Showing papers in "Vadose Zone Journal in 2015"

Organic Growing Media: Constituents and Properties

Abstract: Organic growing media are essentially bulk products. Availability in large quantity allied to its excellent air and water retention, low pH and salinity, and freedom from pests and diseases has led to peat being the dominant organic constituent of growing media in many parts of the world for the last 50 yr. The unique microporous properties of Sphagnum peat and its resistance to degradation are matched by few other growing media constituents. Nevertheless, local scarcity of Sphagnum peat and the expense of transport has led to the use of other materials in growing media. Notable among these is coir, which unlike peat, a CO2 sink, is widely regarded as a rapidly renewable resource. Indeed, advances in processing and quality control in situ have led to a huge upsurge in the export and use of coir in growing media, particularly in Europe but also in the western United States. Locally available organic materials such as bark, composted materials including green (yard) wastes, municipal solid wastes, and even sewage sludge are also used in growing media. While possessing advantages such as the high air content of bark and nutrient supply of many composted materials, these media components may have disadvantages, from limited supplies due to bioenergy pulls and N lock-up in bark, to physical, chemical, and microbial contaminants in composts. Current innovative approaches involve increasing use of wood fiber in Europe, whole pine-tree thinnings in the United States, and realizing the use and transformation of composted wastes as next-generation constituents of growing media.

Critical Zone Services: Expanding Context, Constraints, and Currency beyond Ecosystem Services

Abstract: Processes within the critical zone—spanning groundwater to the top of the vegetation canopy—have important societal relevance and operate over broad spatial and temporal scales that often are not included in existing frameworks for ecosystem services evaluation. Here we expand the scope of ecosystem services by specifying how critical zone processes extend context both spatially and temporally, determine constraints that limit provision of services, and offer a potentially powerful currency for evaluation. Context : A critical zone perspective extends the context of ecosystem services by expressly addressing how the physical structure of the terrestrial Earth surface (e.g., parent material, topography, and orography) provides a broader spatial and temporal template determining the coevolution of physical and biological systems that result in societal benefits. Constraints : The rates at which many ecosystem services are provided are fundamentally constrained by rate-limited critical zone processes, a phenomenon that we describe as a conceptual “supply chain” that accounts for rate-limiting soil formation, hydrologic partitioning, and streamflow generation. Currency : One of the major challenges in assessing ecosystem services is the evaluation of their importance by linking ecological processes to societal benefits through market and nonmarket valuation. We propose that critical zone processes be integrated into an evaluation currency, useful for valuation, by quantifying the energy flux available to do thermodynamic work on the critical zone. In short, characterization of critical zone processes expands the scope of ecosystem services by providing context, constraints, and currency that enable more effective management needed to respond to impacts of changing climate and disturbances.

Transport and Retention of Polyvinylpyrrolidone-Coated Silver Nanoparticles in Natural Soils

Abstract: Transport and retention of polyvinylpyrrolidone-coated silver nanoparticles (PVP-AgNPs) were examined in water-saturated columns packed with three types of natural soils: a clay loam red soil (RS), a sandy loam fluvo-aquic soil (FS), and a loam huangni soil (HS). Specific factors investigated included monovalent K+ and divalent Ca2+ concentrations and the coupled effect of Ca2+ with humic acid (HA) at a PVP-AgNP concentration of 10 mg L−1 and Darcy velocity of 0.182 cm min−1. Our results showed that transport of PVP-AgNPs decreased with increasing K+ and Ca2+ concentrations, primarily due to enhanced aggregation of PVP-AgNPs and reduced repulsive interactions between PVP-AgNPs and soil grains. Consistent with the Derjaguin–Landau–Verwey–Overbeek theory, retention of PVP-AgNPs was more pronounced in the presence of Ca2+ than K+. However, the presence of HA significantly weakened the mobility of PVP-AgNPs with increasing Ca2+ concentration, probably due to increased aggregation of PVP-AgNPs (forming a HA–Ca–PVP-AgNPs complex). Irrespective of solution chemistry, the mobility of PVP-AgNPs in all three soils remained in the order of RS < FS < HS. Principal components analysis indicated that the mobility of PVP-AgNPs was positively correlated with pH, cation exchange capacity, and soil organic matter content and was negatively correlated with Fe oxide content and specific surface area of the soils. Overall, our findings highlight the dominant role of soil physicochemical properties in the mobility of Ag nanoparticles in natural soils and the importance of investigating these factors to better understand the fate of engineered nanoparticles in natural environments at the point of disposal and protecting groundwater resources.

