Calcium isotope fractionation in alpine plants

In order to develop Ca isotopes as a tracer for biogeochemical Ca cycling in terrestrial environments and for Ca utilisation in plants, stable calcium isotope ratios were measured in various species of alpine plants, including woody species, grasses and herbs. Analysis of plant parts (root, stem, leaf and flower samples) provided information on Ca isotope fractionation within plants and seasonal sampling of leaves revealed temporal variation in leaf Ca isotopic composition. There was significant Ca isotope fractionation between soil and root tissue \Updelta^{44/42}\hbox{Ca}_{\rm root-soil} \approx -0.40\,\permille in all investigated species, whereas Ca isotope fractionation between roots and leaves was species dependent. Samples of leaf tissue collected throughout the growing season also highlighted species differences: Ca isotope ratios increased with leaf age in woody species but remained constant in herbs and grasses. The Ca isotope fractionation between roots and soils can be explained by a preferential binding of light Ca isotopes to root adsorption sites. The observed differences in whole plant Ca isotopic compositions both within and between species may be attributed to several potential factors including root cation exchange capacity, the presence of a woody stem, the presence of Ca oxalate, and the levels of mycorrhizal infection. Thus, the impact of plants on the Ca biogeochemical cycle in soils, and ultimately the Ca isotope signature of the weathering flux from terrestrial environments, will depend on the species present and the stage of vegetation successio

Porewater composition in clay rocks explored by advective displacement and squeezing experiments

Argillaceous rocks are foreseen in many countries as the potential hosts for nuclear waste repositories. The knowledge of the chemical composition of the free porewater in these formations is required for the understanding of the paleo-hydrogeological evolution, for the assessment of radionuclide solubilities and migration parameters and for assessing the long-term stability of the technical barrier system. High pressure squeezing and advective displacement are two methods that aim at direct sampling of this porewater fraction while minimizing experimental artefacts. Within the framework of a recent deep drilling campaign in Switzerland, targeting the Opalinus Clay as the designated host rock, a substantial dataset of porewater compositions was obtained by these two methods. It included 51 squeezing and 30 advective displacement experiments on drillcore samples from the Opalinus Clay and confining units distributed over 8 boreholes in 3 study areas. Porewater compositions obtained by either method reflect the geochemical characteristics of each siting region, such as different salinities and water types as well as depth gradients informing on the diffusive exchange with bounding aquifers. An in depth comparison of the Opalinus Clay porewater compositions obtained by both methods shows a high degree of consistency with regard to the ion ratios or mineral equilibria. The pH/pCO2 system was found to be prone to experimental artefacts, but applying a correction, a fairly consistent dataset was obtained. Porewaters acquired by squeezing exhibit systematically lower salinities by 10–40% when compared to those from advective displacement. It is concluded that this is due to the mobilization of a higher fraction of an anion depleted porewater, either due to the higher mobilization of water from the diffuse layer or due to the expulsion of water from interlayer (-like) pores. The comparison with a geochemical model indicates that the experimental data from both methods can be considered as proxies for in-situ major-ion porewater compositions. It also confirms the robustness of the geochemical model predicting porewaters of the Opalinus Clay and confining units. Differences between in-situ, sample storage and extraction temperatures need to be taken into account when interpreting the porewaters obtained by either method and modelling in-situ porewater compositions. Combining different laboratory and analytical methods for porewater investigations in clay rocks provides an added value as it enables a detailed assessment of natural heterogeneities and experimental uncertainties

On the distribution and speciation of arsenic in the soil-plant-system of a rice field in West-Bengal, India: A mu-synchrotron techniques based case study

Worldwide, West-Bengal is one of the areas most affected by elevated levels of arsenic in groundwater (50-3000 mu g/l). This groundwater does not only endanger humans owing to its use as drinking water. More and above that, irrigation of rice paddies consumes huge quantities of arsenic contaminated groundwater. Consequently, arsenic accumulates in soil and endangers the nutrition chain via arsenic uptake by plants. Rice is one of the staple foods in this region. Lately, there is a considerable intensification of research on the fate of arsenic in affected agricultural systems with most of them resorting to bulk analytical methods. However, so far, knowledge on the it-scale distribution of arsenic in soil and plants in such agricultural systems is rather limited. This case study combined mu-synchrotron studies on soil, rice root and rice grain from a rice paddy irrigated with groundwater containing about 519 mu g/L As. The investigation of a soil aggregate has shown that As is mainly associated with Fe and is not equally distributed over the whole aggregate but occurs in local enrichments of few tens mu m in size. In soil, As was mainly associated with Fe-(oxy)hydroxides. Rice root coatings consisted of a similar assemblage of arsenic bearing minerals. Furthermore the incorporation of soil matter in the coating could be shown. On mu m-scale, As concentrations in rice root coatings showed an inhomogeneous, patchy distribution (100-2400 mg/kg; median 500 mg/kg) and correlated with Fe Concentrations. Some small amounts of arsenic could also be detected in the interior of the root (3-60 mg/kg; median 21 mg/kg). In the rice grain, trace elements such as Zn and Cu were mainly enriched along the grain coating, while As in contrast showed the highest concentrations in the germ and some hot spots in the coating (up to 13 mg/kg). Thus, peeling of rice grain would remove some, but not all of the arsenic. (C) 2015 Elsevier Ltd. All rights reserved