On glyphosate-kaolinite surface interactions. A molecular dynamic study.

Glyphosate is an important and widely used herbicide, its environmental behaviour being of scientific and public interest. Computational models of clay minerals and their interactions with small organic molecules are valuable in studying adsorption processes at an atomistic resolution. We analysed the adsorption process of glyphosate on kaolinite, a clay mineral with a high abundance in several types of soils (e.g., of subtropical or tropical origin), in terms of the adsorption strength. The molecular interactions are characterized by monitoring the occurrence of hydrogen bonds, the orientation of the molecular dipole relative to the interface and the interaction energy. Two different ionic forms of glyphosate were considered: neutral and anionic (-1). It was shown that the main mechanism of the binding of both glyphosate forms to the aluminol surface of kaolinite is through multiple hydrogen bonds. The standard free energy of adsorption of neutral glyphosate from water solution to the basal octahedral surface of kaolinite was computed at -5 kJ mol-1, whereas for the anionic form this quantity amounted to -14 kJ mol-1. Our finding showed that kaolinite has an important contribution to overall adsorption capacity of soils for glyphosate, specifically in its anionic form. HIGHLIGHTS: The adsorption free energy of glyphosate on a kaolinite surface is quantifiedInteractions are computed by quantum mechanics and by classical force fieldMolecular interactions are characterized in terms of hydrogen bonds and orientationsThe effect of polarization of the medium on the calculations is analysed.

Molecular modelling of sorption processes of a range of diverse small organic molecules in Leonardite humic acid.

Soil organic matter (SOM) is abundant in the environment and plays an important role in several biogeochemical processes, including microbial activity, soil aggregation, plant growth and carbon storage. One of its key functions is the retention and release of various chemical compounds, primarily governed by the sorption process, which strongly affects the environmental fate of nutrients and pollutants. Sorption largely depends on the composition of SOM, as well as its structure, dynamics and the thermodynamic conditions. Although several approaches are available, experimental characterization of sorption mechanisms is not easy. Computational models for predicting sorption coefficients often require a wealth of experimental data for training and are only applicable to compounds and conditions related to the training dataset. Here, we use molecular dynamics (MD) simulations to study the sorption of a range of small organic compounds. As a model SOM system we use the standard Leonardite humic acid (LHA) sample, which physicochemical properties have recently been characterized computationally in detail. This model allowed us to estimate sorption propensities of the systems at two different hydration levels (water activities close to 0 and 1), showing a remarkable correlation with experimental data. Importantly, this molecular modelling approach based on perturbation free-energy calculations is rigorously derived from statistical thermodynamics and requires no experimental sorption data for training. It is therefore in principle applicable to any SOM model or thermodynamic condition. Moreover, the power of MD simulations to provide high-resolution insight into atomistic and molecular interactions was employed to explore how sorbate molecules associate with the LHA matrix and which contacts they form. The heteroatoms of both sorbate and sorbent play an important role and water molecules are identified as further key players in facilitating the sorption process. HIGHLIGHTS: Modelling of the sorption processes in soil organic matter at atomistic level.Rigorous, physics-based approach applicable to a range of SOM systems and conditions.Remarkable level of matching with experimental data with additional insight into the molecular mechanism.Interactions between the sorbate and local environment strongly affects the sorption process.

Polarization Effects in Simulations of Kaolinite-Water Interfaces.

Computational models of clay minerals and their interactions with the surrounding medium are highly valuable to study adsorption processes at an atomistic resolution, which may be relevant in different areas such as chromatography, environmental chemistry, and so forth. In this work, we analyzed the effect of the treatment of long-range interactions on the polarization of kaolinite-water interfacial systems in terms of structural, electric and dynamic properties, and hydrogen bonds. When using conventional three-dimensional (3D) Ewald summation, water molecules were more structured on the alumina interface of the kaolinite compared to simulations, in which the periodicity perpendicular to the plane was effectively removed. By applying an external electric field to the latter simulations, we were able to reproduce results obtained with the 3D Ewald summation. We proved that the induced polarization promotes adsorption processes of polar and charged species from aqueous solutions using a glyphosate molecule (widely used herbicide) as a particular example. The polarization phenomena raised from the simulation setup should be accounted for carefully in simulations of adsorption processes, which involve periodic images of polar interfaces. An inappropriate treatment of long-range electrostatic interactions can easily lead to artifacts and/or erroneous results.