Calibration and Evaluation of a Frequency Domain Reflectometry Sensor for Real-Time Soil Moisture Monitoring

Abstract: Soil spatial heterogeneity poses a challenge to accurate soil moisture determination. Remote sensing, in particular, using sensors that acquire data at microwave frequencies, is being used to overcome this challenge. In situ soil moisture monitoring can be used to validate remotely sensed surface soil moisture estimates and as inputs for agronomic and hydrologic models. Nine in situ soil moisture stations were established in Manitoba (Canada) and instrumented with Stevens Hydra Probes. The sensors were installed in triplicate with vertical orientation at the surface and with horizontal orientation at the 5-, 20-, 50-, and 100-cm depths. To ensure accuracy of the measured soil moisture, both laboratory and field calibrations were conducted. These calibrated soil moisture values were compared with the probe default values and those generated using published calibrations. Overall, the results showed that the field calibration was superior (coefficient of determination r 2 of 0.95) to the laboratory calibration ( r 2 of 0.89). In addition, coarse-textured sites generally performed better than the fine-textured, high cation exchange capacity (CEC) sites. At the Kelburn site with high clay and CEC, the use of field calibration reduced the root mean square error from 0.188 to 0.026 m3 m−3. However, at the low clay and CEC Treherne site, gains in accuracy were minimal, about 0.005 m3 m−3. The laboratory calibration consistently underestimated soil moisture at all the evaluation sites, whereas both Topp and Logsdon calibrations overestimated soil moisture.

Spatial and Temporal Dynamics of Hillslope-Scale Soil Moisture Patterns: Characteristic States and Transition Mechanisms

Abstract: Recent advances in wireless sensor technology allow monitoring of soil moisture dynamics with high temporal resolution at varying spatial scales. The objectives of this study were to: (i) develop an efficient strategy for monitoring soil moisture dynamics at the hillslope scale using a wireless sensor network; and (ii) characterize spatial patterns of soil moisture and infer hydrological processes controlling the dynamics of such patterns, using a method of analysis that allows the identification of the relevant hydrological dynamics within large data sets. We combined soil hydrological and pedological expertise with geophysical measurements and methods from digital soil mapping for designing the monitoring setup for a grassland hillslope in the Schafertal catchment, central Germany. Hypothesizing a wet and a dry soil moisture state to be characteristic of the spatial pattern of soil moisture, we described the spatial and temporal evolution of such patterns using a method of analysis based on the Spearman rank correlation coefficient. We described the persistence and switching mechanisms of the two characteristic states, inferring the local properties that control the observed spatial patterns and the hydrological processes driving the transitions. The spatial organization of soil moisture appears to be controlled by different processes in different soil horizons, and the topsoil’s moisture does not mirror processes that take place within the soil profile. The results will help to improve conceptual understanding for hydrological model studies at similar or smaller scales and to transfer observation concepts and process understanding to larger or less instrumented areas.

Identifying the Functional Macropore Network Related to Preferential Flow in Structured Soils

Abstract: Understanding the processes and mechanisms that control preferential flow in soils in relation to the properties of their structures is still challenging since fast flow and transport occur in a small fraction of the porosity, that is, the functional macropore network, making it difficult to image and characterize these processes at decimeter scales. The aim of the paper was therefore to propose a new image acquisition and analysis methodology to characterize preferential flow at the core scale and identify the resulting active macropore network. Water infiltration was monitored by a sequence of three-dimensional images (taken at 5-, 10-, or 15-min intervals) with an X-ray scanner that allows very fast acquisitions (10 s for a 135-mm diameter). A simultaneous dye tracer experiment was also conducted. Water infiltration was then imaged at each acquisition time by the voxels impacted by water during infiltration, named the water voxels. The number of times a voxel was impacted by water during the experiment was converted into data reflecting the water detection frequency at the given position in the soil column, named the local detection frequency. Compared with dye staining, the active macropore network was defined by macropores in which water voxels were the most frequently detected during the experiment (local detection frequency above 65%). The geometric properties of this active network, such as the connectivity, were significantly different from those of the total structure. This image processing methodology coupled to dynamic acquisitions can be used to improve the analysis of preferential flow processes related to soil structures at the core scale.

Effects of CT Number Derived Matrix Density on Preferential Flow and Transport in a Macroporous Agricultural Soil

Abstract: 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.

Management Impacts on Soil Organic Matter of Tropical Soils

Abstract: Increased soil organic matter (SOM) improves the cation exchange capacity of tropical weathered soils, and liming is required to achieve high yields in these soils. Despite a decrease in SOM in the short term, liming may increase SOM with time by improving cation chemical bonds with soil colloids. Soil C may also be increased in high dry matter input cropping systems. We evaluated C changes in a Typic Rhodudalf as affected by four production systems with increasing residue inputs, with or without limestone or silicate. Soil use intensification by increasing the number of species in rotation as well as acidity remediation resulted in higher plant residue production. Introducing a green manure or a second crop in the system increased plant residue by 89% over fallow, but when a forage crop was used, plant residues more than doubled. Soil acidity amelioration increased plant residue deposition by 21% over the control. The introduction of a forage crop increased labile SOM and C contents in the particulate fraction, and lime or silicate application led to increases in the more stable SOM fraction. High amounts of plant residues (>70 Mg ha−1 in 5 yr) are effective in raising soil labile C, but the alleviation of soil acidity results in increased soil stable C irrespective of crop rotations in tropical weathered soils, and in this case plant residue deposition can be lower. Lime and silicate are equally effective in alleviating soil acidity and increasing soil C, probably due to the formation of cation bridges with soil colloids.

