Teamrat A. Ghezzehei

Soil science is at a critical juncture in defining its disciplinary identity. This paper critically examines a recent proposal to define the field primarily through its societal contributions, arguing that such an approach risks constraining soil science’s scientific identity. By analyzing historical perspectives and drawing parallels with other scientific disciplines, we demonstrate that transformative solutions often emerge from fundamental research. We propose a definition that positions soil science as a natural science studying the complex planetary surfaces, encompassing both living and nonliving systems, and maintaining intellectual freedom while remaining responsive to environmental challenges.

Aggregation is a vital characteristic of soil structure that affects its physical and biogeochemical properties. Aggregation results from the cohesion of primary minerals with organic or inorganic constituents. It depends on the dynamic balance between binding and fragmentation. There are two classes of aggregation. Mechanical aggregates are formed instantly by external forces and are often unstable. Hierarchical aggregates result from slow binding and are stable. Characterization of aggregation includes size, shape, stability, configuration, and their arrangement within soil. The nature of hierarchical aggregation is inferred from size separation of aggregates. Aggregate stability indicates their ability to persist under disruptive forces.

Keywords: Aeration, Aggregate, Cementation, Clod, Imaging, Organic matter, Rhizosheath, Rhizosphere, Root, Sequestration, Soil health, Tillage, X-racy CT

Soil water status is one of the most important environmental factors that control microbial activity and rate of soil organic matter decomposition (SOM). Its effect can be partitioned into effect of water energy status (water potential) on cellular activity, effect of water volume on cellular motility and aqueous diffusion of substrate and nutrients, as well as effect of air content and gas-diffusion pathways on concentration of dissolved oxygen. However, moisture functions widely used in SOM decomposition models are often based on empirical functions rather than robust physical foundations that account for these disparate impacts of soil water. The contributions of soil water content and water potential vary from soil to soil according to the soil water characteristic (SWC), which in turn is strongly dependent on soil texture and structure.The overall goal of this study is to introducea physically based modelling framework of aerobic microbial respiration that incorporates the role of SWC under arbitrary soil moisture status.The model was tested by compariing it with published datasets of SOM decomposition under laboratory conditions.

Plant roots alter soil properties at an expensive physiological cost by releasing large quantities of organic carbon (rhizodeposition). The role of rhizodeposits in enhancing beneficial microbial activity and bio-geochemical nutrient mobilization is widely appreciated. But the role of rhizodeposits in water uptake has started gaining modest attention only recently. In this study we present a single root model, which demonstrates the possibility for rhizodeposits to create built-in water potential gradient. The conceptual basis for this model rests on three premises: (a) rhizodeposits are distributed in declining profile with distance from the root surface, (b) considerable fraction of rhizodeposits are strongly adhered to soil particles, and (c) rhizodeposits have the ability to retain water. Thus, variable concentration of affixed rhizodeposits results in a gradient of water potential without commensurate decline in water content with proximity to root surface. To corroborate premises (b) and (c), we conducted experiments using synthetic analog of rhizodeposits (Polygalacturonic Acid, PGA) and glass-bead and sand media. Envi-ronmental scanning electron microscopy was utilized to show affixation of PGA on glass beads during drying as well as pore-scale enhanced water retention. Macroscopic enhancement of water retention was characterized by dew-point potentiametry. We simulated water uptake by a root at constant potential transpiration rates representing high atmospheric demand and considered three distinct spatial distri-bution patterns of rhizodeposits as well as a control (without rhizodeposition). The model simulations indicate that the benefit of such variable distribution of exudates is more pronounced when (a) the poten-tial water uptake rate is high or (b) the rhizodeposits are constrained to a narrow volume of rhizosphere soil.

