ALBERT

Description: Eminent environmental challenges such as remediation of contaminated sites, the establishment and maintenance of nuclear waste repositories, or the design of surface landfill covers all require accurate quantification of the soil water characteristic (SWC) at low water contents. Furthermore, several essential but difficult-to-measure soil properties, including clay content and specific surface area, are intimately related to water vapor sorption. Until recently, it was a major challenge to measure detailed water vapor sorption isotherms accurately within a reasonable time frame. This priority communication illustrates potential applications of a new, fully automated, and rapid Vapor Sorption Analyzer (VSA) to pertinent issues in vadose zone research. Detailed vapor sorption isotherms for 25 variably textured soils were measured with the VSA within 1 to 3 d. Links between generated isotherms and pesticide volatilization, toxic organic vapor sorption kinetics, and soil water repellency are illustrated. Several methods to quantify hysteresis effects and to derive soil clay content and specific surface area from VSA-measured isotherms are presented. Besides above mentioned applications, potential relationships to percolation threshold for solute diffusion in unsaturated soil and to soil cation exchange capacity are discussed to stimulate new and much-needed vadose zone research.

Description: Soils function as Earth's life support system, a thin layer full of life covering most of the terrestrial surfaces. Soils form the foundation of society. Norman Borlaug stated in his Nobel laureate lecture that “the first essential component of social justice is adequate food for all mankind.” If we are to provide this component while sustaining environmental quality in the midst of a growing population and rapidly diminishing resources, it is imperative to study and obtain a deeper level of understanding of soil functions using state-of-the-art technologies as well as provide the next generation of environmentalists, soil scientists, and environmental engineers with the best education possible. The 16 papers in this special section on soil architecture and physicochemical functions in the Vadose Zone Journal contribute to these goals by improving and linking measurement, visualization, and modeling of soil structure (architecture) and physical, chemical, and biological processes in different porous media systems and at different scales. Several studies in this special section also outline and discuss emerging and exciting interdisciplinary challenges for the rapidly growing vadose zone research community, including the need for enhanced public awareness of the soil's essential life-support functions, putting value on soil ecosystem services (“capital of soil”), and design of optimal soil-based growth media for long-term missions in space.

Description: Soil is part of the Earth's life support system, but how should we convey the value of this and of soil as a resource? Consideration of the ecosystem services and natural capital of soils offers a framework going beyond performance indicators of soil health and quality, and recognizes the broad value that soil contributes to human wellbeing. This approach provides links and synergies between soil science and other disciplines such as ecology, hydrology, and economics, recognizing the importance of soils alongside other natural resources in sustaining the functioning of the Earth system. We articulate why an ecosystems approach is important for soil science in the context of natural capital, ecosystem services, and soil change. Soil change is defined as change on anthropogenic time scales and is an important way of conveying dynamic changes occurring in soils that are relevant to current political decision-making time scales. We identify four important areas of research: (i) framework development; (ii) quantifying the soil resource, stocks, fluxes, transformations, and identifying indicators; (iii) valuing the soil resource for its ecosystem services; and (iv) developing decision-support tools. Furthermore, we propose contributions that soil science can make to address these research challenges.

Description: An increasing number of electromagnetic (EM) sensors are deployed to measure volumetric soil water content () for agricultural, ecological, and geotechnical applications. While impedance and capacitance sensors generally operate at frequencies between 20–300 MHz, time domain-reflectometry (TDR) and-transmissometry (TDT) function in the GHz range. In general, lower frequency sensors are less expensive but more sensitive to confounding effects of salinity, temperature, and soil textural variations. To simplify sensor application, factory-supplied calibrations are often provided for different porous media types such as mineral, organic, and saline soils, or soilless-substrates. The objective of the presented study was to evaluate the performance of eight commercially available EM moisture sensing systems (TDR 100, CS616, Theta Probe, Hydra Probe, SM300, Wet2, 5TE, 10HS) in seven well-characterized and texturally varying soils using a standardized approach. The validity of factory supplied-calibration relationships was evaluated and the influence of soil properties on the EM responses for measurements was observed. Results indicate that the factory-supplied calibration relationships for groups of mineral and organic soils in general performed well, but some inconsistences were identified and suggestions for improvement are discussed. Soil-specific calibrations from this study yielded accuracies of around 0.015 m 3 m –3 for 10HS, SM300, and Theta Probe, while lower accuracies of about 0.025 m 3 m –3 were found for TDR100, CS616, Wet2, 5TE, and the Hydra Probe. These results are based on mineral soils having a large variation in texture, electrical conductivities below 2 dS m –1 , organic matter below 10%, and specific surface areas of less than 50 m 2 g –1 .

