Sophie Opfergelt

Distinct silicon and germanium pathways in the soil-plant system: Evidence from banana and horsetail

Plants strongly impact the continental silicon cycle by taking up Si and precipitating opal phytoliths which are recycled into the soil. Studying Ge incorporation, a chemical analog of Si, relative to Si may provide a useful tracer of Si pathways. However, Ge uptake and transport through plants and the impact on Ge/Si of phytoliths remain poorly understood. Here, we report Ge uptake and accumulation and Ge/Si fractionation in all plant parts and solutions from: (1) hydroponic banana, (2) in situ sampled banana, and (3) horsetails. We further combine these data with delta Si-29 from banana plants. Our data reconcile opposite conclusions drawn from previous studies on Ge uptake and pathways. No discrimination of Ge occurred at the root-solution interface. Banana and horsetails were shown to accumulate Ge in roots: a previous study provided evidence of low Ge/Si ratios in root phytoliths which contrasts with high bulk Ge/Si ratios in roots we report here. This suggests that Ge is organically trapped in roots. Consequently, shoots display lower Ge/Si ratios, without fractionation between shoot parts since Ge would follow transpiration stream as silicon, and is not discriminated between shoot parts. This contrasts with the two-step discrimination against heavy Si isotopes, at the root-solution interface, and then within the shoots. The soil composition (clays versus Fe oxides) has a leading role on the Ge/Si signatures of plants which may in turn impact on the Si and Ge fluxes to the global ocean.

Impact of Root-Induced Mobilization of Zinc on Stable Zn Isotope Variation in the Soil-Plant System.

Stable Zn isotopes are increasingly used to trace the source of metal pollution in the environment and to gain a better understanding of the biogeochemical cycle of Zn. In this work, we investigated the effect of plants on Zn isotope fractionation in the soil-plant system of the surface horizon of two Zn-rich Technosols (pH 6.73-7.51, total Zn concentration = 9470-56600 mg kg(-1)). In a column experiment, the presence of Agrostis capillaris L. significantly increased the mobilization of Zn from soil to leachate, predominantly as a result of root-induced soil acidification. The zinc isotope compositions of plants and leachates indicated that the Zn uptake by A. capillaris did not fractionate Zn isotopes as compared to the leachates. Within the plant, heavier Zn isotopes were preferentially retained in roots (Δ66Znroot - shoot=+0.24 to +0.40 ‰). More importantly, the Zn released in leachates due to root-induced mobilization was isotopically heavier than the Zn released in the absence of plants (Δ66Zn=+0.16 to +0.18 ‰). This indicates that the rhizosphere activity of A. capillaris mobilized Zn from another pool than the one that spontaneously releases Zn upon contact with the percolating solution. Mobilization of Zn by the roots might thus exert a stronger influence on the Zn isotope composition in the soil solution than the Zn uptake by the plant. This study highlights the key role of the rhizosphere activity in Zn release in soil and demonstrates that stable Zn isotopes provide a useful proxy for the detection of Zn mobilization in soil-plant systems.

The 1998 debris avalanche at Casita volcano, Nicaragua: Investigation of the role of hydrothermal smectite in promoting slope instability

Buildup of excess pore water pressure within highly fractured rocks is held responsible for the initiation of the disastrous 1998 debris avalanche at Casita volcano, Nicaragua. Here, we postulate that the presence of smectite clay ( up to 50 wt.%) in the hydrothermally-altered bedrock contributed to slope instability. Over decades or more, the clayey material probably (i) acted as an efficient barrier to water infiltration, (ii) progressively decreased shear-strength, and (iii) gradually destabilized the overlying rock mass. These effects are explained by the shrink-swell behavior of the clay-rich bedrock. During intense rainfall, formation of incipient weak failure surfaces in the superficial rock mass could have been favoured due to rapid alteration in the mechanical properties of smectite-containing clays deposited in fracture, joint and gouge interfaces.

Kinetically limited weathering at low denudation rates in semiarid climatic conditions.

Biogeochemical cycling within the Critical Zone depends on the interactions between minerals and fluids controlling chemical weathering and physical erosion rates. In this study, we explore the role of water availability in controlling soil chemical weathering in semiarid climatic conditions. Weathering rates and intensities were evaluated for nine soil profiles located on convex ridge crests of three mountain ranges in the Spanish Betic Cordillera. We combine a geochemical mass balance with 10Be cosmogenic nuclides to constrain chemical weathering intensities and long-term denudation rates. As such, this study presents new data on chemical weathering and 10Be-derived denudation for understudied semiarid climate systems. In the Betic Cordillera, chemical weathering intensities are relatively low (~5 to 30% of the total denudation of the soil) and negatively correlated with the magnitude of the water deficit in soils. Chemical mass losses are inversely related to denudation rates (14–109 mm/kyr) and positively to soil thickness (14–58 cm); these results are consistent with kinetic limitation of chemical weathering rates. A worldwide compilation of chemical weathering data suggests that soil water balance may regulate the coupling between chemical weathering and physical erosion by modulating soil solute fluxes. Therefore, future landscape evolution models that seek to link chemical weathering and physical erosion should include soil water flux as an essential driver of weathering.