Molecular Dynamics Simulations of the Standard Leonardite Humic Acid: Microscopic Analysis of the Structure and Dynamics.

Humic substances (HS) are abundant in the environment and play an important role in a number of biogeochemical processes including microbial activity, soil aggregation, plant growth, the retention and release of nutrients, the environmental fate of pollutants, and carbon storage. They are flexible, relatively small molecules forming supramolecular structures through weak interactions. Despite the great importance of understanding their behavior at the atomic level, computational modeling, a premier high-resolution technique providing great level of detail, has been surprisingly little-employed to study humic substances. Here, we use the recently developed Vienna Soil Organic-Matter Modeler to create representative models of a real HS sample, the standard Leonardite humic acid. Molecular dynamics simulations were used to probe the structure and dynamics of the system at a range of hydration levels. The studied systems were characterized in terms of their physicochemical properties, including density, dielectric properties, hydrogen bonding, etc. Moreover, the strength of sorption was estimated for three small organic compounds: benzaldehyde, propan-2-ol, and acetone. Strikingly, the HS models were validated against experimental data showing a remarkable agreement with calculated properties. Finally, we make the equilibrated models of the standard Leonardite humic acid, together with corresponding force-field parameters, available at the Vienna Soil Organic-Matter Modeler.

Non-destructive soil amendment application techniques on heavy metal-contaminated grassland: Success and long-term immobilising efficiency.

Extensive contamination of grassland with cadmium (Cd), lead (Pb) and zinc (Zn) is a typical problem close to Pb/Zn smelter sites. The entry of Cd or Pb into the food chain is very likely, as are toxicity effects of Zn in plants. Previous promising results from pot and field experiments showed the high potential of using amendments for immobilisation to reduce metal input into the food chain via crops grown on smelter-contaminated soils at Arnoldstein (Austria) (Friesl et al., 2006). The aim of this study was to find a practical solution for large-scale contaminations in hilly regions that avoids erosion. Field application of amendments without destroying the vegetation cover (grassland) involved two approaches: (a) slurrying (Slu) the amendments into cut gaps in the vegetation cover and (b) injecting (Inj) the amendments through the vegetation cover. Here, we investigate the immobilising and long-term efficiency of treatments [gravel sludge (2.5%) + red mud (0.5%) (GS + RM)]. Risk assessment was based on soil, plant and water samples taken over a period of 10 years. Ammonium-nitrate-extractable Cd was reduced up to 50%, Pb up to 90%, and Zn over 90%. Plant uptake into the grass mixture and narrow leaf plantain was significantly reduced for Cd, Pb, and Zn. Harvesting early in vegetation period can further reduce uptake and meet the threshold for fodder crops. The reduction of these elements in the seepage water in 24 samplings within these 10 years reached 40%, 45% and 50%, respectively. Immobilisation increased microbial biomass and decreased human bioaccessibility for Pb. Our investigation of the long-term efficiency of GS + RM in all treatments shows that the Slu and Inj amendment application techniques have promising potential as a realistic and practical method for extensively contaminated hilly land. Slurrying performed best. We conclude that grassland remediation methods involving tillage are counterproductive from the viewpoint of bioaccessibility and soil protection and therefore should be avoided.

Decomposition of beech (Fagus sylvatica) and pine (Pinus nigra) litter along an Alpine elevation gradient: Decay and nutrient release.

Litter decomposition is an important process for cycling of nutrients in terrestrial ecosystems. The objective of this study was to evaluate direct and indirect effects of climate on litter decomposition along an altitudinal gradient in a temperate Alpine region. Foliar litter of European beech (Fagus sylvatica) and Black pine (Pinus nigra) was incubated in litterbags during two years in the Hochschwab massif of the Northern Limestone Alps of Austria. Eight incubation sites were selected following an altitudinal/climatic transect from 1900 to 900 m asl. The average remaining mass after two years of decomposition amounted to 54% (beech) and 50% (pine). Net release of N, P, Na, Al, Fe and Mn was higher in pine than in beech litter due to high immobilization (retention) rates of beech litter. However, pine litter retained more Ca than beech litter. Altitude retarded decay (mass loss and associated C release) in beech litter during the first year only but had a longer lasting effect on decaying pine litter. Altitude comprises a suite of highly auto-correlated characteristics (climate, vegetation, litter, soil chemistry, soil microbiology, snow cover) that influence litter decomposition. Hence, decay and nutrient release of incubated litter is difficult to predict by altitude, except during the early stage of decomposition, which seemed to be controlled by climate. Reciprocal litter transplant along the elevation gradient yielded even relatively higher decay of pine litter on beech forest sites after a two-year adaptation period of the microbial community.

