Chemical and Biological Gradients along the Damma Glacier Soil Chronosequence, Switzerland

Soils are the product of a complex suite of chemical, biological, and physical processes. In spite of the importance of soils for society and for sustaining life on earth, our knowledge of soil formation rates and of the influence of biological activity on mineral weathering and geochemical cycles is still limited. In this paper we provide a description of the Damma Glacier Critical Zone Observatory and present a first synthesis of our multi disciplinary studies of the 150-yr soil chronosequence. The aim of our research was to improve our understanding of ecosystem development on a barren substrate and the early evolution of soils and to evaluate the influence of biological activity on weathering rates. Soil pH, cation exchange capacity, biomass, bacterial and fungal populations, and soil organic matt er show clear gradients related to soil age, in spite of the extreme heterogeneity of the ecosystem. The bulk mineralogy and inorganic geochemistry of the soils, in contrast, are independent of soil age and only in older soils (>100 yr) is incipient weathering observed, mainly as a decreasing content in albite and biotite by coincidental formation of secondary chlorites in the clay fraction. Further, we document the rapid evolution of microbial and plant communities along the chronosequence.

Hydrogeochemical Modelling of Variably Saturated Soils

Laurin Wissmeier

This thesis is about the simulation of flow, transport and geochemical reactions in variably saturated soils. Motivated by the problematic disposal and revegetation of residue from aluminium refining, novel modelling tools for general geochemical reactions in variably saturated flow conditions were developed. In addition, a new model for the effect of mineral reactions on unsaturated hydraulic properties was established. The most sophisticated of the presented series of models was applied in a comprehensive simulation of the geochemical evolution of residue from aluminium refining in response to rainfall- and evaporation-induced leaching. Model development commenced with the direct implementation of the one-dimensional moisture-based form of Richards' equation into the geochemical modelling framework PHREEQC for the simulation of biogeochemical reactions in the vadose zone. The implementation gives access to the full suite of reactions in PHREEQC with activity corrected solution speciation, advanced adsorption models and the flexible definition of kinetic mineral reactions. The numerical approach is based on the conceptual treatment of liquid phase flow and solute transport as time- and space-dependent reactions. The accuracy of the applied finite difference scheme profits from the Kirchhoff transformation on the diffusivity term. In case of the van Genuchten hydraulic model, the integration of soil moisture diffusivity led to analytical series solutions that necessitate numerical approximation. The model's novel capabilities for the simulation of unsaturated flow and ion complexation with variable charge surface sites as well as water content modifying mineral reactions, such as dissolution of hydrated minerals were demonstrated in examples. In a second step, the applicability of this simulation tool was extended to soil profiles with heterogeneous hydraulic properties and variably saturated conditions where parts of the domain may be fully saturated. For this, the head-based form of Richards' equation was implemented using a mass balancing, implicit finite difference scheme with Picard iteration. The use of a total variation-diminishing scheme for solute transport yields second order accurate yet oscillation free solutions for the advective transport of sharp concentration fronts. Applications of the scheme were illustrated for (i) infiltration with saturation-dependent cation exchange capacity, (ii) changes in hydraulic properties due to mineral reactions and (iii) transport of mobile organic substances with complexation of heavy metals. Using this extended simulation tool, an advanced model for the effect of mineral reactions on unsaturated hydraulic properties was developed. The selective radius shift model accounts for the fact that mineral reactions in unsaturated conditions only affect the water filled pore size fraction. In addition, the volume change in each pore due to dissolution/precipitation is proportional to the pore volume. Changes in mineral volume according to geochemical speciation are translated to the pore-size distribution considering the actual moisture content. The resulting discontinuous pore-size distribution is related to the water retention curve by the pore-bundle concept. From the updated discontinuous water retention curve, a smooth water retention function is deduced by means of parameter optimization. Applications of the selective radius shift model were presented for kinetic halite dissolution and calcite precipitation as a consequence of cation exchange. In a further development, the simulation of geochemical reactions in variably saturated soils was extended to two and three-dimensional domains by coupling the general finite element solver COMSOL Multiphysics® to IPhreeqc. IPhreeqc is an object-oriented version of PHREEQC that is specifically designed for couplings to flow and solute transport simulators. The resulting model combines COMSOL's capabilities for the computation of variably saturated flow and solute transport in domains of arbitrary complexity with the full suite of geochemical reaction calculations in PHREEQC. Code verification was performed for point source infiltration and degradation of the pesticide Aldicarb via a kinetic reaction chain. An application example of the new software features subsurface irrigation and fertigation, with cation exchange, proton adsorption and pressure- and space dependent root-water uptake. Using the coupled software, the coarse fraction of the leftover from aluminium refining using Bayer's process was investigated as a covering substrate for residue disposal areas and substrate for plant growth. The solid phase mineral assembly in the residue sand was derived from measured acid neutralization curves. Ion adsorption was modelled and parameterized as an exchange process. A column experiment under saturated flow conditions partially validated the model. The parameterized model was used to predict the geochemical evolution of fertigated and gypsum amended residue sand as response to rainfall- and evaporation-induced leaching in the climatic conditions of southwest Western Australia. Scenarios were calculated for a vertical flow lysimeter and a sloping cover layer for a disposal facility with dimensions 3 m × 50 m. Results show poor long-term retention of fertilizers and the stabilization of the pH at around 10 for at least 5 years of leaching. Soil moisture conditions in the cover layer could be ameliorated by appropriate design of the layer thickness, slope and distance between lateral drains.