Towards Retrieving Soil Hydraulic Properties by Hyperspectral Remote Sensing

Abstract: In this study, we developed spectrotransfer functions (STFs) that relate soil hydraulic properties (SHPs) to spectral reflectance values to estimate hydraulic parameters of the Mualem–van Genuchten (MvG) model. We investigated the general potential of airborne as well as space-borne remote sensors to retrieve MvG hydraulic parameters of a bare soil agricultural field. Based on the ASD full spectrum (Scenario I), simple spectral signatures were generated mimicking the hyperspectral EnMAP sensor (Scenario II), and the multispectral Sentinel-2 sensor (Scenario III). A stepwise multiple linear regression method was used for each scenario to derive STFs. We further tested laboratory- and soil-map-based HYPRES and Rosetta pedotransfer functions (PTFs) to parameterize MvG parameters and thus provide soil water characteristics and hydraulic conductivity functions in the region. The best results were obtained for Scenarios I and II, with similar R 2 values for shape parameters α* and n and the lognormal saturated hydraulic conductivity ( K s*). The R 2 values were highest for K s* in Scenarios I and II (0.58 and 0.57, respectively). The R 2 values for α* and n were 0.30 and 0.34 in Scenario I and 0.39 and 0.31 in Scenario II, respectively. In all scenarios, the lowest R 2 values were obtained for saturated water content (θs), with values around 0.10 for Scenarios I and II and almost zero in Scenario III. Compared with HYPRES and Rosetta PTFs, the spectral approach performed reasonably well in terms of predicting soil water retention characteristics and unsaturated hydraulic conductivity. These findings suggest that spectral reflectance data provide a promising indirect and quick method for large-scale parameter estimation.

Physical Properties of Organic Soil: Adapting Mineral Soil Concepts to Horticultural Growing Media and Histosol Characterization

Abstract: Growing media are used in a broad range of applications, for which special consideration must be given to their physical and hydraulic character. Because they are relatively fragile, dominantly consisting of dried plant remnants, their preparation, processing, and handling before potting affect their properties. This is complicated by their subsidence and decomposition during use, which leads to a reduction of their initial bulk volume. Organic growing media show many similarities to Histosols because of the common botanical origin of some of their components. For both growing media and Histosols, classical concepts and values related to physical properties like air-filled porosity, bulk density, available water, hydraulic conductivity, gas diffusivity, and field capacity need to be adapted to reflect distinct differences in their composition, structure, and stability compared with mineral soils. Their use in containers with a variety of shapes and sizes influences water and air storage and exchange as well. They can subside extensively as they undergo decomposition. They shrink. Hence, the range of values observed for the physical properties of organic media differs from those of mineral soils. The methods to be used for measuring such properties must be adapted to that specific context of use and to account for their fragile and dynamic nature. Finally, specific norms to guide substrate manufacturing and for diagnosis of plant growth problems have been derived specifically and should be used in such a situation.

Potential Use of Biochar in Growing Media

Abstract: One of the greatest challenges in the growing media (GM) industry is sourcing superior quality, inexpensive, readily available, and environmentally friendly constituents. Biochar has been widely considered for its potential use in agriculture, in the energy sector, and for environmental purposes, but little attention has been paid to the use of biochar in GM. The objective of this study was to evaluate the potential use of biochar as an alternative for aggregates (e.g., perlite) and peat moss in the GM industry. A laboratory experiment was conducted comparing five organic substrates composed of different combinations of peat moss, perlite, and three types of biochar. The main physical and chemical properties of the biochars and organic substrates were measured. A leaching experiment was also performed to evaluate the nutrient-holding capacity of the investigated substrates. Biochar showed a good potential for replacement of perlite and, to a lesser extent, peat moss in GM. Biochar increases cation-exchange capacity (CEC) and pH, and it decreases nutrient leaching (11% reduction) in GM. Biochar affected the physical properties of GM, and this was mainly related to its particle-size distribution (PSD). In spite of all of its benefits, biochar is still not a standardized product, and its properties may differ from one source to another. However, the GM industry requires high quality, homogeneous, and consistent components. To define suitable properties for biochar products in the GM industry, a standardization program should be put in place. Economically, biochar presents a greater potential in the replacement of aggregates than peat moss. Special attention should be paid to the presence of fine dust particles in some biochar.

Quantifying Topographic and Vegetation Effects on the Transfer of Energy and Mass to the Critical Zone

Abstract: Critical zone evolution, structure, and function are driven by energy and mass fluxes into and through the terrestrial subsurface. We have developed an approach to quantifying the effective energy and mass transfer (EEMT, MJ m−2 yr−1) to the subsurface that accounts for local variations in topography, water and energy balances, and primary production. Our objectives were to quantify how (i) local topography controls coupled energy and water transfer to the subsurface, and (ii) vegetation effects on local-scale evapotranspiration and primary production controls of energy and mass transfer to the critical zone, both at the pedon- to hillslope-scale resolution, in the context of quantifying controls on EEMT. The model was tested across a semiarid environmental gradient in southern Arizona, spanning desert scrub to mixed conifer ecosystems. Data indicated clear variations in EEMT by topography, via both aspect and local water redistribution, and with current vegetative cover. Key findings include: (i) greater values of EEMT on north-facing slopes in a given elevation zone, with a north-facing aspect equivalent to an ∼300-m elevation gain; (ii) a power law relationship between aboveground biomass and EEMT, with disturbance in the form of stand-replacing wildfire substantially reducing estimates of EEMT; and (iii) improved correlation of EEMT to pedon-scale variations in critical zone structure with EEMT values that include topography. Incorporating greater levels of environmental variation and complexity presents an improved approach to estimating the transfer of energy and mass to the subsurface, which is important to our understanding of critical zone structure and function.

