Using stable and radioactive isotopes to trace atmospherically deposited Pb in montane forest soils

Atmospheric deposition of lead (Pb) throughout the 1900s resulted in elevated amounts of this toxic metal even in remote forest soils of the northeastern United States. Soils can act as a net sink for metals and thus minimize groundwater and surface water contamination. Recent studies utilizing forest floor temporal data and models of total Pb in precipitation, surface soils, and streams have estimated the time scale of Pb release from soils. However, due to the limited availability and spatial variability of forest floor survey data, other methods for quantifying anthropogenic Pb movement are needed. This study uses the isotopic composition (206Pb/207Pb) of soil Pb and measurements of 210Pb and 226Ra to directly trace the transit of atmospherically deposited Pb in the soil profile. We also report on the recovery of an enriched 207Pb dose applied in 1984 to the surface of a soil plot in the coniferous forest at Camels Hump in Vermont. The isotopic composition of soil Pb in low elevation deciduous forests suggests that approximately 65% of the original atmospheric Pb load has migrated from the forest floor to the upper 10 cm of the mineral soil. Higher elevation sites with coniferous vegetation have thicker forest floors, which have prevented significant amounts of Pb from entering the mineral soil. After 17 years, the soil organic horizon in the coniferous zone prevented any penetration of the applied Pb into the mineral soil. Using 210Pb budgets in different soil compartments, we determine forest floor response times for atmospherically delivered Pb to be approximately 60 years in the low elevation deciduous forest zone and 150 years for the high elevation spruce-fir forest zone at Camels Hump. According to its distribution in the soil profile, we conclude that a dispersed release of anthropogenic Pb to groundwater and surface water is possible this century. Our results also offer independent confirmation of Pb deposition models previously generated for the region.     

Threshold increases in soil lead and mercury from tropospheric deposition across an elevational gradient

Atmospheric deposition is the primary mechanism by which remote ecosystems are contaminated, but few data sets show how fluxes change and control soil metal burdens at the landform scale. We present mercury (Hg), lead ((210)Pb and total Pb), and cosmogenic beryllium-7 ((7)Be) measurements in organic (O) soil horizons at high-resolution elevation intervals of ～60 m from 540 to 1160 m on Camels Hump in northern Vermont, USA. Across this gradient, average O horizon Hg ranges from 0.99 mg m(-2) in the low elevation deciduous forest zone to 7.6 mg m(-2) in the higher elevation coniferous forest at 1030 m. We measure two pronounced threshold increases in soil metal burdens above 801 and 934 m, corresponding to the two most common altitudes of cloud base, which coincide with changes in vegetation species. Lead-210, a unique tracer of tropospheric deposition, also increased from 3200 Bq m(-2) to 11?500 Bq m(-2) in O horizons, exhibiting threshold responses at the same elevations as Hg and total Pb. Concentrations of (210)Pb and Hg in foliage double from 760 to 900 m elevation, indicating enhanced deposition across the transition from deciduous to coniferous forest. In contrast, (7)Be is constant across the entire elevational gradient because of its upper atmospheric source. This indicates that the effects of orographic precipitation have a smaller control on soil contaminant burdens than the coupled cloudwater deposition-vegetation scavenging effect in the presence of upwind sources. By measuring soil contaminants and unique tracers of atmospheric deposition, we show that tropospheric fluxes of Hg and Pb are higher by a factor of 2 in high-elevation coniferous forests than in adjacent lowlands. Total O horizon Hg and Pb burdens increase by over 4-fold with elevation because of the compounding effects of enhanced deposition and longer metal residence times at higher elevations (>50 years).     

Partitioning CO2 effluxes from an Atlantic pine forest soil between endogenous soil organic matter and recently incorporated 13C-enriched plant material

