Mitra Asadollahi

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Unités associées (4)

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Publications associées (4)

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Personnes menant des recherches similaires

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Spatially Explicit Linkages Between Redox Potential Cycles and Soil Moisture Fluctuations

Mitra Asadollahi, Paolo Benettin, Rizlan Bernier-Latmani, Manon Frutschi, Filippo Miele, Andrea Rinaldo, Simiao Wang

Reduction-oxidation cycles measured through soil redox potential (Eh) are associated with dynamic soil microbial activity. Understanding changes in the composition of, and resource use by, soil microbial communities requires Eh predictability under shifting hydrologic drivers. Here, 50-cm soil column installations are manipulated to vary hydrologic and geochemical conditions, and are extensively monitored by a dense instrumental deployment to record the depth-time variation of physical and biogeochemical conditions. We contrast measurements of Eh, soil saturation and key compounds in water samples (probing the majority of soil microbial metabolisms) with computations of the relevant state variables, to investigate the interplay between soil moisture and redox potential dynamics. Our results highlight the importance of joint spatially resolved hydrologic flow/transport and redox processes, the worth of contrasting experiments and computations for a sufficient understanding of the Eh dynamics, and the minimum amount of biogeochemistry needed to characterize the dynamics of electron donors/acceptors that are responsible for the patterns of Eh not directly explained by physical oxic/anoxic transitions. As an example, measured concentrations of sulfate, ammonium and iron II suggest coexistence of both oxic and anoxic conditions. We find that the local saturation velocity (a threshold value of the time derivative of soil saturation) exerts a significant hysteretic control on oxygen intrusion and on the cycling of redox potentials, in contrast with approaches using a single threshold saturation level as the determinant of anoxic conditions. Our findings improve our ability to target how and where hotspots of activity develop within soil microbial communities.

2023

Chargement

Crossing properties of soil saturation, defined as the duration and excursion of soil saturation below and above certain thresholds, are key variables to ecosystem functioning and evolution by primarily influencing the plant and soil microbes physiological dynamics. Under climate change, the variability in the precipitation patterns is expected to be more ubiquitous and directly translate to a shift in soil saturation levels. This, in turn, will influence crossing properties of soil saturation and thus be expected to leave a mark on ecosystems. However, the lack of a deep understanding of the links between biological processes and the change in soil saturation patterns hinders foretelling the impact of climate variability on ecohydrological systems even at the smallest scale, that is, the level of a single plant.To that end, this thesis presents experimental results from laboratory soil column experiments and extensive computational studies on the interactions between soil saturation and biological activities, that is, plants and microbes. To this aim, the depth-time soil saturation patterns are mapped by means of modeling the transport processes of relevance and experimental tools at a minimal hydrological control volume in search for upscaling methods. In this context, two modeling approaches can be employed. Physically based models that explicitly represent transport processes with high costs of application at larger scales. Age-based models that rely on generic functions with a focus on time past since arrival, that is, the age of event water in the storage. This thesis work combines models (HYDRUS-1D as physically based and tran-SAS as age-based) with soil column experimental data and Bayesian inference methods to explore the links between age characteristics and transport processes. In this thesis, data from four different experimental designs on monthly to yearly timescale under varied environmental conditions are examined in terms of tracer type, precipitation pattern, vegetation, soil type, resolution and type of collected data. Of the modeled data, two sets of results from specific experiments were carried out within the framework of this thesis.This thesis addresses also the role of the balance between dispersive and convective forces, known as Péclet number, in controlling the shape of age functions in drainage. While sole reliance on drainage flow and tracer data (reflective of Péclet number) suffice to capture age dynamics in drainage, un- certainties may arise for predictions on (evapo)transpiration. This thesis deals also with the role of root distributions as controls in determining the age to root uptake and shows plants with similar uniform-equivalent root distribution render very similar age dynamics. It was shown in this thesis that nitrate consumption by microbes can be linked to age characteristics of a conservative tracer and that the relation between microbial respiration and soil saturation may bear more complexity than is currently integrated in most models. Overall, this thesis lays steps to the development of modeling tools with high predictive power for soil saturation by originating upscaling methods and bridging the gaps between physically based and age-based models with a view on the consequences of (possi- bly changing) variable soil saturation on adaptations of microbial communities and their metabolic product.

EPFL2022

Chargement

Transport and Water Age Dynamics in Soils: A Comparative Study of Spatially Integrated and Spatially Explicit Models

Mitra Asadollahi, Paolo Benettin, Andrea Rinaldo

We address transport by transit times (i.e., the age of water parcels leaving a storage as discharge, deep loss, or evapotranspiration) in subvertical soil systems, key to our understanding of water quality in catchments and streams. While the use of field and lysimeter observations to constrain and validate modeling approaches is generally accepted, different views exist on the relative ranges of applicability of spatially integrated or spatially explicit approaches. This study specifically illustrates how one class of spatially integrated models of transport, based on StorAge Selection (SAS) functions, fares with respect to spatially explicit hydrologic models in a case where detailed tracer data from experimental lysimeters exist and for which both approaches are viable. Data from two lysimeters experiments that differ in atmospheric conditions, size of the installation, tracer type, soil texture, and vegetation are used to contrast results from two transport models: tran-SAS (space implicit) and HYDRUS-1D (space explicit). Results suggest that although the two lysimeters are characterized by different transit time distributions, their underlying transport mechanisms are similar and represented well by both models. The comparison between the two models results in robust estimates of the transport timescales and clearly shows that percolation fluxes at the bottom of a lysimeter tend to drain the relatively old components of the soil storage. We conclude that the convergence of the approaches in a geomorphologically simple and data-rich problem supports extensive uses of the spatially integrated approach in cases where the scale of the problem, or subgrid parameterization needs, may limit the applicability of detailed modeling approaches.

2020

Chargement

EPFL2022