Characterization of Water Retention Curves for a Series of Cultivated Histosols

Abstract: Water retention curves are essential for the parameterization of soil water models such as HYDRUS. Although hydraulic parameters are known for a large number of mineral and natural organic soils, our knowledge on the hydraulic behavior of cultivated Histosols is rather limited. The objective of this study was to derive characteristic water retention curves for a large cultivated peatland with lettuce ( Lactuca sativa L.) and vegetable farming in southern Quebec, Canada. A comparison showed that the van Genuchten model fits better to the water retention data obtained with a Tempe pressure cell experiment than the Groenevelt–Grant model in terms of residual sum of squares; however, the difference in performance was quite small due to the high number of iterations used for fitting. Finally, an agglomerative cluster analysis of 85 peat samples allowed us to define two distinct water retention curves, where the first water retention curve described samples of relatively shallow ( 0.3 g cm −3 , and the second curve characterized samples of the deepest (depth 150–230 cm) Histosols with an organic content of up to 0.97 and a bulk density >0.3 g cm −3 , which are the soils that suffered a more dramatic transformation as a result of agriculture. This characterization allows for a multitude of applications, including parameterization of the HYDRUS model for soil water movement, and presents an essential tool for the optimization of water management in cultivated peatlands.

Determining Soil Ice Contents during Freezing and Thawing with Thermo-Time Domain Reflectometry

Abstract: Determining soil ice content during freezing and thawing is important and challenging for both engineering and environmental issues. The thermo-time domain reflectometry (T-TDR) probe, which can monitor unfrozen soil water content and soil thermal properties simultaneously, has the potential to measure ice content in partially frozen soils. The objective of this study was to identify an optimum heat application strategy for measuring soil thermal properties with T-TDR probes in partially frozen soil while minimizing ice melting during the process. The optimized heating schemes were then applied for monitoring soil ice content dynamics during freezing and thawing. The results indicated that the heat pulse method failed at temperatures between −5 and 0°C because of temperature field disturbances from latent heat of fusion. When soil temperatures were ≤ −5°C, ice melting during heat pulse applications could be limited effectively with a combination of 60-s heat-pulse duration and 450 J m −1 heating strength, or a 90-s heat-pulse duration and heating strength of 450 to 900 J m −1 . With the optimized heating scheme, T-TDR probes were able to measure soil ice content changes at ≤ −5°C during freezing and thawing, and the errors were within ±0.05 m 3 m −3 in sandy loams and within ±0.1 m 3 m −3 in soils with high clay content. At temperatures between −5 and 0°C, soil ice contents could not be measured accurately with the heat-pulse method directly, but they could be estimated coarsely from water content before freezing, TDR measured unfrozen water content, and T-TDR measured total water content at temperatures below −5°C.

Field observations of regional controls of soil hydraulic properties on soil moisture spatial variability in different climate zones

Abstract: Knowledge of soil moisture spatial variability (SMSP) is important for many practical reasons. However, a significant gap still exists in our understanding of different controls on SMSP, especially the roles of soil hydraulic parameters due to their limited availability. Although modeling approaches have been used to assess the impacts of those parameters on SMSP, they have led to inconsistent findings. In this study, soil moisture data from Utah (5 yr) and the US Southeast (2 yr) were obtained from the Soil Climate Analysis Network (SCAN), along with estimated van Genuchten parameters. The method of mean relative difference (MRD) of soil moisture was used as a diagnostic tool for assessing different climate and soil controls on SMSP. The results show that instead of being controlled by climate variables (e.g., precipitation and potential evapotranspiration) as traditionally believed at regional scales (∼10 5 km 2 ), MRD is mainly dependent on soil hydraulic properties. In Utah with a drier climate, the residual soil moisture content (θ r ) is the dominant control on MRD, followed by the saturated soil moisture content (θ s ). With wetter climates in the US Southeast, the impacts of θ r and θ s on MRD become comparable, mostly due to the high correlation between θ r and θ s in this region, and there exists a nonlinear negative relationship between MRD and the parameter n for coarser soils, indicated by larger n values tending to have lower MRDs. The findings of this study have important implications for verifying remotely sensed moisture data and initializing and parameterizing regional land surface and climate models.