Soil CO2 effluxes from recently added 13C-labeled phytomass versus endogenous soil organic matter (SOM) were studied in an acid soil from Atlantic pine forests (NW Spain). After several cultures to incorporate fresh 13C-enriched Lolium perenne to a Humic Cambisol with predominance of humus--Al over humus--Fe complexes, potential soil C mineralization was determined by laboratory aerobic incubation (84 days). Isotopic 13C analyses of SOM fractions were assessed to know in which organic compartments the 13C was preferentially incorporated. Although in the 13C-labeled soil the C mineralization coefficient totalized less than 3% of soil C, the 13C mineralization coefficient exceeded 14%, indicating a greater lability of the newly incorporated organic matter. Organic compounds coming from added phytomass showed a higher lability and contributed considerably to the total soil CO2 effluxes (52% of total soil CO2 evolved during the first decomposition stages and 27% at the end), even though added-C comprised less than 4% of total soil C. Good determination coefficients, when values of CO2--C released were fitted to a first-order double exponential kinetic model, support the existence of two C pools of different lability. Kinetic parameters obtained with this model indicated that phytomass addition augmented the biodegradability of the labile pool (instantaneous mineralization rate k increased from 0.07 d(-1) to 0.12 d(-1)) but diminished that of the recalcitrant pool (instantaneous mineralization rate h decreased from 2.7 x 10(-4) d(-1) to 1.6 x 10(-4) d(-1)). Consequently, the differentiation between both SOM pools increased, showing the importance of SOM quality on CO2 emissions from this kind of soil to the atmosphere.     

Kinetics of Ni sorption in soils: roles of soil organic matter and Ni precipitation

The kinetics of Ni sorption to two Delaware agricultural soils were studied to quantitatively assess the relative importance of Ni adsorption on soil organic matter (SOM) and the formation of Ni layered double hydroxide (Ni-LDH) precipitates using both experimental studies and kinetic modeling. Batch sorption kinetic experiments were conducted with both soils at pH 6.0, 7.0, and 7.5 from 24 h up to 1 month. Time-resolved Ni speciation in soils was determined by X-ray absorption spectroscopy (XAS) during the kinetic experiments. A kinetics model was developed to describe Ni kinetic reactions under various reaction conditions and time scales, which integrated Ni adsorption on SOM with Ni-LDH precipitation in soils. The soil Ni speciation (adsorbed phases and Ni-LDH) calculated using the kinetics model was consistent with that obtained through XAS analysis during the sorption processes. Under our experimental conditions, both modeling and XAS results demonstrated that Ni adsorption on SOM was dominant in the short term and the formation of Ni-LDH precipitates accounted for the long-term Ni sequestration in soils, and, more interestingly, that the adsorbed Ni may slowly transfer to Ni-LDH phases with longer reaction times.     

Complexation with dissolved organic matter and solubility control of heavy metals in a sandy soil

The complexation of heavy metals with dissolved organic matter (DOM) in the environment influences the solubility and mobility of these metals. In this paper, we measured the complexation of Cu, Cd, Zn, Ni, and Pb with DOM in the soil solution at pH 3.7-6.1 using a Donnan membrane technique. The results show that the DOM-complexed species is generally more significant for Cu and Pb than for Cd, Zn, and Ni. The ability of two advanced models for ion binding to humic substances, e.g., model VI and NICA-Donnan, in the simulation of metal binding to natural DOM was assessed by comparing the model predictions with the measurements. Using the default parameters of fulvic and humic acid, the predicted concentrations of free metal ions from the solution speciation calculation using the two models are mostly within 1 order of magnitude difference from the measured concentrations, except for Ni and Pb in a few samples. Furthermore, the solid-solution partitioning of the metals was simulated using a multisurface model, in which metal binding to soil organic matter, dissolved organic matter, clay, and iron hydroxides was accounted for using adsorption and cation exchange models (NICA-Donnan, Donnan, DDL, CD-MUSIC). The model estimation of the dissolved concentration of the metals is mostly within 1 order of magnitude difference from those measured except for Ni in some samples and Pb. The solubility of the metals depends mainly on the metal loading over soil sorbents, pH, and the concentration of inorganic ligands and DOM in the soil solution.     

Extended X-ray absorption fine structure spectroscopy evidence for the complexation of cadmium by reduced sulfur groups in natural organic matter

It is widely accepted that the bioavailability, toxicity, and mobility of trace metals are highly dependent on complexation reactions with functional groups in natural organic matter (NOM). In this study, the coordination chemistry of Cd in NOM was investigated by extended X-ray absorption fine structure spectroscopy. Soil organic matter (SOM) from different types of organic soils and dissolved organic matter (DOM) from an organic and a mineral soil horizon of a Spodosol and aquatic DOM from Suwannee River were investigated. In SOM samples (1000-25000 microg of Cd g(-1), pH 4.6-6.6), Cd was coordinated by 1.0-2.5 S atoms at a distance of 2.49-2.55 A and by 3.0-4.5 O/N atoms at a distance of 2.22-2.25 A. In DOM samples (1750-4250 microg of Cd g(-1), pH 5.4-6.3), Cd was coordinated by 0.3-1.8 S atoms at a distance of 2.51-2.56 A and 3.6-4.5 O/N atoms at a distance of 2.23-2.26 A. In both SOM and DOM samples a second coordination shell of 1.7-6.0 carbon atoms was found at an average distance of 3.12 A. This is direct evidence for inner-sphere complexation of Cd by functional groups in NOM. Furthermore, ion activity measurements showed that less than 1% of total Cd was in the form of free Cd2+ in our samples. Bond distances and coordination numbers suggest that Cd complexed in SOM and DOM is a mixture of a 4-coordination with S (thiols) and 4- and 6-coordinations with O/N ligands. Given that Cd-S associations on average are stronger than Cd-O/N associations, our results strongly indicate that reduced S ligands are involved in the complexation of Cd by NOM also at native concentrations of metal in oxidized organic-rich soils and in humic streams.     