Keywords: Rhizosphere, Roots, Exudates, Water-uptake

Recently, the potential for biochar use to recapture excess nutrients from dairy wastewater has been a focus of a growing number of studies. It is suggested that biochar produced from locally available excess biomass can be important in reducing release of excess nutrient elements from agricultural runoff, improving soil productivity, and long-term carbon (C) sequestration. Here we present a review of a new approach that is showing promise for the use of biochar for nutrient capture. Using batch sorption experiments, it has been shown that biochar can adsorb up to 20–43% of ammonium and 19–65% of the phosphate in flushed dairy manure in 24 h. These results suggest a potential of biochar for recovering essential nutrients from dairy wastewater and improving soil fertility if the enriched biochar is returned to soil. Based on the sorption capacity of 2.86 and 0.23 mg ammonium and phosphate, respectively, per gram of biochar and 10–50% utilization of available excess biomass, in the state of California (US) alone, 11 440 to 57 200 tonnes of ammonium-N and 920–4600 tonnes of phosphate can be captured from dairy waste each year while at the same time disposing up to 8–40 million tons of excess biomass.

Subsurface geochemical and biological transformations often influence fluid flow by altering the pore space morphology and related hydrologic properties such as porosity and permeability. In most coupled-processes models changes in porosity are inferred from geochemical and biological process models using mass-balance. The corresponding evolution of permeability is estimated using (semi-) empirical porosity–permeability functions such as the Kozeny–Carman equation or power-law functions. These equations typically do not account for the heterogeneous spatial distribution and morphological irregularities of the geochemical precipitates and biomass. As a result, predictions of permeability evolution are generally unsatisfactory. In this communication, we demonstrate the significance of pore-scale precipitate distribution on porosity–permeability relations using high resolution simulations of fluid flow through a single pore interspersed with crystals. Based on these simulations, we propose a modification to the Kozeny–Carman model that accounts for the shape of the deposits. Limited comparison with published experimental data suggests the plausibility of the proposed conceptual model.

Keywords: Porosity, Permeability, Clogging, Coupled processes, Mineral precipitation

Methane hydrate present in permafrost and sub oceanic sediments has been identified as a potentially large energy source. Producing natural gas from hydrate results in the hydrate dissociating into gas and water, which then become distributed in the pore space according to gravitational, viscous, and capillary forces. Capillary pressure is a function of the medium (wettability, geometry) and the saturations of all phases (e.g. gas, hydrate, water) in the pore space. The presence of hydrate alters the geometry of the pore space, changing the capillary pressure-saturation relationship from the hydrate-free condition. Understanding the capillary pressure-saturation relationship of hydrate-bearing media is important for modeling the flow of gas and water through that medium, and predicting natural gas production from hydrate-bearing reservoirs. We have developed a method for measuring the capillary pressure-saturation relationship in methane hydrate-bearing sand, and our measurements and modeling aid in understanding the behavior of the gas and water in hydrate-bearing sediment. Our experiments involve hydrate formation in unsaturated sand, saturating the sample, followed by step-wise drainage from full water saturation to residual water saturation while measuring the pressure difference between the water and gas phases. During drainage, a number of intermediate static equilibrium conditions were established during which flow was discontinued. The static equilibrium observations provide water saturation vs. capillary pressure relations. For selected samples, drainage was followed by step-wise imbibition to full saturation.

Application of time domain reflectometry (TDR) in soil hydrology often involves the conversion of TDR‐measured dielectric permittivity to water content using universal calibration equations (empirical or physically based). Deviations of soil‐specific calibrations from the universal calibrations have been noted and are usually attributed to peculiar composition of soil constituents, such as high content of clay and/or organic matter. Although it is recognized that soil disturbance by TDR waveguides may have impact on measurement errors, to our knowledge, there has not been any quantification of this effect. In this paper, we introduce a method that estimates this error by combining two models: one that describes soil compaction around cylindrical objects and another that translates change in bulk density to evolution of soil water retention characteristics. Our analysis indicates that the compaction pattern depends on the mechanical properties of the soil at the time of installation. The relative error in water content measurement depends on the compaction pattern as well as the water content and water retention properties of the soil. Illustrative calculations based on measured soil mechanical and hydrologic properties from the literature indicate that the measurement errors of using a standard three‐prong TDR waveguide could be up to 10%. We also show that the error scales linearly with the ratio of rod radius to the interradius spacing.