Estimation of Catchment-Scale Soil Moisture Patterns Based on Terrain Data and Sparse TDR Measurements Using a Fuzzy C-Means Clustering Approach

Abstract: Accurate characterization of spatial soil moisture patterns and their temporal dynamics is important to infer hydrological fluxes and flow pathways and to improve the description and prediction of hydrological models. Recent advances in ground-based and remote sensing technologies provide new opportunities for temporal information on soil moisture patterns. However, spatial monitoring of soil moisture at the small catchment scale (0.1–1 km 2 ) remains challenging and traditional in situ soil moisture measurements are still indispensable. This paper presents a strategic soil moisture sampling framework for a low-mountain catchment. The objectives were to: (i) find a priori a representative number of measurement locations, (ii) estimate the soil moisture pattern on the measurement date, and (iii) assess the relative importance of topography for explaining soil moisture pattern dynamics. The fuzzy c-means sampling and estimation approach (FCM SEA) was used to identify representative measurement locations for in situ soil moisture measurements. The sampling was based on terrain attributes derived from a digital elevation model (DEM). Five time-domain reflectometry (TDR) measurement campaigns were conducted from April to October 2013. The TDR measurements were used to calibrate the FCM SEA to estimate the soil moisture pattern. For wet conditions the FCM SEA performed better than under intermediate conditions and was able to reproduce a substantial part of the soil moisture pattern. A temporal stability analysis shows a transition between states characterized by a reorganization of the soil moisture pattern. This indicates that, at the investigated site, under wet conditions, topography is a major control that drives water redistribution, whereas for the intermediate state, other factors become increasingly important.

Managing Phosphorus Leaching in Mid-Atlantic Soils: Importance of Legacy Sources

Abstract: Application of phosphorus (P) inputs to soils saturated with legacy P can significantly increase the risk of P leaching and deteriorate water quality. Our objectives were to quantify the effect of soil type and extent of P saturation on P leaching and determine the suitability of a rapid, inexpensive soil P saturation test for use in P leaching risk assessment protocols. We collected 18 undisturbed lysimeters (30-cm diameter, 50 cm deep), using a tractor-mounted corer, from three typical mid-Atlantic soils in Delaware, where P leaching is a concern. The soils were selected on the basis of Mehlich-3 P saturation ratio (M3-PSR) of optimum ( 0.15) thresholds. Lysimeters were irrigated with the equivalent of 50 mm of water each week for a total of 16 wk. Concentrations of dissolved reactive P (DRP) and total P in leachate were not significantly different between optimum and environmental M3-PSR lysimeters before fertilizer application (Weeks 1–8). However, concentrations of DRP and total P significantly increased after fertilizer application at 85 kg P ha −1 (Weeks 9–16) in the environmental M3-PSR lysimeters. Among three soils in both M3-PSR categories, concentrations and loads of P leached were higher from the Matapeake silt loam and Pocomoke sandy loam due to preferential leaching that limited contact of flowing water and P with the soil P-fixing constituents (Fe, Al, and Ca), while P leaching was lower from Woodstown sandy loam due to matrix flow that resulted in greater interaction of water and P with the soil constituents. These results provide clear evidence of a greater risk of P leaching from P-saturated soils with preferential flow pathways and show that the M3-PSR along with information about preferential flow pathways should be used to predict the risk of P leaching.

Diffusive–Dispersive and Reactive Fronts in Porous Media: Iron(II) Oxidation at the Unsaturated–Saturated Interface

Abstract: Diffusive–dispersive mass transfer is important for many groundwater quality problems as it drives the interaction between different reactants, thus influencing a wide variety of biogeochemical processes. In this study, we performed laboratory experiments to quantify O 2 transport in porous media, across the unsaturated–saturated interface, under both conservative and reactive transport conditions. As reactive system we considered the abiotic oxidation of Fe 2+ in the presence of O 2 . We studied the reaction kinetics in batch experiments and its coupling with diffusive and dispersive transport processes by means of one-dimensional columns and two-dimensional flow-through experiments, respectively. A noninvasive optode technique was used to track O 2 transport into the initially anoxic porous medium at highly resolved spatial and temporal scales. The results show significant differences in the propagation of the conservative and reactive O 2 fronts. Under reactive conditions, O 2 , continuously provided from the atmosphere, was considerably retarded due to the interaction with dissolved Fe(II), initially present in the anoxic groundwater. The reaction between dissolved O 2 and Fe 2+ led to the formation of an Fe(III) precipitation zone in the experiments. Reactive transport modeling based on a kinetic PHREEQC module tested in controlled batch experiments allowed a quantitative interpretation of the experimental results in both one- and two-dimensional setups.