Metal solubility and speciation in the rhizosphere of Lupinus albus cluster roots

The objective of this study was to investigate the influence of root exudation of organic acid anions on the speciation of major and trace metal cations in the rhizosphere of Lupinus albus cluster roots. Plants were grown in rhizoboxes containing repacked weakly acidic loam. Bulk soil solutions and, during the lifetime of cluster roots, rhizosphere solutions were collected using micro suction cups. During organic acid anion exudation bursts, metals in the rhizosphere of cluster roots were strongly mobilized. The concentrations of dissolved organic carbon derived from soil organic matter increased parallel to organic acid anions. Speciation calculations revealed that, during exudation, Al, Ca, Mn, and Zn in the cluster root rhizosphere were mainly bound with citrate, while Cu and Pb were always strongly bound to soil-derived dissolved organic matter. Our results indicate that cluster root exudation led on one hand to direct mobilization and complexation of metals like Al, Fe, and Zn by citrate and on the other hand to the mobilization of soil organic matter which complexes and solubilizes Cu and Pb.     

Formation and fate of bound residues from microbial biomass during 2,4-D degradation in soil

During organic contaminant degradation in soil, bound or nonextractable residues (NER) are formed. Part of these residues may be biogenic, because degrading microorganisms assimilate carbon derived from the pollutant and mineralized CO(2) to form cellular components for example, [fatty acids (FA) and amino acids (AA)], which are subsequently stabilized within soil organic matter (SOM). We investigated the formation and fate of FA and AA from biodegradation of (13)C(6)-2,4-D in soil and the incorporation of the (13)C-label into living biomass via (13)CO(2) fixation. After 64 days of incubation, (13)C-AA in SOM indicated that 44% of the initially applied (13)C(6)-2,4-D equivalents had been converted to microbial biomass and finally to biogenic residues. The intermediate maximum of (13)C-FA in SOM indicated a 20% conversion of (13)C(6)-2,4-D to biomass, but (13)C-FA decreased to 50% of that value whereas (13)C-AA in the SOM remained stable. We provide the first evidence that nearly all bound residues from 2,4-D are biogenic, containing natural microbial residues stabilized in SOM. Because of biogenic residue formation, the potential risk of bound residues from readily metabolized xenobiotics in soils is highly overestimated. Hence, the formation of biogenic residues must be considered in general when performing mass balances of pollutant biodegradation in soils.     

重庆典型烟区土壤有机质及其活性组分的分布特征

为了解重庆典型烟区土壤有机质数量和质量状况,采用现场调查与室内分析相结合的方法,对重庆彭水、丰都和巫山3个典型烟区土壤有机质及其活性组分的分布特征及相互关系进行了研究。结果表明,3个典型烟区土壤有机质(SOM)含量为3.35～53.94 g/kg,在适宜植烟范围(15～30 g/kg)内的比例呈现出丰都>巫山>彭水的趋势,变异系数为巫山烟区大于丰都和彭水烟区。用33、167和333 mmol/L KMnO₄测得的高活性有机质(HLOM)、中活性有机质(MLOM)、活性有机质(LOM)占SOM的比例分别表现为巫山>丰都>彭水、丰都>巫山>彭水、丰都>彭水>巫山,与各烟区的SOM含量均呈极显著正相关,说明土壤活性组分不仅可以很好的反映土壤碳素动态变化,还可用作评价重庆植烟土壤肥力和土壤质量的指标之一。     

Speciation of Cu in a contaminated agricultural soil measured by XAFS, micro-XAFS, and micro-XRF