Keywords: TDR, water content, compaction, measurement error

The Gardner and van Genuchten models of relativepermeability are widely used in analytical and numerical solutions toflow problems. However, the applicability of the Gardner model to realproblems is usually limited, because empirical relative permeability datato calibrate the model are not routinely available. In contrast, vanGenuchten parameters can be estimated using more routinely availablematric potential and saturation data. However, the van Genuchten model isnot amenable to analytical solutions. In this paper, we introducegeneralized conversion formulae that reconcile these two models. Ingeneral, we find that the Gardner parameter alpha G is related to the vanGenuchten parameters alpha vG and n by alpha G/alpha vG  ; 1.3 n. Thisconversion rule will allow direct recasting of Gardner-based analyticalsolutions in the van Genuchten parameter space. The validity of theproposed formulae was tested by comparing the predicted relativepermeability of various porous media with measured values.

The Nopal I mine in Pena Blanca, Chihuahua, Mexico, contains a uranium ore deposit within fractured tuff. Previous mining activities exposed a level ground surface 8 m above an excavated mining adit. In this paper, we report results of ongoing research to understand and model percolation through the fractured tuff and seepage into a mined adit both of which are important processes for the performance of the proposed nuclear waste repository at Yucca Mountain. Travel of water plumes was modeled using one-dimensional numerical and analytical approaches. Most of the hydrologic properly estimates were calculated from mean fracture apertures and fracture density. Based on the modeling results, we presented constraints for the arrival time and temporal pattern of seepage at the adit.

Flow diversion around subsurface cavities in unsaturated fractured media is important to numerous environmental and engineering applications. This paper provides analytical solutions to partial and complete flow diversion around cavities intersected by fractures under steady state conditions. It is focused on a typical trifracture junction located upstream from a cavity surface. Fractures are modeled as two‐dimensional porous media with an exponential relationship between the capillary pressure and unsaturated hydraulic conductivity. The solutions show that the vertical distance between the fracture end and the nearest junction (Z) and the slope of the unsaturated hydraulic conductivity (α) are by far the most important determinants of flow diversion. In fact, the product of Z and α enters the threshold flux and liquid entry flux equations as a dimensionless sorptive length (s). This relationship between Z and α is shown to have important implications for uncertainty and scalability of calibrated model parameters. The solutions given in this paper are expected to be directly applicable to cavities on the order of the fracture spacing.

In recent years, significant advances have been made in our understanding of the complex flow processes in individual fractures, aided by flow visualization experiments and conceptual modeling efforts. These advances have led to the recognition of several flow regimes in unsaturated individual fractures subjected to different initial and boundary conditions. For an idealized smooth fracture surface the most important regimes are film flow, rivulet flow, and sliding of droplets. The existence of such significantly dissimilar flow regimes has been a major hindrance in the development of self-consistent conceptual models of flow for single fracture surfaces that encompass all the flow regimes. The objective of this study is to delineate the existence of the different flow regimes in individual fracture surfaces. For steady state flow conditions, we developed physical constraints on the different flow regimes that satisfy minimum energy configurations, which enabled us to segregate the wide range of fracture flux (volumetric flow rate per fracture width) into several flow regimes. These are, in increasing order of flow rate, flow of adsorbed films, flow of sliding drops, rivulet flow, stable film flow, and unstable (turbulent) film flow. The scope of this study is limited to wide-aperture smooth fractures with the flow on the opposing sides of fracture being independent.

