ALBERT

Description: Author(s): L. Biadala, H. Frederich, L. Coolen, S. Buil, X. Quélin, C. Javaux, M. Nasilowski, B. Dubertret, and J.-P. Hermier The emission spectrum of the trion state in very thick shell CdSe/CdS nanocrystals is characterized at 4 K by photon correlation Fourier spectroscopy. A value of 50 μeV for the width of the zero phonon line is measured. The absence of blinking and the high photostability of these emitters offer the ... [Phys. Rev. B 91, 085416] Published Wed Feb 18, 2015

Description: An automated method for root system architecture reconstruction from three-dimensional volume data sets obtained from magnetic resonance imaging (MRI) was developed and validated with a three-dimensional semimanual reconstruction using virtual reality and a two-dimensional reconstruction using SmartRoot. It was tested on the basis of an MRI image of a 25-d-old lupin ( Lupinus albus L.) grown in natural sand with a resolution of 0.39 by 0.39 by 1.1 mm. The automated reconstruction algorithm was inspired by methods for blood vessel detection in MRI images. It describes the root system by a hierarchical network of nodes, which are connected by segments of defined length and thickness, and also allows the calculation of root parameter profiles such as root length, surface, and apex density The obtained root system architecture (RSA) varied in number of branches, segments, and connectivity of the segments but did not vary in the average diameter of the segments (0.137 cm for semimanual and 0.143 cm for automatic RSA), total root surface (127 cm 2 for semimanual and 124 cm 2 for automatic RSA), total root length (293 cm for semimanual and 282 cm for automatic RSA), and total root volume (4.7 cm 3 for semimanual and 4.7 cm 3 for automatic RSA). The difference in performance of the automated and semimanual reconstructions was checked by using the root system as input for water uptake modeling with the Doussan model. Both systems worked well and allowed for continuous water flow. Slight differences in the connectivity appeared to be leading to locally different water flow velocities, which were 30% smaller for the semimanual method.

Description: The study of water pathways from the soil to the atmosphere through plants—the so-called soil–plant–atmosphere continuum (SPAC)—has always been central to agronomy, hydrology, plant physiology, and other disciplines, using a wide range of approaches and tools. In recent years, we have been witnessing a rapid expansion of interweaving monitoring activities and model development related to SPAC in climatic, ecological, and applications other than the traditional agrohydrological, and it is therefore timely to review the current status of this topic and outline future directions of research. The initiative for the special section of Vadose Zone Journal on SPAC emanated from several sessions we recently organized in international conferences and meetings. With a view to the specific research questions covered in this special section, this article introduces and reviews SPAC underlying issues and then provides a brief overview of the invited contributions. We have grouped together the 15 contributions under three main sections related to the local, field, and landscape spatial scales of interests. Within these sections, the papers present their innovative results using different measuring techniques (from classic tensiometers and TDR sensors to more advanced and sophisticated equipment based on tomography and geophysics) and different modeling tools (from mechanistic models based on the Richards equation to more parametrically parsimonious hydrologic balance models). They provide a snapshot of the current state of the art while emphasizing the significant progress attained in this field of research. New technological developments and applications are also highlighted.

Description: Chicory (Cichorium intybus L.) is a cash crop cultivated in Western Europe for inulin production. Due to actual and future climate changes, this plant could be exposed to severe water stress at the end of its growing period, leading to a decrease of its yield. The aim of this work was to investigate the chicory root water uptake dynamics and the plant ability to compensate a lack of water in the upper horizons. We performed a controlled experiment with 3 replicates under contrasted irrigation scenarios. We observed that, in case of drought, total root length decreased and root profiles developed deeper. We successfully used a one-dimensional Richards-based model with a stress function and a compensation mechanism (Hydrus 1-D) to inversely characterize the dynamics of the actual sink-term profiles under both irrigation scenarios. We could also use the model to assess the compensation thanks to a weighted stress index that is consistent between replicates. The extraction profiles evolved differently under water-deficit and controlled situations. The passive compensation mechanism allowed chicory roots under water-limited conditions to take water deeper in the soil, where they had only few lateral roots. We found that, in case of drought, compensation started before the plants had to reduce their transpiration rate. Because the soil kept drying out, compensation was not sufficient anymore, and the plants had to decrease their transpiration some days later. However, chicories maintained their metabolism and continued to transpire and to growth slowly. This allowed them to adapt thanks to an active compensation mechanism, by generating new lateral roots in wetter horizons. This study also showed that there was no unique Feddes stress parameter set able to describe plant behavior under contrasted irrigation conditions or even under different plant development stages.