Contamination of agricultural soils with Cu as a result of fungicide application and spills threatens environmental quality and reduces soil quality for crop growth. In this paper advanced spectroscopic and microscopic methods were used to elucidate the Cu speciation in a calcareous soil contaminated since the 1940s. Microscopically focused synchrotron-based XRF (micro-SXRF) was used to map the elemental distribution in the soils. Results indicated that most of the Cu was not associated with metal oxides, silicates, phosphates, or carbonates. Bulk and microscopically focused X-absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra indicated thatthe Cu in the soil was predominantly Cu adsorbed on soil organic matter (SOM). Interpretation of the fitting results suggests that the Cu is complexed to SOM via bidentate inner-sphere coordination with carboxyl or amine ligands. Results presented in this paper provide detailed information on the molecular coordination of Cu in a contaminated soil. Such information is critical for understanding the long-term fate and best management practices for Cu in the environment.     

Ambivalent role of water in thermodesorption of hydrocarbons from contaminated soil

Thermodesorption studies with soil samples from a former filling station for light crude oil contaminated with mineral oil hydrocarbons (mainly benzene, toluene, ethylbenzene, xylenes, naphthalene, alkylnaphthalenes, and C(10) to C(14) alkanes) have revealed an ambivalent influence of water on desorption rates. Particularly, the influences of soil moisture content, humidity of the purge gas, temperature, and content of soil organic matter (SOM) were studied. At low temperature, purge gas humidity strongly affected the mobility of hydrocarbons in the soil organic matter (SOM) leading to an enhanced release of contaminants at higher moisture contents. Heating resulted in a decrease of thermodesorption when connected with desiccation of soil, in spite of the strong temperature impact on the vapor pressure of contaminants. At high water content of the SOM, the transfer of the pollutant molecules into the gas phase was found to be markedly hindered by the formation of water films or pore-filling by bulk water, both acting as diffusion barriers.     

Quantifying the availability of clay surfaces in soils for adsorption of nitrocyanobenzene and diuron

Coverage of clay surfaces by soil organic matter (SOM) may limitthe efficacy of the soil mineral fractions for adsorption of organic contaminants and pesticides. Two methods were scrutinized for quantitatively assessing the availability of clay surfaces in a smectitic Webster A-horizon soil for sorption of p-nitrocyanobenzene (p-NCB) and diuron. One method, described previously, involves the summation of independent contributions of SOM and swelling clays to sorption of organic solutes. For this method, several assumptions must be made and/or procedural difficulties overcome in the determination of certain terms in the equation proposed for calculating the fractional availability of mineral surfaces (fa). To alleviate the methodological limitations, we developed an alternative approach for determining fa. Good agreement between fa values was obtained from both methods for p-NCB but not diuron. For p-NCB sorption, fa values varied between 0.55 and 0.71. For diuron sorption, our alternative equation estimated fa values varied between 0.41 and 0.61; the other approach yielded negative values. The results demonstrate that SOM does reduce the availability of clay surfaces, hence, suppressing sorption by the Webster A-horizon soil. Our newly developed method provides more reasonable estimates of the availability of soil-clay surfaces for sorption than an earlier published approach.     

Fractions affected and probabilistic risk assessment of Cu, Zn, Cd, and Pb in soils using the free ion approach

The free ion approach quantifies the toxic effects of cationic metals on soil biota as a function of chemistry. The approach is here extended to calculate the general relationship among toxic effects as the Fraction Affected (FA), soil solution pH, and soil organic matter content (SOM) for Cu, Zn, Cd, and Pb within toxicity data sets from literature. The dependence of FA on SOM is strong, with the FA decreasing as the SOM increases. The dependence of FA upon pH varies; Cd and Zn show strong dependences while for Cu and Pb dependences are weaker. The FA usually decreases as the soil pH increases. When the free ion approach is applied, risks across soils due to different metals can be compared using the FA. The free ion approach may also be applied to probabilistic risk assessment. Risk values, using the joint probability curve approach, were derived for Pb using two field soil data sets. One data set, with higher SOM than that of the Pb toxicity data set, gave a lower risk with the free ion approach than that when the soil chemistry was not considered. The other data set had lower SOM than that of the toxicity data set and gave a higher risk with the free ion approach. Since literature toxicity tests are biased toward low SOM soils of circumneutral pH, using such data to perform classical risk assessment for soils of differing chemical composition can lead to misestimation of risk due to neglecting soil chemistry, especially in soils with extreme pH and/or SOM.     