Flow processes in unsaturated fractures considerably differ from flow in rock matrix because of the dominance of gravitational forces, accentuated by variations in fracture geometry. This gives rise to liquid fragmentation, fingering, and intermittent flow regimes that are not amenable to standard continuum representation. We develop an alternative modeling framework to describe the onset of liquid fragmentation and subsequent flow behavior of discrete liquid clusters. The transition from a slowly growing anchored liquid element to a finger‐forming mobile liquid element is estimated from the force balance between retarding capillary forces dominated by contact angle hysteresis and suspended liquid weight. A model for liquid fragmentation within the fracture plane (smooth and parallel walled fractures) for given a steady input flux and aperture size is developed and tested. Predictions of sizes and detachment intervals of liquid elements are in good agreement with experimental results. The results show that the mass of detached liquid element is only weakly related to flow rate but increases with fracture aperture size. Periodic discharge similar to that experimentally observed is a result of the interplay between capillary, viscous, and gravitational forces. We show that the presence of even a few irregularities in a fracture plane may induce complicated flux patterns downstream. Similar erratic fluxes are observed in studies involving gravity‐driven unsaturated flow.

Keywords: fracture, intermittent, episodic, finger, dripping, vadose zone

Cavities excavated in unsaturated geological formations are important to activities such as nuclear waste disposal and mining. Such cavities provide a unique setting for simultaneous occurrence of seepage and evaporation. Previously, inverse numerical modeling of field liquid-release tests and associated seepage into cavities were used to provide seepage-related large-scale formation properties, ignoring the impact of evaporation. The applicability of such models was limited to the narrow range of ventilation conditions under which the models were calibrated. The objective of this study was to alleviate this limitation by incorporating evaporation into the seepage models. We modeled evaporation as an isothermal vapor diffusion process. The semiphysical model accounts for the relative humidity (RH), temperature, and ventilation conditions of the cavities. The evaporation boundary layer thickness (BLT) over which diffusion occurs was estimated by calibration against free-water evaporation data collected inside the experimental cavities. The estimated values of BLT were 5 to 7 mm for the open underground drifts and 20 mm for niches closed off by bulkheads. Compared with previous models that neglected the effect of evaporation, this new approach showed significant improvement in capturing seepage fluctuations into open cavities of low RH. At high relative-humidity values (>85%), the effect of evaporation on seepage was very small.

This study deals with deformation of small pores in wet soils of relatively high bulk density such as in the final settlement phase of tilled or disturbed soils. Pore deformation was modeled by volume reduction of spherical voids embedded in a homogenous soil matrix. External constant stress and overburden were considered as steady stresses because the change in interaggregate contact stress under overburden is slow compared to the associated strain rate. In contrast, stress due to passage of farm implements was considered as transient because the rate of change of interaggregate stress is comparable with the strain rate. Rheological behavior of the soil matrix under steady and transient stresses was obtained from independent rheological measurements. Experimental data from the literature were used to illustrate the model. Model predictions of relative density compared favorably with experimental data for constant stress application as well as for constant strain rate experiments. Results showed that the rate of densification decreased as the relative density approached unity (complete pore closure) and the relative stress required for driving densification increased exponentially with increasing relative density.

Evaporation from the surface of a porous medium is a complex process, governed by interplay between (1) coupled liquid and vapor flow in the porous medium, and (2) relative humidity, temperature, and aerodynamic conditions in the surrounding air. In order to avoid the computational expense of explicitly simulating liquid, gas, and heat flow in the porous medium (and the possible further expense of simulating the flow of water vapor in the atmosphere), evaporative potentials can be treated in a simplified manner within a model where liquid is the only active phase. In the case of limited air mixing, evaporation can be approximated as a diffusion process with a linear vapor-concentration gradient. We have incorporated a simplified scheme into the EOS9 module of iTOUGH2 to represent evaporation as isothermal Fickian diffusion. This is notable because the EOS9 module solves a single equation describing saturated and unsaturated flow, i.e., phase transitions and vapor flow are not explicitly simulated. The new approach was applied to three simple problems and the results were compared to those obtained with analytical solutions or the EOS4 module, which explicitly considers advective and diffusive vapor flow. Where vapor flow within the porous medium can be neglected, this new scheme represents significant improvement over the computational expense of explicitly simulating liquid, gas, and heat flow, while providing an adequate reproduction of the overall hydrologic system. The scheme is set up to allow parallel flow of liquid and vapor, so that evaporation from an actively seeping face can be simulated. In addition, dynamic relative humidity boundary conditions can be simulated using standard iTOUGH2 features.