EPFL2010

Land management trumps the effects of climate change and elevated CO2 on grassland functioning

Pierre Rémi Mariotte, Aurélie Thébault

Grasslands cover similar to 30% of the Earth's terrestrial surface and provide many ecosystem services. Many grasslands are heavily managed to maximize these services for human benefit, but the outcome of management is anticipated to be increasingly influenced by various aspects of climate change and elevated atmospheric CO2. The relative importance of global change vs. land management on grasslands is largely unknown. 2. A meta-analysis is used here to examine drivers at both scales primarily targeting services provided by grasslands relating to plant productivity (above- and below-ground biomass) and soil processes (nutrients and soil respiration) in 38 manipulative experiments published in the last decade. We specifically target effects of (i) single and combined land management practices (LMs), (ii) single and combined factors relating to broad-scale climate change and elevated CO2, and (iii) combined management practices and changes to climate and CO2. Collectively, this examines the general efficacy of global change models in predicting changes to grassland functioning. 3. We found that combinations of management practices had approximately double the explanatory power for variation in grassland services compared with individual or interactive effects of factors associated with climate change and CO2. These interacting management practices such as nutrient additions and defoliation predominantly influenced functions associated with productivity or biomass both below and above ground. The effects of interacting factors of climate and CO2 influenced a wider range of ecosystem functions, but the magnitude of these effects was relatively smaller. 4. Interactions between management practices or between climate change/CO2 factors always had higher explanatory power than any factor in isolation indicating that multivariate synergistic models of environmental change can better describe impacts on ecosystem function in plant communities (e.g. relative to univariate climate-based models). Given that the magnitude and direction (positive or negative) of the interactions varied widely, this also implies that the outcomes of these multivariate interactions can vary spatially, temporally or by immediate context (e.g. management prescriptions). 5. Synthesis. Although our work confirms how climate change and CO2 can affect many ecosystem-based functional attributes, it suggests that combinations of LMs remain the dominant set of factors in determining the performance of grassland plant communities. Land management may thus be critical for influencing projected responses to future climate change and elevated CO2 in models of grassland function at least for factors relating to primary production.

Wiley-Blackwell2014

Soil protistology rebooted: 30 fundamental questions to start with

Bertrand Fournier, Thierry Jean Heger, Vincent Eric Jules Jassey, Alexandre Jousset, Santosh Kumar, Mariusz Lamentowicz, Enrique Lara, Edward Mitchell

Protists are the most diverse eukaryotes. These microbes are keystone organisms of soil ecosystems and regulate essential processes of soil fertility such as nutrient cycling and plant growth. Despite this, protists have received little scientific attention, especially compared to bacteria, fungi and nematodes in soil studies. Recent methodological advances, particularly in molecular biology techniques, have made the study of soil protists more accessible, and have created a resurgence of interest in soil protistology. This ongoing revolution now enables comprehensive investigations of the structure and functioning of soil protist communities, paving the way to a new era in soil biology. Instead of providing an exhaustive review, we provide a synthesis of research gaps that should be prioritized in future studies of soil protistology to guide this rapidly developing research area. Based on a synthesis of expert opinion we propose 30 key questions covering a broad range of topics including evolution, phylogenetics, functional ecology, macroecology, paleoecology, and methodologies. These questions highlight a diversity of topics that will establish soil protistology as a hub discipline connecting different fundamental and applied fields such as ecology, biogeography, evolution, plant-microbe interactions, agronomy, and conservation biology. We are convinced that soil protistology has the potential to be one of the most exciting frontiers in biology. (C) 2017 Elsevier Ltd. All rights reserved.

Pergamon-Elsevier Science Ltd2017