Lignin decomposition along an Alpine elevation gradient in relation to physicochemical and soil microbial parameters.

Lignin is an aromatic plant compound that decomposes more slowly than other organic matter compounds; however, it was recently shown that lignin could decompose as fast as litter bulk carbon in minerals soils. In alpine Histosols, where organic matter dynamics is largely unaffected by mineral constituents, lignin may be an important part of soil organic matter (SOM). These soils are expected to experience alterations in temperature and/or physicochemical parameters as a result of global climate change. The effect of these changes on lignin dynamics remains to be examined and the importance of lignin as SOM compound in these soils evaluated. Here, we investigated the decomposition of individual lignin phenols of maize litter incubated for 2 years in-situ in Histosols on an Alpine elevation gradient (900, 1300, and 1900 m above sea level); to this end, we used the cupric oxide oxidation method and determined the phenols' (13) C signature. Maize lignin decomposed faster than bulk maize carbon in the first year (86 vs. 78% decomposed); however, after the second year, lignin and bulk C decomposition did not differ significantly. Lignin mass loss did not correlate with soil temperature after the first year, and even correlated negatively at the end of the second year. Lignin mass loss also correlated negatively with the remaining maize N at the end of the second year, and we interpreted this result as a possible negative influence of nitrogen on lignin degradation, although other factors (notably the depletion of easily degradable carbon sources) may also have played a role at this stage of decomposition. Microbial community composition did not correlate with lignin mass loss, but it did so with the lignin degradation indicators (Ac/Al)s and S/V after 2 years of decomposition. Progressing substrate decomposition toward the final stages thus appears to be linked with microbial community differentiation.

The multiradical character of one- and two-dimensional graphene nanoribbons.

When is an acene stable? The pronounced multiradical character of graphene nanoribbons of different size and shape was investigated with high-level multireference methods. Quantitative information based on the number of effectively unpaired electrons leads to specific estimates of the chemical stability of graphene nanostructures.

Chemical imaging reveals environmental risk of minor tungsten and lead shotgun pellet constituents during weathering in soil.

Tungsten (W)-based shots are considered more environmentally safe than lead (Pb)-based shots, but knowledge about the W-shot fate in the soil environment is still limited, especially in terms of minor constituents such as iron, copper, and nickel (Ni). Contaminant behaviour in soil strongly depends on pH; in turn, the corrosion of metal composites may affect the pH locally. The aim of this study was to compare Pb- and W-shot weathering dynamics in soil (silt loam, pH 6.3) and reveal the interplay of shot weathering-induced pH-changes on the mobility of elements using in situ chemical imaging (Diffusive gradients in thin films for labile elements, planar optodes for soil pH) and batch incubation experiments over time (16 months). Despite our expectation to find acidification due to W oxidation, we observed a pH increase by 0.2 units in extracted soil solutions and by 0.6 units in the soil around W-shots as Ni dissolved from the binder phase of the shot. After 10 weeks, release of labile Ni was 3-times higher compared to W despite the low Ni content in the shot (7 %, m/m). Pb-shot oxidation increased soil solution pH by 0.5 units which likely supported mobility of Pb-shot-derived antimony (Sb). Steep gradients of labile W and Pb and soil solution concentrations <0.8 µmol L-1 indicated that transfer from shot to soil was low. Contrastingly, labile Ni and Sb were found up to ~4 mm from the shot surface and in higher soil solution concentrations as suggested by the shot constitution, indicating higher mobility of minor as compared to major shot constituents. After 16 months, 36 % of total Ni were dissolved in the soil solution highlighting the environmental relevance of minor shot constituents in Pb-shot alternatives after short term weathering in soil.

Adsorption of the hydrophobic organic pollutant hexachlorobenzene to phyllosilicate minerals.

