ALBERT

Description: In porous media, the nonwetting phase is trapped on water saturation due to capillary forces acting in a heterogeneous porous structure. Within the capillary fringe, the gas phase is trapped and released along with the fluctuation of the water table, creating a highly active zone for biological transformations and mass transport. We conducted column experiments to observe and quantify the magnitude and structure of the trapped gas phase at the pore scale using computed microtomography. Different grain size distributions of glass beads were used to study the effect of the pore structure on trapping at various capillary numbers. Viscous forces were found to have negligible impact on phase trapping compared with capillary and buoyancy forces. Residual gas saturations ranged from 0.5 to 10%, while residual saturation increased with decreasing grain size. The gas phase was trapped by snap-off in single pores but also in pore clusters, while this single-pore trapping was dominant for grains larger than 1 mm in diameter. Gas surface area was found to increase linearly with increasing gas volume and with decreasing grain size.

Description: Processes in capillary fringes (CFs) have a complex nature due to the interactions between the solid, liquid, and gaseous environments. Despite a considerable body of literature on CFs coming from different disciplines, the ongoing processes and their complex interactions are yet only partially understood.

Description: Water flow and solute transport in unsaturated porous media are affected by the highly nonlinear material properties and nonequilibrium effects. This makes experimental procedures and modeling of water flow and solute transport challenging. In this study, we present an extension to the well-known multistep-outflow (MS-O) and the newly introduced multistep-flux (MS-F) approaches to measure solute dispersivity as a function of water content under well-defined conditions (i.e., constant pressure head and uniform water content). The new approach is termed multistep-transport (MS-T) and complements the MS-O and MS-F approaches. Our setup allows for applying all three approaches in a single experimental setting. Hence, it provides a comprehensive data set to parameterize unsaturated flow and transport processes in a consistent way. We demonstrate this combined approach (MS-OFT) for sand (grain diameter: 0.1–0.3 mm) and complemented the experimental results with an analysis of the underlying pore structure using X-ray computed tomography (CT). The results show that dispersivity is a nonlinear function of water content, and a critical water content (0.2) exists at which dispersivity increased significantly. The results could be explained by marked change in the geometry of the flow field as derived from X-ray CT measurements. It is characterized by a reduced connectivity of the water phase. The results demonstrate the potential of a combined approach linking pore structure, hydraulic functions, and transport characteristic.

Description: Knowledge of local water fluxes across the soil–root interface is essential to understand and model root water uptake. Despite its importance, there is a lack of direct methods to measure the location of water uptake along roots. The aim of this study was to develop a technique to quantify local flux of water from soil to the roots of living plants. To this end, we used neutron radiography to trace the transport of deuterated water (D2O) into individual roots. We grew lupin (Lupinus albus L.) in 30- by 25- by 1-cm containers filled with a sandy soil, which was partitioned into different compartments using 1-cm-thick layers of coarse sand. We locally injected D2O into a selected soil compartment near the roots of 18-d-old lupin during the day (transpiring) and night (not transpiring). The transport of D2O into roots was then monitored using time-series neutron radiography. The results show that: (i) the transport of D2O into roots was faster during the day than at night; and (ii) during the day, D2O was quickly transported along the roots toward the shoots, while at night this transport was insignificant. The differences between day and night measurements were explained by convective transport of D2O into the roots driven by transpiration. To quantify the local transport of D2O into roots, we developed a simple convection–diffusion model that assumed the endodermis as the main resistance to water transport. The D2O uptake predicted by the model was in agreement with the axial flow within the roots as derived from D2O transport behind the capillary barrier. This new method allows quantification of local water uptake in different parts of the root system.

Description: Prediction of fluid phase dynamics in the vadose zone is hampered by the presence of different types of sharp interfaces, questioning the validity of standard theory that is based on assumption of continuity of air and water phases, uniform distribution of fluids in a control volume, local equilibrium of phase contents and pressures, and slow process velocities. Complexity of fluid front morphology and burst-like redistribution processes may be accentuated across material contrasts or at the interface between soil and atmosphere. This special section presents eleven contributions highlighting the role of interfaces on water and air distributions and outlining methods to improve prediction of interfacial displacement.