Description: Near-surface soil conditions (i.e., moisture and temperature) moderate mass and energy exchange at the soil–atmosphere interface. While remote sensing offers an effective means for mapping near-surface moisture content across large areas, in situ measurements, targeting those specific remotely sensed soil depths, are poorly understood and high-resolution near-surface measurement capabilities are lacking. Time domain reflectometry (TDR) is a well-established, accurate measurement method for soil dielectric permittivity and moisture content. A TDR array was designed to provide centimeter-resolution measurements of near-surface soil moisture. The array consists of nine stainless steel TDR rods spaced 1 cm apart, acting as waveguide pairs to form eight two-rod TDR probes in series. A critical aspect of the design was matching the spacing of the coaxial cable–TDR rod transition to avoid unwanted reflections in the waveforms. The accuracy of the TDR array permittivity measurement (±1 permittivity unit) was similar to that of conventional TDR as verified in dielectric liquids. Electric field numerical simulations showed minimal influence of adjacent rods during a given rod-pair measurement. The evaporation rate determined by the TDR array compared well with mass balance data in a laboratory test. Near-surface soil moisture profile dynamics were monitored at centimeter-depth resolution using the TDR array in a field experiment where volumetric moisture content estimates (0–8 cm) were within 2% of conventional three-rod TDR probes averaging across 0 to 8 cm and from 1- to 3-cm depths.

Description: Growing plants in containerized substrates has long been common practice in horticulture. Containerized plants (e.g., greenhouse tomatoes) have restricted access to essential growth resources such as oxygen, water, and nutrients. Since a wide range of inorganic and organic materials, and different combinations thereof, are commonly used as growth media, detailed and comparable physical characterization is key to identify the best performing media. In this study, five potential growth media and two mixtures thereof were characterized based on soil gas diffusivity ( D p / D o , where D p and D o are gas diffusion coefficients in soil air and free air, respectively) and an operationally defined critical window of diffusivity (CWD) representing the interval of air-filled porosity between critical air filled porosity where D p / D o 0.02 and interaggregate porosity. The D p measurements were conducted with 100-cm 3 samples from wet to complete dry conditions achieved by stepwise air drying and equilibration of initially water-saturated samples. A previously developed inactive pore and density-corrected (IPDC) model was able to describe gas diffusivities for media with distinct inactive pore space in the interaggregate pore region reasonably well. An extended IPDC model was introduced for media exhibiting a second percolation threshold in the intra-aggregate pore region. The analysis revealed comparable CWD values for the majority of the investigated media. The results further highlighted the importance of other major aspects (physical, chemical, and biological) of growth media characterization for optimal growth media design. A simple approach toward designing a gas diffusivity mixing model is presented to assist with selection of optimal mixing ratios of growth media with markedly different D p / D o behavior.

Description: The gas diffusion coefficient, air permeability, and their interrelations with air-filled porosity are essential for characterization of diffusive and convective transport of gases in soils. Variations in soil bulk density can affect water retention, air-filled pore space, and pore-network connectivity and tortuosity and, thereby, control gas diffusion and air permeability. Considering 86 undisturbed core samples with variable bulk densities that were extracted on a 15 by 15 m grid from the top layer of a sandy field, the effects of soil bulk density on gas transport parameters and the soil water characteristic were investigated. Interactions with soil organic matter, sand, and clay fractions were also examined. To evaluate bulk density effects, two constitutive parameters were derived from each of the three measured relationships. The Campbell pore-size distribution index ( b ) and the air-entry matric potential ( ae ) were derived from the soil water characteristic; the diffusive percolation threshold ( DPT ), the air-filled porosity where gas diffusivity ceases to almost zero because of interconnected water films creating isolated–inactive air content, and a pore-network connectivity index ( A 2 ) were derived from the gas diffusivity curve, and the analogous parameters convective percolation threshold ( CPT ) and convective pore-network connectivity index ( B 2 ) from the air permeability curve. All six parameters showed significant negative correlations with bulk density. To further account for the effects of both bulk density and macroporosity in parametric gas transport models, a diffusive-analog macroporosity–dependent model (DAMP) for gas diffusivity and a generalized Kawamoto et al. model (GK) for air permeability, which yielded improved predictive capabilities when compared with previous models, were developed. Both new models apply a reference point of prediction at –100 cm H 2 O matric potential (macroporosity drained), corresponding to the point where analysis of pore-network tortuosity (T) and equivalent pore diameter for gas transport ( d g ) showed diminishing effects of water blockage on gas transport in the sandy soil.

Description: Preferential flow and transport in structured soils can be intimately linked to numerous environmental problems. Surface-applied chemicals are susceptible to rapid transport to deeper depths in structural soil pores, thereby potentially contaminating valuable environmental resources and posing risks to public health. This study focused on establishing links between the structural pore space and preferential transport using a combination of standard physical measurement methods for air and water permeabilities, breakthrough experiments, and X-ray computed tomography (CT) on large soil columns. Substantial structural heterogeneity that resulted in significant variations in flow and tracer transport was observed, despite the textural similarity of the investigated samples. Quantification of macropore characteristics with X-ray CT was useful but not sufficient to explain the variability in air permeability, saturated hydraulic conductivity, and solute transport. This was due to the limited CT scan resolution and large structural variability below this resolution. However, CT matrix , a new parameter derived from the CT number of the matrix excluding stones and large mostly air-filled macropores, was found to be useful for determining the magnitude of preferential flow under boundary conditions of constant, near-saturated flow.