The silicon isotope composition of Ethmodiscus rex laminated diatom mats from the tropical West Pacific: Implications for silicate cycling during the Last Glacial Maximum

The cause of massive blooms of Ethmodiscus rex laminated diatom mats (LDMs) in the eastern Philippine Sea (EPS) during the Last Glacial Maximum (LGM) remains uncertain. In order to better understand the mechanism of formation of E. rex LDMs from the perspective of dissolved silicon (DSi) utilization, we determined the silicon isotopic composition of single E. rex diatom frustules (δ30SiE. rex) from two sediment cores in the Parece Vela Basin of the EPS. In the study cores, δ30SiE. rex varies from −1.23‰ to −0.83‰ (average −1.04‰), a range that is atypical of marine diatom δ30Si and that corresponds to the lower limit of reported diatom δ30Si values of any age. A binary mixing model (upwelled silicon versus eolian silicon) accounting for silicon isotopic fractionation during DSi uptake by diatoms was constructed. The binary mixing model demonstrates that E. rex dominantly utilized DSi from eolian sources (i.e., Asian dust) with only minor contributions from upwelled seawater sources (i.e., advected from Subantarctic Mode Water, Antarctic Intermediate Water, or North Pacific Intermediate Water). E. rex utilized only ~24% of available DSi, indicating that surface waters of the EPS were eutrophic with respect to silicon during the LGM. Our results suggest that giant diatoms did not always use a buoyancy strategy to obtain nutrients from the deep nutrient pool, thus revising previously proposed models for the formation of E. rex LDMs.

Summit acid crater lakes and flank instability in composite volcanoes

Volcanic landslides, including flank and sector collapses, constitute a major hazard in many parts of the world. While composite volcanoes are innately unstable, the presence of a hydrothermal system maintained by a magmatic source at depth is recognized as a key factor increasing the risk of failure. This relates to the formation of hydrothermally altered rock masses within the core and upper flanks of the volcano which leads to heterogeneous distribution of rock strength properties and pore fluid pressures. Here an emphasis is placed on acid crater lakes perched high on active volcanoes. By acting as a trap for magmatic heat and gas flows, these lakes localize extreme acid attack on their surrounds, thereby creating a source of instability. We outline how acid crater lakes form in relation to magmatic hydrothermal systems hosted within composite volcanoes, and describe the associated hydrothermal alteration and its relationships to flank instability. The sustainability of a volcanic slope is partly governed by the degree of rock alteration, which in turn reflects the time-integrated flux of acidic gases (SO2 and HCl) released from the subsurface magmatic source. Transient or longer-term changes in pore fluid pressure linked to hydrothermal system activity also readily affect the slope stability of composite volcanoes. Such fluctuations can be initiated by both magmatic and external non-magmatic processes such as major rainfall events and regional seismicity. Kawah Ijen hyper-acid crater lake, Indonesia, is used as a case study to illustrate the cascade of effects that may ensue following slope rupture linked to hydrothermal alteration.

Understanding Root Uptake of Nutrients, Toxic and Polluting Elements in Hydroponic Culture

The understanding of plant uptake (nutrients, toxic and polluting elements) is crucial for the future food needs of humanity given the explosive growth of the world population and the anthropogenic pressure on the environment which significantly modify the homeostasis of the balanced global cycles. Rice and banana are of fundamental interest for development policy since they are two major foods for the world population. The understanding of the mechanisms and the optimal conditions of nutrient uptake by these plants is thus important to ensure biomass production. Furthermore, the transfer of toxic and polluting elements in the soil-plant system can influence the nutrient uptake and plant growth, and thus has strong agronomic consequences, in addition to the large environmental consequences.

Impacts of forest conversion and agriculture practices on water pathways in Southern Brazil

Division of Geography and Tourism, Department of Earth and Environmental Sciences, KU Leuven, Leuven, Belgium Department of Soil, Universidade Federal de Santa Maria, Rio Grande do Sul, Brazil Department of Agronomy, Universidade Federal do Rio Grande do Sul, Rio Grande do Sul, Brazil 4 Institute of Bio‐ and Geosciences, IBG‐3: Agrosphere, Forschungszentrum Jülich GmbH Jülich, Jülich, Germany Earth and Life Institute, Environmental Sciences, Université catholique de Louvain, Louvain, Belgium Correspondence Jérémy Robinet, Division of Geography and Tourism, Department of Earth and Environmental Sciences, KU Leuven, Leuven, Belgium. Email: jeremy.robinet@kuleuven.be

Land use change affects biogenic silica pool distribution in a subtropical soil toposequence

Abstract. Land use change (deforestation) has several negative consequences for the soil system. It is known to increase erosion rates, which affect the distribution of elements in soils. In this context, the crucial nutrient Si has received little attention, especially in a tropical context. Therefore, we studied the effect of land conversion and erosion intensity on the biogenic silica pools in a subtropical soil in the south of Brazil. Biogenic silica (BSi) was determined using a novel alkaline continuous extraction where Si ∕ Al ratios of the fractions extracted are used to distinguish BSi and other soluble fractions: Si ∕ Al > 5 for the biogenic AlkExSi (alkaline-extractable Si) and Si ∕ Al