showed inconsistent patterns and localized soil TE increases (Ni, Pb) were measured. Temporal changes in soil TE were also detected in control plots and sometimes exceeded changes in planted plots. Discrepancies existed between the amount of soil TE change and the amount of TE uptake by willows, except for Cd and Zn. Phytoremediation with willows could reduce soil Cd and Zn within a decadal timeframe indicating that they can be remediated by willows in moderately contaminated soils. However, the time needed to reduce soil As, Cu, Ni and Pb was too long to be efficient. We submit that soil leaching contributed to the TE decrease in controls and the TE discrepancies, and that the plantation could have secondary effects such as the accelerated leaching of soil TE.

The speciation of water-soluble Al and Zn in the rhizosphere of forest soils.

This study focuses on the relationships of dissolved Al and Zn speciation with microbial and chemical soil properties in the bulk and rhizosphere of forest soils. The soil components were sampled under Populus tremuloides Michx. at six sites located close to industrial facilities. Total water-soluble (Al(WS), Zn(WS)) and reactive (Al(R), Zn(R)) Al and Zn concentrations measured in soil water extracts, speciation data modeled by WHAM 6, chemical properties (pH, DOC, major cations and anions) and microbial properties (microbial biomass and enzyme activities) were measured on all soils. Enrichment in Al(R) and Zn(R) was observed in the rhizosphere compared to bulk soils. In a given soil material, the speciation of Al and Zn varied according to solution pH and Al-organic as well as Zn-organic complexes or Zn(2+) were generally the dominant species. The factors controlling the Al(WS), Zn(WS), Al(R) and Zn(R) concentrations differed between soil components, shifting from strictly chemical in the bulk (78%) to interactions among microbial and chemical variables in the rhizosphere (87%). Results further indicate that organic matter and pH were significantly linked to these response variables in the rhizosphere. Involvement of rhizospheric microorganisms occurred via pH changes induced by either the microbial assimilation of nitrogen or through the release of metals during the mineralization of roots. Our results therefore suggest that microbial activity is an important component of the biogeochemistry of Al and Zn in the rhizosphere. The study further provides key information to improve the assessment of ecological risk associated to Al and Zn in forest soils.

Mass Balance of Organic Carbon in the Soils of Forested Watersheds from Northeastern North America

The objective of this chapter is to establish the functional links between the organic carbon (C) dynamics in soils, the biogeochemical C cycle of forested watershed and the potential changes in the sequestration of atmospheric C by forest soils in response to changing climatic conditions. After an introductory statement on greenhouse gases and their effects on climate, the second part of the chapter describes the properties, functions and biogeochemical cycling of organic carbon (C) in forest soils. The third part of the chapter presents the results of an extensive review of organic C mass balance studies conducted in forested watersheds of northeastern North America. The soil and plant C pools and fluxes are quantified and results are compared to those obtained from other environments, such as southeastern United States and Western and Central Europe. The potential contribution of soils to the emission of greenhouse gases is critically discussed through an evaluation of the net role of soils on organic carbon (C) cycling and on its transport from terrestrial to aquatic environments. Based on the available data and evidences, it appears that the question as to whether soils from northern forests will behave as a net source or sink of C under a warmer climate cannot yet be answer unequivocally.