The nature of soil organic matter affects sorption of pesticides. 1. Relationships with carbon chemistry as determined by 13C CPMAS NMR spectroscopy

The structural composition of soil organic matter (SOM) was determined in twenty-seven soils with different vegetation from several ecological zones of Australia and Pakistan using solid-state CPMAS 13C NMR. The SOM was characterized using carbon types derived from the NMR spectra. Relationships were determined between Koc (sorption per unit organic C) of carbaryl(1-naphthylmethylcarbamate) and phosalone (S-6-chloro-2,3-dihydro-2-oxobenzoxazol-3-ylmethyl O,O-diethyl phosphorodithioate) and the nature of organic matter in the soils. Substantial variations were revealed in the structural composition of organic matter in the soils studied. The variations in Koc values of the pesticides observed for the soils could be explained only when variations in the aromatic components of SOM were taken into consideration. The highly significant positive correlations of aromaticity of SOM and Koc values of carbaryl and phosalone revealed that the aromatic component of SOM is a good predictor of a soil's ability to bind such nonionic pesticides.     

Understanding the effects of soil characteristics on phytotoxicity and bioavailability of nickel using speciation models

Acidity (pH) has been realized to be the most important soil characteristic that modulates bioavailability of heavy metals by affecting both the chemical speciation of metals in soil and the metal binding to the active sites on biota. In this work, we show that besides soil pH, metal bioavailability also depends to a certain extent on the type of soil. A better understanding of the role of soil type in regulating metal availability can be achieved with the analysis of soil composition and with calculations using chemical speciation models. Results of pot experiments, in which three different soils were spiked with nickel, show that the EC50 of total nickel in decreasing the biomass production of oats varies widely (0.7-22.5 mmol kg(-1) soil, more than 30 times). pH (4.7-7.0) is the most important factor, explaining up to a factor of 14 difference of nickel bioavailability in the soils. The remaining variation is caused by other differences in soil composition (soil type). The bioavailability and toxicity of nickel in the organic matter-rich soil studied is less than half of that in the sandy and clay soil studied at a similar pH. The chemical calculations using a multi-surface speciation model show that soil organic matter binds Ni much stronger than clay silicates and iron (hydr)oxides within the acidic pH range, which supports the experimental findings. In all three soils, the EC50 of Ni expressed in terms of Ni in 0.01 M CaCl2 soil extraction is rather stable (24-58 microM), suggesting the possibility to use this extraction as an estimation of metal availability in soil.     

Sequestration and remobilization of radioiodine (129I) by soil organic matter and possible consequences of the remedial action at Savannah River Site

In order to investigate the distributions and speciation of (129)I (and (127)I) in a contaminated F-Area groundwater plume of the Savannah River Site that cannot be explained by simple transport models, soil resuspension experiments simulating surface runoff or stormflow and erosion events were conducted. Results showed that 72-77% of the newly introduced I(-) or IO(3)(-) were irreversibly sequestered into the organic-rich riparian soil, while the rest was transformed by the soil into colloidal and truly dissolved organo-iodine, resulting in (129)I remobilization from the soil greatly exceeding the 1 pCi/L drinking water permit. This contradicts the conventional view that only considers I(-) or IO(3)(-) as the mobile forms. Laboratory iodination experiments indicate that iodine likely covalently binds to aromatic structures of the soil organic matter (SOM). Under very acidic conditions, abiotic iodination of SOM was predominant, whereas under less acidic conditions (pH ≥5), microbial enzymatically assisted iodination of SOM was predominant. The organic-rich soil in the vadose zone of F-Area thus acts primarily as a "sink," but may also behave as a potentially important vector for mobile radioiodine in an on-off carrying mechanism. Generally the riparian zone provides as a natural attenuation zone that greatly reduces radioiodine release.     

Solid-solution partitioning of Cd, Cu, Ni, Pb, and Zn in the organic horizons of a forest soil

We report the solid-liquid partitioning of Cd, Cu, Ni, Pb, and Zn in 60 organic horizon samples of forest soils from the Hermine Watershed (St-Hippolyte, PQ, Canada). The mean Kd values are respectively 1132, 966, 802, 3337 and 561. Comparison of those Kd coefficients to published compilation values show that the Kd values are lower in acidic organic soil horizons relative to the overall mean Kd values compiled for mineral soils. But, once normalized to a mean pH of 4.4, the Kd values in organic soil horizons demonstrate the high sorption affinity of organic matter, which is either as good as or up to 30 times higher than mineral soil materials for sorbing trace metals. Regression analysis shows that, within our data set, pH and total metal contents are not consistent predictors of metal partitioning. Indeed, metal sorption by the solid phase must be studied in relation to complexation by dissolved organic ligands, and both processes may sometime counteract one another.     