Hexachlorobenzene (HCB), a representative of hydrophobic organic chemicals (HOC), belongs to the group of persistent organic pollutants (POPs) that can have harmful effects on humans and other biota. Sorption processes in soils and sediments largely determine the fate of HCB and the risks arising from the compound in the environment. In this context, especially HOC-organic matter interactions are intensively studied, whereas knowledge of HOC adsorption to mineral phases (e.g., clay minerals) is comparatively limited. In this work, we performed batch adsorption experiments of HCB on a set of twelve phyllosilicate mineral sorbents that comprised several smectites, kaolinite, hectorite, chlorite, vermiculite, and illite. The effect of charge and size of exchangeable cations on HCB adsorption was studied using the source clay montmorillonite STx-1b after treatment with nine types of alkali (M+: Li, K, Na, Rb, Cs) and alkaline earth metal cations (M2+: Mg, Ca, Sr, Ba). Molecular modeling simulations based on density functional theory (DFT) calculations to reveal the effect of different cations on the adsorption energy in a selected HCB-clay mineral system accompanied this study. Results for HCB adsorption to minerals showed a large variation of solid-liquid adsorption constants Kd over four orders of magnitude (log Kd 0.9-3.3). Experiments with cation-modified montmorillonite resulted in increasing HCB adsorption with decreasing hydrated radii of exchangeable cations (log Kd 1.3-3.8 for M+ and 1.3-1.4 for M2+). DFT calculations predicted (gas phase) adsorption energies (- 76 to - 24 kJ mol-1 for M+ and - 96 to - 71 kJ mol-1 for M2+) showing a good correlation with Kd values for M2+-modified montmorillonite, whereas a discrepancy was observed for M+-modified montmorillonite. Supported by further calculations, this indicated that the solvent effect plays a relevant role in the adsorption process. Our results provide insight into the influence of minerals on HOC adsorption using HCB as an example and support the relevance of minerals for the environmental fate of HOCs such as for long-term source/sink phenomena in soils and sediments.

The stability of the acetic acid dimer in microhydrated environments and in aqueous solution.

The thermodynamic stability of the acetic acid dimer conformers in microhydrated environments and in aqueous solution was studied by means of molecular dynamics simulations using the density functional based tight binding (DFTB) method. To confirm the reliability of this method for the system studied, density functional theory (DFT) and second order Møller-Plesset perturbation theory (MP2) calculations were performed for comparison. Classical optimized potentials for liquid simulations (OPLS) force field dynamics was used as well. One focus of this work was laid on the study of the capabilities of water molecules to break the hydrogen bonds of the acetic acid dimer. The barrier for insertion of one water molecule into the most stable cyclic dimer is found to lie between 3.25 and 4.8 kcal mol(-1) for the quantum mechanical methods, but only at 1.2 kcal mol(-1) for OPLS. Starting from different acetic acid dimer structures optimized in gas phase, DFTB dynamics simulations give a different picture of the stability in the microhydrated environment (4 to 12 water molecules) as compared to aqueous solution. In the former case all conformers are converted to the hydrated cyclic dimer, which remains stable over the entire simulation time of 1 ns. These results demonstrate that the considered microhydrated environment is not sufficient to dissociate the acetic acid dimer. In aqueous solution, however, the DFTB dynamics shows dissociation of all dimer structures (or processes leading thereto) starting after about 50 ps, demonstrating the capability of the water environment to break up the relatively strong hydrogen bridges. The OPLS dynamics in the aqueous environment shows--in contrast to the DFTB results--immediate dissociation, but a similar long-term behavior.

Characterization of slow pyrolysis biochars: effects of feedstocks and pyrolysis temperature on biochar properties.

