François Courchesne

Changes in water extractable metals, pH and organic carbon concentrations at the soil-root interface of forested soils

Soluble metals are of nutritional and ecotoxicological interest as they are the most readily available form to the biota. Metal solubility in soils is mostly controlled by pH and the organic matter content. The rhizosphere is generally considered as an environment enriched in organic matter and often more acidic (depending on nutritional status of the plant) than the bulk soil. Yet, there is a lack of consensus on the distribution of metals at the soil-root interface. Consequently, the specific objectives of this paper are to compare the chemical properties and the water extractable metal concentrations of the rhizosphere and the bulk soil of forest soil (1) along a gradient in soil contamination and (2) under different tree species. Two study areas were used: (1) Rouyn-Noranda (Canada) where samples were collected along a gradient in metal contamination at a distance of 0.5, 2 and 8 km downwind from a copper smelter; (2) Saint-Hippolyte (Canada) where the effect of three tree species (Abies balsamea, Acer saccharum and Betula papyrifera) was studied. In the field, the rhizosphere was operationally defined as the soil adhering to the roots after agitation, soil falling from the roots and the rest of the soil composing the bulk soil. Once in laboratory, a second agitation was performed to separate the rhizosphere into an inner and an outer component. Water extractable metal concentrations (Al, Ca, Cd, Co, Cr, Cu, Fe, Li, Mg, Mn, Ni, Pb and Zn) were quantified either with an ICP-AES or a GFAAS. Measurements of pH, electrical conductivity (EC), water-extractable organic carbon (WEOC) and solid phase organic carbon (SPOC) were performed. Results systematically indicate that EC, WEOC and SPOC follow the sequence inner rhizosphere > outer rhizosphere > bulk soil. The pH is always lower in the inner rhizosphere than in the bulk soil, while the outer rhizosphere frequently shows an inconstant behaviour. The results also show a clear gradient following inner rhizosphere > outer rhizosphere > bulk soil for water extractable Al, Ca, Cd, Cu, Fe, Mg, Mn, Ni, Pb and Zn. Li, Co and Cr levels were below method detection limit in all cases. WEOC seems to be the main variable related to the water-extractable metals concentrations. The gradient in metal contamination at Rouyn-Noranda was not as expected in the water extracts with the site at 2 km frequently presenting higher metal concentrations than the sites at 0.5 and 8 km. Moreover, a tree species effect did not clearly immerge for any of the chemical properties studied. However, the water extractable Ca concentrations were higher in the soil under Acer saccharum. The effects of the metal gradient and of the tree species may be more pronounced if stronger extractants are used. The addition of an outer rhizosphere component is useful as its behaviour is not consistently intermediate between the inner rhizosphere and bulk soil.

Rhizospheric processes influencing the biogeochemistry of forest ecosystems

In the rhizosphere, biotic and abiotic processes interact to create a zone distinct from the bulk soil that may strongly influence the biogeochemistry of forest ecosystems. This paper presents a conceptual model based upon three operationally defined soil-root compartments (bulk soil, rhizosphere and soil-root interface) to assess nutrient availability in the mineral soil-root system. The model is supported by chemical and mineralogical analyses from bulk and rhizosphere soils collected from a Norway spruce forest. The rhizosphere was more intensively weathered and had accumulated more acidity, base cations and phosphorus than the bulk soil. The quantity and quality of organic matter regulate the reciprocal relationships between soil and roots with their associated biota. However, the biogeochemical role of organic matter in the rhizosphere still remains as an area in which more future research is needed. The mechanisms that may regulate nutrient availability in the rhizosphere are also discussed and related to nutrient cycling and adaptation of forests growing under nutrient poor or perturbed conditions. We suggest that the rhizosphere is not an ephemeral environment in the soil, but persists over time and is resilient against perturbation as evinced by consistent differences between rhizosphere and bulk chemistry and mineralogy over wide range of field treatments.