Leaching of heavy metals from contaminated soils: an experimental and modeling study

In this paper, we characterize the leaching of heavy metals (Ni, Cu, Zn, Cd, and Pb) from eight contaminated soils over a wide range of pH (pH 0.4-12) using an original approach based on batch pH-static leaching experiments in combination with selective chemical extractions and geochemical modeling. The leached concentrations of the heavy metals are generally much lower than the total concentrations and show a strong pH dependency, resulting in "V-shaped" leaching curves with orders of magnitude changes in solution concentrations. The "multisurface" model used incorporates adsorption to dissolved and solid organic matter (NICA-Donnan), iron/aluminum (hydr)oxide (generalized two-layer model) and clay (Donnan model). These models were applied without modifications, and only the standard set of binding constants and parameters was used (i.e., without any fitting). The model predictions of heavy metal leaching are generally adequate and sometimes excellent. Results from speciation calculations are consistent with the well-recognized importance of organic matter as the dominant reactive solid phase in soils. The observed differences between soils with respect to element speciation in the solid phase correspond to the relative amounts of the reactive surfaces present in the soils. In the solution phase, complexes with dissolved organic matter (DOM) are predominant over most of the pH range. Free metal ions (Me2+) are generally the dominant species below pH 4. The combination of the experimental and modeling approach as used in this study is shown to be promising because it leads to a more fundamental understanding of the pH-dependent leaching processes in soils. The "multisurface" modeling approach, with the selected sorption models, is shown to be able to adequately predict the leaching of heavy metals from contaminated soils over a wide range of conditions, without any fitting of parameters.     

Deriving soil critical limits for Cu, Zn, Cd, and Pb: a method based on free ion concentrations

We present a method to calculate critical limits of cationic heavy metals accounting for variations in soil chemistry. We assume the free metal ion concentration (Mfree) to be the most appropriate indicator of toxicity, combined with a protective effect of soil cations (e.g., H+, Ca2+). Because soil metal cations tend to covary with pH, the concentration of Mfree exerting a given level of toxic effect (Mfree,toxic) can be expressed as a function of pH alone. We use linear regression equations to derive Mfree,toxic in toxicity experiments from soil pH, organic matter content, and endpoint soil metal. Chronic toxicity data from the literature, for plants, invertebrates, microbial processes, and fungi are interpreted in terms of an average log Mfree,toxic together with distributions of species sensitivity. This leads to critical limit functions to protect 95% of species, of the form log Mfree,CRIT = (pH + gamma. Appreciable effects of soil pH upon log Mfree,CRIT are found, with alpha = -1.21 (Cu), -0.34 (Zn), -0.43 (Cd), and -0.83 (Pb). Critical limit functions in terms of the geochemically active soil metal (Msoil,CRIT), that pool of metal which controls the free ion concentration, have also been derived, with soil pH and organic matter content as variables. The pH effect on Msoil,CRIT is relatively small, with slopes of 0.05 (Cu), 0.19 (Zn), 0.16 (Cd), and 0.20 (Pb), since the effect of pH on Mfree,CRIT is countered by the variation of Mfree with pH.     

Copper dynamics and impact on microbial communities in soils of variable organic status

The effect of soil organic status on copper impact was investigated by means of a microcosm study carried out on a vineyard soil that had been amended with varying types of organic matter during a previous long-term field experiment. Soil microcosms were contaminated at 250 mg Cu kg(-1) and incubated for 35 days. Copper distribution and dynamics were assessed in the solid matrix by a sequential extraction procedure and in the soil solution by measuring total and free exchangeable copper concentrations. Copper bioavailability was also measured with a whole-cell biosensor. Modifications of microbial communities were assessed by means of biomass-C measurements and characterization of genetic structure using ARISA (automated-ribosomal-intergenic-spacer-analysis). The results showed that copper distribution, speciation, and bioavailability are strongly different between organically amended and nonamended soils. Surprisingly, in solution, bioavailable copper correlated with total copper but not with free copper. Similarly the observed differential copper impact on micro-organisms suggested that organic matter controlled copper toxicity. Bacterial-ARISA modifications also correlated with the estimated metal bioavailability and corresponded to the enrichment of the Actinobacteria. Contrarily, biomass-C and fungal-ARISA measurements did not relate trivially to copper speciation and bioavailability, suggesting that the specific composition of the indigenous-soil communities controls its sensitivity to this metal.