Biochars are increasingly used as soil amendment and for C sequestration in soils. The influence of feedstock differences and pyrolysis temperature on biochar characteristics has been widely studied. However, there is a lack of knowledge about the formation of potentially toxic compounds that remain in the biochars after pyrolysis. We investigated biochars from three feedstocks (wheat straw, poplar wood, and spruce wood) that were slowly pyrolyzed at 400, 460, and 525°C for 5 h (straw) and 10 h (woodchips), respectively. We characterized the biochars' pH, electrical conductivity, elemental composition (by dry combustion and X-ray fluorescence), surface area (by N adsorption), water-extractable major elements, and cation exchange capacity (CEC). We further conducted differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), and X-ray diffractometry to obtain information on the biochars' molecular characteristics and mineralogical composition. We investigated trace metal content, total polycyclic aromatic hydrocarbon (PAH) content, and PAH composition in the biochars. The highest salt (4.92 mS cm) and ash (12.7%) contents were found in straw-derived biochars. The H/C ratios of biochars with highest treatment temperature (HTT) 525°C were 0.46 to 0.40. Surface areas were low but increased (1.8-56 m g) with increasing HTT, whereas CEC decreased (162-52 mmol kg) with increasing HTT. The results of DSC and FTIR suggested a loss of labile, aliphatic compounds during pyrolysis and the formation of more recalcitrant, aromatic constituents. X-ray diffractometry patterns indicated a mineralogical restructuring of biochars with increasing HTT. Water-extractable major and trace elements varied considerably with feedstock composition, with trace elements also affected by HTT. Total PAH contents (sum of EPA 16 PAHs) were highly variable with values up to 33.7 mg kg; irrespective of feedstock type, the composition of PAHs showed increasing dominance of naphthalene with increasing HTT. The results demonstrate that biochars are highly heterogeneous materials that, depending on feedstock and HTT, may be suitable for soil application by contributing to the nutrient status and adding recalcitrant C to the soil but also potentially pose ecotoxicological challenges.

14C-labeled organic amendments: Characterization in different particle size fractions and humic acids in a long-term field experiment.

Knowledge about the stabilization of organic matter input to soil is essential for understanding the influence of different agricultural practices on turnover characteristics in agricultural soil systems. In this study, soil samples from a long-term field experiment were separated into silt- and clay-sized particles. In 1967, 14C labeled farmyard manure was applied to three different cropping systems: crop rotation, monoculture and permanent bare fallow. Humic acids (HAs) were extracted from silt- and clay-sized fractions and characterized using photometry, mid-infrared and fluorescence spectroscopy. Remaining 14C was determined in size fractions as well as in their extracted HAs. Yields of carbon and remaining 14C in HAs from silt-sized particles and Corg in clay-sized particles decreased significantly in the order: crop rotation > monoculture â‰« bare fallow. Thus, crop rotation not only had the largest overall C-pool in the experiment, but it also best stabilized the added manure. Mid-infrared spectroscopy could distinguish between HAs from different particle size soil fractions. With spectroscopic methods significant differences between the cropping systems were detectable in fewer cases compared to quantitative results of HAs (yields, 14C, Corg and Nt). The trends, however, pointed towards increased humification of HAs from bare fallow systems compared to crop rotation and monoculture as well as of HAs from clay-sized particles compared to silt-sized particles. Our study clearly shows that the largest differences were observed between bare fallow on one hand and monoculture and crop rotation on the other.

Differences in sorption behavior of the herbicide 4-chloro-2-methylphenoxyacetic acid on artificial soils as a function of soil pre-aging.

PURPOSE: The sorption behavior of the herbicide 4-chloro-2-methylphenoxyacetic acid (MCPA) to three different artificial soil mixtures was investigated. Artificial soils serve as model systems for improving understanding of sorption phenomena. MATERIALS AND METHODS: The soils consisted of quartz, ferrihydrite, illite, montmorillonite, and charcoal. In a previous study, several selected mixtures had been inoculated with organic matter, and microbial aging (incubation) had been performed for different periods of time (3, 12, and 18 months) before conducting the sorption experiments. The effect of this pre-incubation time on the sorption behavior was determined. Interaction of MCPA with soil surfaces was monitored by aqueous phase sorption experiments, using high-performance liquid chromatography/ultraviolet and in selected cases Fourier-transformed infrared spectroscopy. RESULTS AND DISCUSSION: The sorption behavior showed large differences between differently aged soils; Freundlich and linear sorption model fits (with sorption constants Kf , 1/n exponents, and Kd values, respectively) were given for pH = 3 and the unbuffered pH of ∼7. The largest extent of sorption from diluted solutions was found on the surfaces with a pre-incubation time of 3 months. Sorption increased at acidic pH values. CONCLUSIONS: Regarding the influence of aging of artificial soils, the following conclusions were drawn: young artificial soils exhibit stronger sorption at lower concentrations, with a larger Kf value than aged soils. A correlation with organic carbon content was not confirmed. Thus, the sorption characteristics of the soils are more influenced by the aging of the organic carbon than by the organic carbon content itself.

Molecular dynamics simulations of water molecule-bridges in polar domains of humic acids.