The phenology of fine root growth in a maple-dominated ecosystem: relationships with some soil properties

A two-year study was undertaken in a maple-dominated watershed of southern Québec, Canada, to examine relationships between trends in fine root growth, stem diameter growth, soil moisture, soil temperature, mineralized-N and extractable-P. Until September, soil temperature was consistently higher in 1995 than in 1994. Apart from the first sampling in mid-May, soil moisture was higher in 1994 than in 1995. In 1994, most fine roots were produced before leaf expansion, whereas in 1995, fine root production peaked in July. Annual fine root production was estimated to be 2.7 times higher in 1994 than in 1995. Stem growth was strongly associated with the seasonal and annual variation in soil temperature. Root and diameter growth were asynchronous in 1994 but not in 1995. Fine root production was associated with two groups of variables: a soil fertility (mineralized-N and extractable-P) group and a physical soil environment (moisture and temperature) group. Our results are consistent with the negative effect of high soil-N fertility on fine root production but are inconclusive as to the positive effect of high soil-P fertility. Soil conditions that are detrimental to root growth such as high N availability and anaerobiosis could modify the normal dynamics of fine root growth.

Speciation of zinc in contaminated soils.

The chemical speciation of zinc in soil solutions is critical to the understanding of its bioavailability and potential toxic effects. We studied the speciation of Zn in soil solution extracts from 66 contaminated soils representative of a wide range of field conditions in both North America and Europe. Within this dataset, we evaluated the links among the dissolved concentrations of zinc and the speciation of Zn(2+), soil solution pH, total soil Zn, dissolved organic matter (DOM), soil organic matter (SOM) and the concentrations of different inorganic anions. The solid-liquid partitioning coefficient (K(d)) for Zn ranged from 17 to 13,100 L kg(-1) soil. The fraction of dissolved Zn bound to DOM varied from 60% to 98% and the soil solution free Zn(2+) varied from 40% to 60% of the labile Zn. Multiple regression equations to predict free Zn(2+), dissolved Zn and the solid-liquid partitioning of Zn are given for potential use in environmental fate modeling and risk assessment. The multiple regressions also highlight some of the most important soil properties controlling the solubility and chemical speciation of zinc in contaminated soils.

Simulation of stream-water chemistry with soil solution and groundwater flow contributions

Abstract The mass balance equation was used to assess the contribution of water flowing from the solum (the upper 80 cm of soil) and subsoil to a first-order stream in the southern Laurentians, Quebec, during spring snowmelt. Dissolved reactive silicon was used as a tracer and showed that during high flow the solum contributed 50–95% of the stream discharge. During low flow, the stream was mainly fed by ground water. Based on this hydrograph separation and soil solution chemistry, a simple model was developed to predict stream chemistry. The simulations obtained for calcium, sodium, sulphate, nitrate, chlorine, and electrical conductivity showed good agreement with the stream chemistry. The predicted values of hydrogen and total aluminium followed the same pattern as the measured values but overestimated concentrations in the stream during high flow due to their reactive, non-conservative behaviour.

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.

Bioaccumulation of manganese by plants: influence of MMT as a gasoline additive

Abstract This study on the bioaccumulation of manganese (Mn) by plants was motivated by the increased use of Methylcyclopentadienyl manganese tricarbonyl (MMT) as a replacement for lead in gasoline. Oats and beans were grown in sandy and organic soils at a control site (E o ) and at two other sites weakly (E + ) and highly exposed (E ++ ) to Mn contamination, potentially from a MMT source. Total Mn, Ca, Mg, Fe, Zn and Al were measured in the soils and in the plants (roots, stems/leaves and fruits). Exchangeable Mn was measured in the soils at the beginning and at the end of the exposure period. The pH of the organic and sandy silty soils were found to be lower outdoors (E + and E ++ ) than in the greenhouse (E o ) and exchangeable Mn was found to be significantly higher in the organic soil at E ++ (1.03–1.36 ppm). Higher Mn accumulation was also found in the fruits and stems of oats grown in the organic and sandy soils at E ++ . This Mn accumulation is often associated with increased Fe and Al in the plant. These results suggest that the addition of MMT to gasoline may result in an increase in exchangeable Mn in organic soils. However, it has not been proven that the source of the increased Mn accumulation is indeed MMT in gasoline.