The loose and fragmented soil structure that results from tillage operations provides favorable physical conditions for plant growth. This desirable state is structurally unstable and deteriorates with time because of overburden, external stresses, and capillary forces. The objective of this study was to model these structural changes by coupling soil intrinsic rheological properties with geometry and arrangement of aggregates represented as monosized spheres. Calculations of interaggregate stresses and strains, and associated changes in density and porosity, were performed for a rhombohedral unit cell. Soil rheological properties determined by application of steady shear stress were used for calculations of strains under steady interaggregate stresses. The models developed herein correspond to the initial stage of deformation when discrete aggregates exist. At strains exceeding 0.12 the interaggregate voids are isolated and the current model no longer applies and an alternative approach is presented elsewhere. Unit cell calculations were up scaled to an aggregate-bed scale by considering a one-dimensional stack of unit cells, which allows only vertical stress transmission. The stress acting at an interaggregate contact is fully accommodated (dissipated) by viscous flow when it exceeds the yield stress (strength) of the aggregates. The stress is fully transmitted to subsequent unit cells when it is less than the yield stress. Plausibility of the models was demonstrated by illustrative examples that highlight the different features of the models. The results were in qualitative agreement with observations from the literature for deformation of either loose structure, and for highly dense cases close to maximal bulk density.

Tilled agricultural soils are in a constant state of change induced by variations in soil strength due to wetting and drying and compaction by farm implements. Changes in soil structure affect many hydraulic and transport properties; hence their quantification is critical for accu- rate hydrological and environmental modeling. This study highlights the role of soil rheology in determining time-dependent stress–strain relationships that are essential for prediction and analysis of structural changes in soils. The primary objectives of this study were (i) to extend a previously proposed aggregate-pair model to prediction of compaction under external steady or transient stresses and (ii) to provide experimentally determined rheological information for the above models. Rheological properties of soils and clay minerals were measured with a rotational rheometer with parallel-plate sensors. These measurements, under controlled steady shear stress application, have shown that wet soils have viscoplastic behavior with well-defined yield stress and nearly constant plastic viscosity. In contrast, rapid transient loading (e.g., passage of a tractor) is often too short for complete viscous dissipation of applied stress, resulting in an elastic (recoverable) component of deformation (viscoelastic behavior). Measured viscoelastic properties were expressed by complex viscosity and shear modulus whose components denote viscous energy dissipa- tion, and energy storage (elastic). Results show that for low water contents and fast loading (tractor speed), the elastic component of deformation increases, whereas with higher water contents, viscosity and shear modulus decrease. Steady and oscillatory stress application to an aggregate pair model illustrates potential use of rheological properties towards obtaining predictions of strains in soils.

The desired soil structure following tillage of agricultural soils is often unstable and susceptible to coalescence of aggregates and reduction of interaggregate porosity due to wetting and drying cycles. This process of aggregate rejoining was modeled by equating the rate of work done by liquid‐vapor menisci, to the rate of energy dissipation due to viscous deformation of a pair of spherical aggregates. The nonlinearity of wet soil viscous flow behavior was accounted for by introducing a Bingham rheological model. A natural outcome of the analysis was the formulation of a mathematical condition for the onset and termination of coalescence based on soil strength at specified water content. The condition states that sufficient energy in excess of soil strength (yield stress) must be available for coalescence to proceed. The rate of aggregate coalescence is proportional to available energy and is inversely related to the coefficient of plastic viscosity. Transport of wet soil to the periphery of the interaggregate contact by viscous flow leads to smoothing of the neck, resulting in pore closure, on the one hand, and restricting the minimum matric potential that can be achieved, on the other. The interplay between rheology and geometry prevent coalescence from proceeding indefinitely. Independently determined soil rheological properties were used to illustrate the use of the model. Coalescence under constant water content and during wetting‐drying cycles was calculated. Comparison of data from experiments on one‐dimensional, aggregate bed settlement has shown reasonable agreement with the model predictions.