The stabilizing effect of water molecule bridges on polar regions in humic substances (HSs) has been investigated by means of molecular dynamics (MD) simulations. The purpose of these investigations was to show the effect of water molecular bridges (WAMB) for cross-linking distant locations of hydrophilic groups. For this purpose, a tetramer of undecanoid fatty acids connected to a network of water molecules has been constructed, which serve as a model for spatially fixed aliphatic chains in HSs terminated by a polar (carboxyl) group. The effect of environmental polarity has been investigated by using solvents of low and medium polarity in force-field MD. A nonpolar environment simulated by n-hexane was chosen to mimic the stability of WAMB in a hydrophilic hotspot surrounded by a nonpolar environment, while the more polar acetonitrile environment was chosen to simulate a more even distribution of polarity around the carboxylic groups and the water molecules. The dynamics simulations show that the rigidity of the oligomer chains is significantly enhanced as soon as the water cluster is large enough to comprise all four carboxyl groups. Increasing the temperature leads to evaporization processes which destabilize the rigidity of the tetramer-water cluster. Embedding it into the nonpolar environment introduces a pronounced cage effect which significantly impedes removal of water molecules from the cluster region. On the other hand, a polar environment facilitates their diffusion from the polar region. One important consequence of these simulations is that although the local water network is the stabilizing factor for the organic matter matrix, the degree of stabilization is additionally affected by the presence of nonpolar surroundings.

Heavy metal contents, mobility and origin in agricultural topsoils of the Galápagos Islands.

While the Galápagos Islands have been renowned for their unique flora and fauna since the time of Charles Darwin, the soils of the isolated island chain have been mostly overlooked and little information on their heavy metal contents is available. The aim of this study was therefore to examine the total heavy metal (Cd, Co, Cr, Cu, Ni, Pb, U, Zn) contents of soils from the agricultural areas on islands Isabela, Santa Cruz and San Cristóbal, and identify trends with duration of exposure to weathering processes. Additionally, the mobility of these elements was assessed using ammonium nitrate extraction. In general, levels of Cd, Co, Cr, Cu, Ni and Zn were high compared to other world locations, while Pb levels were low and U levels were similar. Ni, Co, Cr, and to a lesser extent Pb and U tended to accumulate with increasing weathering duration. Soil concentrations of Cd, Zn, Cu, and possibly Pb and U, may have been influenced by use of agrochemicals, particularly on Santa Cruz Island. Mobility of Cd displayed an increasing trend with soil age, while Ni mobility decreased. Many soils had total contents of Cd, Co, Cr, Cu, Ni and Zn above threshold values indicating possible ecological or health risks. Systematic examination of trace element contents in soils from pristine national park areas would further assist in the delineation of background levels and the development of soil quality standards to ensure crop quality, animal and human health on this unique island chain.

Cadmium retention and microbial response in volcanic soils along gradients of soil age and climate on the Galápagos Islands.

The behavior of trace metals may vary strongly in the course of volcanic soil development. Cadmium retention in soils is specifically important for some Galápagos islands where agriculture is leading to anthropogenic Cd contamination. To assess the influence of soil development factors on soil Cd retention and toxicity, we performed Cd sorption-desorption experiments with volcanic topsoils from the Galápagos Islands sampled along gradients of (a) substrate age (chronosequence, 1.5-1,070 ka) and (b) climate (elevation sequence, 47-866 m asl) ranging from arid lowland areas to humid highland areas. Additionally, the effects of Cd toxicity on the soil microbial community composition were evaluated for two soils of the chronosequence. In young volcanic soils, the sorption capacity was very high but decreased rapidly with soil age and increasing elevation. These trends were coupled with decreases in soil weathering indicators (e.g., electrical conductivity, pH, and effective cation exchange capacity) as well as changes in soil mineralogy. Cadmium addition did not influence total phospholipid fatty acids and basal respiration in most soils. However, with increasing Cd concentration, a pronounced reduction in the Gram-negative/Gram-positive bacteria ratio (from 0.32 to 0.12) occurred in an old, highly weathered soil with low Cd retention capacity. Our results show that up to 60% of added Cd was only weakly sorbed in old volcanic soils. As a consequence, the old volcanic soils of Galápagos bear the potential risk that the mobile Cd fraction is taken up by soil microorganisms, transferring this element into the food chain.