Within-Year Changes in Hydraulic Properties of a Shallow Entisol in Farmland and Forestland

Abstract: For regions with shallow soils underlain by fractured parent rocks, it is important to understand the characteristics of soil water retention and hydraulic conductivity. This knowledge could support the development of strategies to manage water for crops. A farmland and a forestland with thin Entisols, located in hilly central Sichuan on the Yangtze River's upper reaches, were chosen as experimental fields. A variety of parameters related to hydraulically effective macroporosity (pore radius > 125 mu m), the single-porosity van Genuchten model, and the dual-porosity bi-exponential (BE) model for water retention characteristics were determined. Tension infiltration data showed that soil macropores were the main pores (accounting for 87.9-99.7%) contributing to fast drainage at all tested soil depths under both land uses in the rainy summer, in spite of their very low percentage of the total porosity (0.61-3.06%). The available soil water content, determined as the drainable soil textural porosity using the bi-exponential model (theta(txt-BE)), was found to be very low (0.032-0.091 m(3) m(-3)) and negatively (P

Water-Entry Pressure and Friction Angle in an Artificially Synthesized Water-Repellent Silty Soil

Abstract: Water-repellent soils possess unique hydraulic and mechanical behaviors that confer large potential for their use in geotechnical applications because particle-scale surface-wettability characteristics significantly influence macroscale manifestations. This study examined the hydraulic and mechanical behavior of an artificially created water-repellent silty soil with four different concentrations of a reactive organo-silane solution. A series of laboratory tests was performed that included measurements of water-droplet penetration time (WDPT), water-entry pressure (WEP), flow rate, and friction angle. Experimental results showed that the artificial treatment produced a unique range of porosity values depending on the concentration and that the WDPT and WEP increased with decreasing porosity and increasing concentration. A gravimetric fraction of 40% water-repellent particles was sufficient for bulk soils to exhibit water repellency. The flow rate of specimens with a high concentration of reactive organo-silane tended to be high due to the resulting high degree of saturation on water permeation. In contrast, friction angles tended to decrease with increasing concentration of organo-silane solution under dry conditions and remained quasi-constant on wetting, regardless of the degree of saturation.

Water Flow and Solute Transport in Unsaturated Sand—A Comprehensive Experimental Approach

Abstract: S.K. Kumahor,* G.H. de Rooij, S. Schluter, and H.-J. Vogel 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 wellknown 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.

Simulated Preferential Water Flow and Solute Transport in Shrinking Soils

Abstract: We present follow-up work to previous work extending the classical rigid (RGD) approach formerly proposed by Gerke and van Genuchten, to account for shrinking effects (SHR) in modeling water flow and solute transport in dual-permeability porous media. In this study we considered three SHR scenarios, assuming that aggregate shrinkage may change either: (i) the hydraulic properties of the two pore domains, (ii) their relative fractions, or (iii) both hydraulic properties and fractions of the two domains. The objective was to compare simulation results obtained under the RGD and the SHR assumptions to illustrate the impact of matrix volume changes on water storage, water fluxes, and solute concentrations during an infiltration process bringing an initially dry soil to saturation and a drainage process starting from an initially saturated soil. For an infiltration process, the simulated wetting front and the solute concentration propagation velocity, as well as the water fluxes and water and solute exchange rates, for the three SHR scenarios significantly deviated from the RGD. By contrast, relatively similar water content profiles evolved under all scenarios during drying. Overall, compared to the RGD approach, the effect of changing the hydraulic properties and the weight of the two domains according to the shrinkage behavior of the soil aggregates induced a much more rapid response in terms of water fluxes and solute travel times, as well as a larger and deeper water and solute transfer from the fractures to the matrix during wetting processes.

A New Method for In Situ Measurements of Oxygen Isotopologues of Soil Water and Carbon Dioxide with High Time Resolution

Abstract: The oxygen isotope composition of atmospheric CO2 (δ18Oac) can be used to disentangle ecosystem component CO2 fluxes, such as soil respiration and plant assimilation, because the δ18O composition of different water pools is transferred to CO2 during isotopic equilibration. The oxygen isotope exchange between CO2 and water in soils has been widely studied with theoretical models, but experimental data are scarce, albeit indispensable to characterization of the role of soils in determining δ18Oac. Here, we present a new methodology to monitor the δ18O of soil CO2 (δ18Osc) and of soil water (δ18Osw) in situ at varying soil water content. Infrared laser spectroscopy was combined with gas-permeable polypropylene (PP) tubing installed at different depths in a sand column. The permeable tubing did not lead to any isotopic fractionation and was suitable for combined δ18Osc and δ18Osw measurements. Soil water became gradually 18O enriched from the top of the sand over several days. Measured and δ18Osc simulated with the model MuSICA indicated incomplete CO2–H2O isotopic equilibrium. Irrigation of the sand column with tapwater resulted in a temporary reset of δ18Osw along the soil column, while δ18Osc was only influenced when the enzyme carbonic anhydrase was added to the irrigation water. Our study demonstrates that δ18Osc and δ18Osw can now be monitored in situ and online with high time resolution with minimum disturbance. With this new tool at hand, research into the oxygen isotope exchange between soil water and CO2 in natural soils has the potential to advance to a new stage and help to constrain the atmospheric CO2 budget.