Description: Root system architecture and associated root–soil interactions exhibit large changes over time. Nondestructive methods for the quantification of root systems and their temporal development are needed to improve our understanding of root activity in natural soils. X-ray computed tomography (X-ray CT) was used to visualize and quantify growth of a single Vicia faba L. root system during a drying period. The plant was grown under controlled conditions in a sandy soil mixture and imaged every second day. Minkowski functionals and Euclidean distance transform were used to quantify root architectural traits. We were able to image the root system with water content decreasing from 29.6 to 6.75%. Root length was slightly underestimated compared with destructive measurements. Based on repeated measurements over time it was possible to quantify the dynamics of root growth and the demography of roots along soil depth. Measurement of Euclidean distances from any point within the soil to the nearest root surface yielded a frequency distribution of travel distances for water and nutrients towards roots. Our results demonstrate that a meaningful quantitative characterization of root systems and their temporal dynamics is possible.

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 hydraulic behavior of soil is determined by its hydraulic properties and their variability in space. In agricultural soils, this heterogeneity may stem from tillage or may have natural origin. The root distribution of plants will adapt to some extent to this soil heterogeneity. However, the combined impact of soil heterogeneity and root water uptake (RWU) on long-term soil water budgets has not received much attention. Numerical experiments helped identify how soil heterogeneity affects plant transpiration, soil evaporation, and groundwater recharge. Two-dimensional virtual soils with hierarchical heterogeneity, both natural and tillage induced, served as a basis for modeling soil water dynamics for a 10-yr climate record from two weather stations in Germany that vastly differ in annual precipitation. The complex interactions between soil and vegetation were explored by (i) comparing different RWU strategies (depth-, structure-, and time-dependent root profiles), (ii) land use types (perennial grass and annual winter crops), (iii) a combination of textures (silt above sand and sand above loam), and (iv) RWU with or without a compensation mechanism. The simulations were repeated with one-dimensional, effective representations of these virtual soils. In the framework of hydropedology, this study shed some light on the interaction between plants and pedological features and its impact on the macroscopic soil water budget. We demonstrated that land use has a major impact on the annual water balance through the partitioning of evapotranspiration into bare soil evaporation and plant transpiration. Compensational RWU becomes important for the annual water balance when the root zone comprises contrasting materials with respect to water holding capacity. Soil heterogeneity has in fact a minor impact on long-term soil water budgets. As a consequence, the relative contribution of plant transpiration, soil evaporation, and groundwater recharge to the total soil water loss was well reproduced by simulations in one-dimensional effective soil profiles. This advocates the application of one-dimensional soil–atmosphere–vegetation transfer (SVAT) models at larger scales. These findings only hold for assumptions made in our numerical simulations including flat area without lateral flow and no macropore flow.

Description: Soils are structured on multiple spatial scales, originating from inhomogeneities of the parent material, pedogenesis, soil organisms, plant roots, or tillage. This leads to heterogeneities that cause variability of local measurements of hydraulic state variables and affects the flow behavior of water in soil. Whereas in real-world systems, the true underlying structures can never be absolutely known, it is appealing to employ synthetic or "virtual" experiments for assessing general properties of flow in porous media and grasping the main physical mechanisms. With this aim, three two-dimensional virtual realities with increasing structural complexity, representing cultivated soils with hierarchical spatial heterogeneity on multiple scales were constructed by the interdisciplinary research group Virtual Institute of the Helmholtz Association (INVEST). At these systems, numerical simulations of water dynamics including a heavy rain, a redistribution, and a long-lasting evaporation period were performed. The technical aspects of the construction of the virtual soils and results of the forward simulations have been presented in a paper by Schlüter et al. (2012) . In this follow-up paper, we use inverse modeling to investigate measurements in virtual vertical soil profiles, mimicking typical field monitoring campaigns with moisture content and matric potential sensors placed at five depths. Contrary to the real situation, we can interpret observed data, their variability, estimated hydraulic properties, and predicted water balance in the light of the known truth. Our results showed that measurements, particularly those of water contents, varied strongly with measuring position. Using data from single profiles in systems similar to our virtual soils thus will lead to very different estimates of the soil hydraulic properties. As a consequence, the correct calculation of the water balance is rather a lucky coincidence than the rule. However, the average of the predicted water balances obtained from the one-dimensional simulations, and the estimated soil hydraulic properties agreed very well with those attained from the two-dimensional systems.