Labile zinc concentration and free copper ion activity in the rhizosphere of forest soils

Water-soluble and acid-extractable Cu and Zn, water-soluble organic carbon (WSOC), pH, differential pulse anodic stripping voltammetry-labile Zn (ZnL), Zn2+ activity (Windemere humic aqueous model [WHAM]; http://chess.ensmp.fr/ chemsites.html), and Cu2+ activity with an ion-selective electrode were compared between the rhizosphere and the bulk components of nine acidic forest sites from southeastern Canada. At all sites, the WSOC contents were higher in the rhizosphere than in the bulk component. Acidity was also higher in the rhizosphere, although pH differences were significant at only five sites. The concentrations of Zn in water extracts and ZnL contents (at six sites) were higher in the rhizosphere, whereas acid-extractable Zn was only marginally increased in the rhizosphere. Calculations with WHAM indicated that free Zn2+ ion activities were higher in the rhizosphere than in the bulk soil but that the fraction of total dissolved Zn in water extracts that is present as free Zn2+ did not differ significantly between the two components. The concentration of Cu in the water extract was higher in the rhizosphere for all sites, but acid-extractable Cu levels did not differ. The fraction of water-soluble Cu present as Cu2+ was higher in the bulk soil, although Cu2+ activities did not significantly vary with proximity to roots. These results showed that the processes acting in the rhizosphere of forest soils strongly affected the concentrations of dissolved Zn and Cu and that this microenvironment should be considered when estimating the bioavailability and the ecological risks of metals in soils.

Simulation of soil chemistry and nutrient availability in a forested ecosytem of southern Quebec. Part II. Application of the SAFE model

The dynamic soil model SAFE was calibrated and validated in a small hardwood forest of southern Quebec as a function of its ability to reproduce current soil chemistry and similar pre-industrial soil conditions despite the difference in forest history. SAFE was relatively accurate for reproducing soil chemistry, but comparison of pre-industrial soil conditions between unburned and burned stands casts doubt as to its applicability at sites where specific processes may be involved in nutrient cycling, e.g. the immobilization of N by microbes. Simulated soil chemistry in the unburned zone reinforced the conclusions of a few historical studies which support the hypothesis that acid-sensitive forest sites of northeastern USA underwent significant acidification when major inputs of acidity from the atmosphere occurred, i.e. during the 1930-1980 time span. Model projections in the mineral soil suggest that a new steady-state should be reached in the 21st century assuming no harvest, but that this equilibrium is broken if timber harvesting is done. Model output also suggests that cation nutrient deficiencies could occur in the long-term, but future Al phytotoxic responses are unlikely to occur due to a relatively high projected pH. Finally, it was demonstrated that the time-series files of nutrient cycling should be prepared with care as they can be the source of some abnormalities in model calibration. (Less)

Mineral Weathering in the Rhizosphere of Forested Soils

ABSTRACT The rhizosphere is a microenvironment enriched in organic matter and generally more acidic than the bulk soil. In this chapter, we submit that mineral weathering and metal fractionation differ in the rhizosphere compared to the bulk soil, a change that could impact on plant nutrition and element toxicity. The objective of the study is to establish the nature of the effect of roots on mineral weathering in the rhizosphere of forested soils based on differences in (1) mineralogical composition and (2) the chemical forms of metals between the rhizosphere and the bulk soil. The study area was located in Rouyn–Noranda (Canada), where samples were collected under Populus tremuloides growing on Luvisolic soils. X-ray diffraction (XRD), time of flight secondary-ion mass spectroscopy (TOF-SIMS) and X-ray absorption near-edge structure (XANES) analyses were performed. The concentrations of Al, Ca, Cd, Co, Cr, Cu, Fe, Li, Mg, Mn, Ni, Pb, Si and Zn were obtained from an acid ammonium oxalate (AAO) extraction. The XRD results show differences in mineralogical abundance, particularly of chlorite and amphibole, which is interpreted as an increase in mineral weathering in the rhizosphere. It was suggested in the literature that the higher alteration in the rhizosphere could be related to K uptake by roots. However, our results show greater BaCl2-extractable K in the rhizosphere, an observation in opposition to this nutrient-depletion hypothesis. The AAO extraction reveals higher contentrations of Fe and Mn in the rhizosphere. These data support XRD results and suggest the formation of secondary oxides in the rhizosphere through weathering. In turn, the greater abundance of oxides creates absorption sites for trace elements such as Cu and Zn as supported by the AAO extractions. The TOF-SIMS mapping also shows an accumulation of total metals at the soil–root interface. The XANES analysis of Mn further indicates that metals tend to be oxidized in the rhizosphere, whereas they are found in organic forms in the root or as a mixture of both at the soil–root interface. The presence of oxidized forms of Mn in the rhizosphere is in agreement with the results of the AAO extraction. In summary, weathering is shown to be higher in the rhizosphere, favors the formation of oxides, notably Mn oxides and, hence, the retention of trace metals.