Three-Dimensional Printing of Macropore Networks of an Undisturbed Soil Sample

Abstract: Macropore systems predominantly determine rapid water flow and solute transport in undisturbed soils. Repeated experiments are needed to investigate the relationship between the nature of the macropore network and the resulting water and solute transport under different hydraulic initial and boundary conditions. However, the large heterogeneity in soil macropore network structures renders each soil sample unique and multiple identical samples impossible. In addition, the fragile nature of soil strongly limits the possible number of repeated experiments on one individual sample. Micromodels that mimic the precise shape and location of the macropores in undisturbed soil are therefore necessary to allow repeated experiments. In this study we investigated whether such micromodels can be obtained using contemporary three-dimensional (3-D) printing techniques and materials. We used X-ray computed tomography to digitize the 3-D macropore structure of an undisturbed soil sample. We printed a subsection of this macropore system in five different materials. Four out of the five investigated materials had essential parts of their macropore system clogged with residual printing or printing-aid material. Only one reprint, namely the prime-gray sample that was printed using stereo lithography, exhibited no pore clogging and had the largest hydraulic conductivity of all investigated reprints. Prime gray showed subcritical water repellency with a medium contact angle of approximately 65°, which is similar to contact angles found in natural soil. We conclude that the 3-D printing of undisturbed soil macropore systems is in principle possible with contemporary 3-D printing systems.

Finite Analytic Method for Solving the Unsaturated Flow Equation

Abstract: The unsaturated zone plays an important role in groundwater recharge, discharge, and the ecological environment. Mathematical models are essential to the investigations of flow processes in the unsaturated zone. The governing equations of the mathematic models for unsaturated flow are nonlinear since their unsaturated hydraulic parameters of the equations depend on their solutions (i.e., pressure or moisture content). The nonlinearity can become very strong with flow in relatively dry soils, and the equation has hyperbolic characteristics. Therefore, the traditional numerical methods, such as the finite difference method and the finite element method, could lead to the numerical oscillation, dispersion, and the divergence of the solution if improper time and space steps were selected. In this study, a finite analytic method (FAM) to solve Richards’ equation was developed. The basic idea of the FAM is the incorporation of the analytic solution in a small local element to formulate the algebraic representation of the partial differential equation of the unsaturated flow. Therefore, the finite analytic numerical method can effectively control the numerical oscillation and dispersion. The convergence and stability of finite analytic numerical scheme are then proven by a rigorous mathematical analysis. Numerical experiments are then used to show that FAM is highly accurate by comparing its results with those obtained using analytical solutions by a modified Picard finite difference method. In addition, we also demonstrate that FAM reproduces results of laboratory experiments. Therefore, it can be considered an appropriate simulation method for the unsaturated flow.

Fate of Effluent-Borne Nitrogen in the Mounded Drainfield of an Onsite Wastewater Treatment System

Abstract: Onsite wastewater treatment systems (OWTS), commonly known as septic systems, are increasingly recognized as a potential source of nitrogen in the shallow groundwater. Our objective was to investigate the effect of hydrological and biogeochemical factors in the vadose zone on the fate of effluent-borne N in the drainfields of a drip-dispersal OWTS. Three lysimeters (152.4 cm long, 91.4 cm wide, and 91.4 cm high) were constructed using pressure-treated wood to mimic OWTS drainfields. Each lysimeter had three distinct layers of gravel–sand mixture, soil, and commercial sand. A drip tube, which was covered with commercial sand before planting St. Augustine grass ( Stenotaphrum Trin.) on the top and sides of the lysimeters, dispersed 9 L of septic tank effluent (STE) per day on top of the stacked layers. Each lysimeter was instrumented with 10 multi-probe sensors to determine the water content, electrical conductivity (EC), and temperature in the center and sides of sand and soil layers. Leachate samples were collected over 67 events, which consisted of one sample every 24 h for 15 d ( n = 15) and weekly flow-weighted composite samples ( n = 52). In all events, the pH, EC, and chloride were lower in the leachate than STE. Daily multi-probe data showed that EC was greater in the center than sides of the lysimeters due to more STE interaction. Sensor water content data were used to calculate water filled porosity (WFP), which was greater in the soil (0.55–0.9) than sand (0.07–0.32) due to the textural differences. Mean total N was 70 mg L−1 in the STE, which reduced to 27.4 mg L−1 in the leachate likely due to the denitrification in the soil layer. The dominance of NO x –N in the leachate (61%) as compared to STE (0.6%) was attributed to the nitrification in the sand layer. Higher proportion of organic N in the leachate (39%) than STE (16%) suggests that organic N was mobile in the vadose zone and leached below the drainfields. We conclude that hydrological and biogeochemical controls in the vadose zone play an important role in N transformations and transport of NO x –N and organic N below OWTS drainfields.