Description: Knowledge of soil moisture dynamics and its spatial variability is essential to improve our understanding of root water uptake and soil moisture redistribution at the local scale and the field scale. We investigated the potential and limitations of electrical resistivity tomography (ERT) to measure three-dimensional soil moisture changes and variability in a large, undisturbed, cropped soil column and examined the interactions between soil and root system. Our analysis sustained the value of ERT as a tool to monitor and quantify water contents and water content changes in the soil, as long as the root biomass does not influence the observed resistivity. This is shown using a global water mass balance and a local validation using time domain reflectometry (TDR) probes. The observed soil moisture variability was rather high compared to values reported in the literature for bare soil. The measured water depletion rate, being the result of combined effects of root water uptake and soil water redistribution, was compared with the evaporative demand and root length densities. We observed a gradual downward movement of the maximum water depletion rate combined with periods of redistribution when there was less transpiration. Finally, the maximum root length density was observed at -70 cm depth, pointing out that root architecture can strongly depend on soil characteristics and states.

Description: Electrical resistance tomography (ERT) can be used for the noninvasive characterization of soil moisture and soil structural heterogeneity. Any attempt to relate electrical resistivity measurements to soil moisture content or soil bulk density, however, must rely on a "pedo-electrical" function, i.e., a conductivity model for soils. This study aimed to test five pedo-electrical models for their ability to reproduce electrical resistivity as measured by ERT in a silt loam soil sample across a range of moisture and bulk density values. The Waxman and Smits model, the Revil model, the volume-averaging (VA) model, the Rhoades model, and the Mojid model were inverted within a Bayesian framework, thereby identifying not only the optimal parameter set but also parameter uncertainty and its effect on model prediction. The VA model outperformed the other models in terms of both fit and parameter consistency with respect to independent estimates of surface conductivity obtained with published pedotransfer functions. Sensitivity of the electrical resistivity was then studied by means of the calibrated VA model, revealing an approximately 1.5 times higher sensitivity to soil moisture content than to soil bulk density. In addition, the sensitivity of electrical resistivity to soil moisture and soil bulk density was found to increase as soil moisture and bulk density decreased. The VA model calibrated on the basis of resistivity measurements appeared to simulate relatively well the measured soil moisture content for electrical resistivity values

Description: The remarkable complexity of soil and its importance to a wide range of ecosystem services presents major challenges to the modeling of soil processes. Although major progress in soil models has occurred in the last decades, models of soil processes remain disjointed between disciplines or ecosystem services, with considerable uncertainty remaining in the quality of predictions and several challenges that remain yet to be addressed. First, there is a need to improve exchange of knowledge and experience among the different disciplines in soil science and to reach out to other Earth science communities. Second, the community needs to develop a new generation of soil models based on a systemic approach comprising relevant physical, chemical, and biological processes to address critical knowledge gaps in our understanding of soil processes and their interactions. Overcoming these challenges will facilitate exchanges between soil modeling and climate, plant, and social science modeling communities. It will allow us to contribute to preserve and improve our assessment of ecosystem services and advance our understanding of climate-change feedback mechanisms, among others, thereby facilitating and strengthening communication among scientific disciplines and society. We review the role of modeling soil processes in quantifying key soil processes that shape ecosystem services, with a focus on provisioning and regulating services. We then identify key challenges in modeling soil processes, including the systematic incorporation of heterogeneity and uncertainty, the integration of data and models, and strategies for effective integration of knowledge on physical, chemical, and biological soil processes. We discuss how the soil modeling community could best interface with modern modeling activities in other disciplines, such as climate, ecology, and plant research, and how to weave novel observation and measurement techniques into soil models. We propose the establishment of an international soil modeling consortium to coherently advance soil modeling activities and foster communication with other Earth science disciplines. Such a consortium should promote soil modeling platforms and data repository for model development, calibration and intercomparison essential for addressing contemporary challenges.

Description: The process description of plant transpiration and soil water uptake in macroscopic root water uptake models is often based on simplifying assumptions that no longer reflect, or even contradict, the current status of knowledge in plant biology. The sink term in the Richards equation for root water uptake generally comprises four terms: (i) a root resistance function, (ii) a soil resistance function, (iii) a stress function, and (iv) a compensation function. Here we propose to use a detailed three-dimensional model, which integrates current knowledge of soil and root water flow equations, to deduct a one-dimensional effective behavior at the plant scale and to propose improvements for the four functions used in the macroscopic sink term. We show that (i) root hydraulic resistance may be well defined by the root length density but only for homogeneous lateral conductances and no limiting xylem conductance—in other cases a new function depending on the root hydraulic architecture should be used; (ii) soil resistance cannot be neglected, in particular in the rhizosphere where specific processes may occur that alter the soil hydraulic properties and therefore affect uptake; (iii) stress and compensation are two different processes, which should not be linked explicitly; (iv) there is a need for a clear definition of compensatory root water uptake independent of water stress; (v) stress functions should be defined as a maximal actual transpiration in function of an integrated root–soil interface water head rather than in terms of local bulk water heads; and (vi) nonlinearity in the stress function is expected to arise if root hydraulic resistances depend on soil matric head or when it is defined as a function of the bulk soil water head.