Thermodynamic stability of hydrogen-bonded systems in polar and nonpolar environments.

The thermodynamic properties of a selected set of benchmark hydrogen-bonded systems (acetic acid dimer and the complexes of acetic acid with acetamide and methanol) was studied with the goal of obtaining detailed information on solvent effects on the hydrogen-bonded interactions using water, chloroform, and n-heptane as representatives for a wide range in the dielectric constant. Solvent effects were investigated using both explicit and implicit solvation models. For the explicit description of the solvent, molecular dynamics and Monte Carlo simulations in the isothermal-isobaric (NpT) ensemble combined with the free energy perturbation technique were performed to determine solvation free energies. Within the implicit solvation approach, the polarizable continuum model and the conductor-like screening model were applied. Combination of gas phase results with the results obtained from the different solvation models through an appropriate thermodynamic cycle allows estimation of complexation free energies, enthalpies, and the respective entropic contributions in solution. Owing to the strong solvation effects of water the cyclic acetic acid dimer is not stable in aqueous solution. In less polar solvents the double hydrogen bond structure of the acetic acid dimer remains stable. This finding is in agreement with previous theoretical and experimental results. A similar trend as for the acetic acid dimer is also observed for the acetamide complex. The methanol complex was found to be thermodynamically unstable in gas phase as well as in any of the three solvents.

Vertical Redistribution of Soil Organic Carbon Pools After Twenty Years of Nitrogen Addition in Two Temperate Coniferous Forests.

Nitrogen (N) inputs from atmospheric deposition can increase soil organic carbon (SOC) storage in temperate and boreal forests, thereby mitigating the adverse effects of anthropogenic CO2 emissions on global climate. However, direct evidence of N-induced SOC sequestration from low-dose, long-term N addition experiments (that is, addition of < 50 kg N ha-1 y-1 for > 10 years) is scarce worldwide and virtually absent for European temperate forests. Here, we examine how tree growth, fine roots, physicochemical soil properties as well as pools of SOC and soil total N responded to 20 years of regular, low-dose N addition in two European coniferous forests in Switzerland and Denmark. At the Swiss site, the addition of 22 kg N ha-1 y-1 (or 1.3 times throughfall deposition) stimulated tree growth, but decreased soil pH and exchangeable calcium. At the Danish site, the addition of 35 kg N ha-1 y-1 (1.5 times throughfall deposition) impaired tree growth, increased fine root biomass and led to an accumulation of N in several belowground pools. At both sites, elevated N inputs increased SOC pools in the moderately decomposed organic horizons, but decreased them in the mineral topsoil. Hence, long-term N addition led to a vertical redistribution of SOC pools, whereas overall SOC storage within 30 cm depth was unaffected. Our results imply that an N-induced shift of SOC from older, mineral-associated pools to younger, unprotected pools might foster the vulnerability of SOC in temperate coniferous forest soils.

Impact of different tillage practices on molecular characteristics of humic acids in a long-term field experiment - an application of three different spectroscopic methods.

The present paper describes changes in soil organic carbon (SOC) and extractable humic acids (HAs) in a long-term field experiment with different tillage treatments (minimum tillage (MT), reduced tillage (RT) and conventional tillage (CT)). This field experiment is located in the east of Vienna in a Pannonian climate and it was started in 1988. The methodological approach included elemental analyses, FT-IR, 13C NMR and fluorimetric measurements. Both MT and RT revealed significant depth gradients of yields of extractable HAs. In CT no depth gradient was observable, neither for HA yields nor for observed molecular characteristics. This indicated a destruction of the gradient by mixing of the soil in CT. Especially MT showed an increase of aromatic moieties with depth, suggesting an increased humification of HAs in the lower soil layers. Gradients with similar trends were indicated for the carbonylic, the amidic and probably the hydroxylic groups in HAs extracted from MT and RT samples. The data revealed with FT-IR and solid-state 13C NMR spectroscopy were convincing, plausible and meaningful, the highly sensitive fluorescence spectroscopy was limited because of strong quenching by inner filter effects, compromising data reliability. However, the fluorescence results based on a defined HAs concentration (and comparing soils from the same site) were in line with results from the other methods (13C NMR and FT-IR). As a consequence, the influence of tillage treatments can be followed by absence or presence of depth gradients of the according molecular characteristics in extracted HAs.