Modeling effects of canopy and roots on soil moisture and deep drainage

Abstract: The spatial variability of rainfall at the soil surface and tree roots in a coastal sand dune forest was investigated. Various scenario simulations were further conducted using calibrated HYDRUS models to assess the effects of these vegetation-related variability patterns on soil moisture dynamics and the water balance. Vadose zone soil moisture and groundwater recharge in forested ecosystems can be influenced by canopy architecture and root systems. Spatial distributions of surface rainfall and tree roots were observed at the canopy and intercanopy areas in a coastal sand dune forest of subtropical Australia. We further explored the effects of these vegetation-associated spatial patterns on the spatiotemporal dynamics of soil moisture and deep drainage in this system using calibrated HYDRUS models. Simulated soil moisture and deep drainage were found higher at the intercanopy area than the canopy area due to the reinforced effects of rainfall interception and root water uptake. Spatial analysis of deep drainage at various locations across the canopy–intercanopy transect resulted in an average deep drainage of 1347 mm yr−1 (65% of annual gross rainfall), with 68% of total deep drainage occurring at the intercanopy area. The scenario analyses indicate that the spatial variability of rainfall at the soil surface is not important for the larger scale groundwater balance in our coastal sand dune forest. However, deep drainage was underestimated by 130 to 162 mm (9.7–12.0% drop compared with baseline scenario) as a consequence of uniform representation of heterogeneous root systems in one- or axisymmetric three-dimensional HYDRUS models. Our HYDRUS simulations indicated that translating the hydrological effects of the two-dimensional tree structure (rainfall redistribution and heterogeneous roots) to a one-dimensional lumped vertical conceptualization in coastal sand dune forests needs to be undertaken with caution.

Mobilization of Microspheres from a Fractured Soil during Intermittent Infiltration Events

Abstract: The vadose zone filters pathogenic microbes from infiltrating water and con sequently protects the groundwater from possible contamination. In some cases, however, the deposited microbes may be mobilized during rainfall and migrate into the groundwater. We examined the mobilization of microspheres, surrogates for microbes, in an intact core of a fractured soil by intermittent simulated rainfall. Fluorescent polystyrene microspheres of two sizes (0.5 and 1.8 o m) and Br − were first applied to the core to deposit the micr ospheres, and then the core was subjected to three intermittent infil tration events to mobilize the deposited microspheres. Collecting effluent samples through a 19-port sampler at the base of the core, we found that water flowed through only five ports, and the flow rates varied among the ports by a factor of 12. These results suggest that flow paths leading to the ports had different permeabilities, partly due to macropores. Although 40 to 69% of injected microspheres were retained in the core during their application, 12 to 30% of the retained microspheres were mobilized during three intermittent infiltration events. The extent of microsphere mobilization was greater in flow paths with greater permeability, which indicates that mac ropores could enhance colloid mobilization during intermittent infiltration events. In all ports, the 1.8-o m microspheres were mobilized to a greater extent than the 0.5-o m microspheres, suggesting that larger colloids are more likely to mobilize. These results are useful in assessing the potential of pathogen mobilization and colloid-facilitated transport of contaminants in the subsurface under natural infiltration events.

Hysteresis in the Soil Water Retention of a Sand–Clay Mixture with Contact Angles Lower than Ninety Degrees

Abstract: Soil water retention curves (SWRCs) are usually obtained on the assumption small water menisci spread in the surface of particles yielding contact angles near zero degrees. While this is true at high water contents, where pores are filled by water or a film of water covers the particles, it is less likely for drier states, where the adhesion of water menisci to the surface of the particles is controlled by the nature of the particle surface (chemistry and surface roughness) and the presence of organic matter. Here, we investigate hysteretic effects in the relation between water content and soil particle wettability versus suction for model samples following drying and wetting paths. A comparison is done for a model material (mixture of sand and clay) with and without water repellent substances, being the samples with water repellent substances in a subcritical water repellent state (contact angles <90°). Wettability was manipulated by treatment with organic acids that mimic the chemistry of natural water repellent substances. Suction was measured directly with a high suction tensiometer and wettability via contact angles by the sessile drop method. The results revealed lower water retention and greater contact angles for the subcritical water repellent samples, following both drying and wetting paths. Hysteresis was present in the relation between the contact angles and suction for the subcritical water repellent samples.

Using X-ray Computed Tomography to Describe the Dynamics of Nitrous Oxide Emissions during Soil Drying

Abstract: Water in soil is known to be a key factor for controlling N2O emissions because N2O is mainly produced by denitrification in anoxic environments. In this study, we proposed a methodology to image the water and soil structure of a soil sample with X-ray computed tomography while controlling the hydric state and monitoring N2O fluxes. We used a multistep outflow system to apply two wetting–drying cycles to an undisturbed soil. The soil core was scanned with coarse-resolution X-ray computed tomography, one time during wetting and several times during drying, to measure quantitative and qualitative indicators of the pore network. Nitrous oxide emissions were higher during the first (C1) than during the second (C2) wetting–drying cycle for both the wetting and the drying phases. Fluxes increased quickly after the beginning of the drying phase to reach a peak after 5 h. Differences in the intensity of N2O emissions between the two cycles were attributed to differences in the water saturation, air-phase connectivity, and relative gas diffusion coefficient, which led to more or less N2O production, consumption, and entrapment in the soil. The speed of the N2O emissions at the beginning of the drying phase depended on the rate of increase of the air-filled pore volume and connectivity, and was especially well described by the estimated relative gas diffusion coefficient. Parameters of the soil structure were not able to explain completely the intensity of N2O emissions during drying; N2O production and consumption factors were also involved.