bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Variable Horizon Thickness on Solute Transport
Authors: van Wesenbeeck I. J. , Kachanoski R. G.
Published: July 2010

Significant variability of solute mass recovery, solute velocity, and dispersion occur in layered field soils. The effect of horizontally variable soil horzion thickness on vertical solute movement has not been examined in detail. The objective of this study was to determine the effect of varying B horizon thickness on solute transport. Steady‐state transport experiments were conducted on a Fox sand (a fine‐loamy over sandy or sandy‐skeletal, mixed, mesic Typic Hapludalf) in Ontario, Canada. A uniform pulse of KCl was applied at the soil surface on a 65‐m2 area and subsequently leached through the soil profile using a drip irrigation system and a constant surface flux density of water. After 13.1 cm of water had been applied, resident Cl− concentration distributions were measured by excavating a 10‐m‐long trench and sampling vertically every 0.1 m to a depth of 1.5 m and horizontally every 0.2 m. A total of eight parallel trenches, 0.15 m apart, were sampled. Moment analysis was used to calculate solute recovery, mean solute travel depth, and solute travel depth variance for each of the individual vertical samplings. Results indicate significant lateral movement of tracer resulting in greater recovery of tracer in deep B horizon tongue areas. The recovery and movement of solutes in this soil was largely controlled by the spatial pattern of B horizon depth, and the spatial variance structure of B horizon thickness was closely related to the scale dependence of in situ solute transport. The results suggest that the horizontal variability of horizon thickness should be taken into account for monitoring and modeling purposes in this soil.
Anion Exclusion‐Pore Water Velocity Interaction Affecting Transport of Bromine through an Oxisol
Authors: Melamed R. , Jurinak J. J. , Dudley L. M.
Published: July 2010

The transport of anions through soils is affected by both chemical and physical processes. We investigated how adsorbed phosphate affected the mobility of Br, at different pore water velocities (PWV), through saturated Oxisol soil columns. The PWVs were 2.9, 8.7, and 48.1 cm h−1. The addition of 16.2, 32.4, and 48.6 mmol P kg−1 phosphate reduced the pH of the point of zero charge from 6.0 (control) to 5.1, 4.6, and 4.3, respectively, as measured by potentiometic titration. The negative surface charge of the soil increased by 0.7 mmolc mmol−1 P added. The mobility of Br increased with P treatment because of Br exclusion induced by the P generated negative surface charge. At the highest P treatment, the exclusion volume of Br was 21% of the saturated water content. With increasing PWV, the P‐induced surface charge had a decreasing effect on Br mobility, whereas the influence of solute dispersion increased.
Soil Phosphorus Fraction Dynamics during 18 Years of Cultivation on a Typic Paleudult
Authors: Beck Michel A. , Sanchez Pedro A.
Published: July 2010

Traditional soil P test methods estimate plant available inorganic P but ignore the less available inorganic and organic P pools. In low‐input systems where fertilizer P additions are very low to nil, these less available P pools may be a better measure of potential plant available P, particularly in highly weathered soils. The objectives of this study were to determine the size and changes in soil P pools in the nonfertilized and fertilized treatments of a long‐term continuous cultivation experiment established on a Typic Paleudult in Yurimaguas, Peru. A modified version of the Hedley et al. procedure was used to sequentially fractionate soil P into increasingly recalcitrant organic and inorganic pools. The use of path analysis highlights the interactions among P pools and the different roles of the pools in P cycling between the nonfertilized and fertilized system. For the fertilized system, the NaOH‐extractable inorganic P pool acts as a sink for fertilizer P but desorption is rapid enough to maintain high levels of plant available P. For this system, inorganic P pools explain 96% of the variation in the level of available P. Organic P is the primary source of plant‐available P in the nonfertilized system and explains 44% of the variation in available P. Available P, measured by anion exchange resin, is dependent on crop residue (b values ± standard error = 4.98 ± 3.57), whereas yield depends most strongly on the available P (0.16 ± 0.11) and on NaOH‐extractable organic P (−0.17 ± 0.11). The lack of stability in organic P levels in the first 10 yr illustrates the need for long‐term experiments. The presented results support the notion that traditional soil P tests are inadequate for low‐ to no‐input systems.
Phosphorus Transformations and Availability under Cropping and Fertilization Assessed by Isotopic Exchange
Authors: Morel C. , Tiessen H. , Moir J. O. , Stewart J. W. B.
Published: July 2010

In fertilized agricultural soils, both fertilizer and soil P are partly depleted by crop export, while residual fertilizer P reacts with the soil. As a result, P availability changes in complex ways that cannot be described by an available P pool. Our objective was to characterize the P availability in two long‐term field experiments on a Mollisol and an Alfisol (near Lethbridge and Breton, Canada), by using the soil solution P (Cp) and the time‐dependent isotopically exchangeable P (Et). The relationship of Cp to Et was further evaluated for short‐term reactions by incubating field samples for 1 d with increasing P rates. Both Cp and Et were related to changes in the soil P budget resulting from continuous cropping and wheat (Triticum aestivum L.)‐fallow rotations with and without fertilization, with lower Cp values in the more depleted soils. In the Alfisol, field variations in pH and texture affected both Et and Cp, obscuring relationships between the field P budgets and availability parameters. In the Mollisol, one single equation described the relationship between Et and Cp for both increases of Cp due to short‐term additions and decreases due to long‐term crop export, indicating that transformations of available P were fully reversible.
Field‐Scale Variability of Soil Properties in Central Iowa Soils
Authors: Cambardella C. A. , Moorman T. B. , Novak J. M. , Parkin T. B. , Karlen D. L. , Turco R. F. , Konopka A. E.
Published: July 2010

Spatial distributions of soil properties at the field and watershed scale may affect yield potential, hydrologic responses, and transport of herbicides and NO−3 to surface or groundwater. Our research describes field‐scale distributions and spatial trends for 28 different soil parameters at two sites within a watershed in central Iowa. Two of 27 parameters measured at one site and 10 of 14 parameters measured at the second site were normally distributed. Spatial variability was investigated using semivariograms and the ratio of nugget to total semivariance, expressed as a percentage, was used to classify spatial dependence. A ratio of <25% indicated strong spatial dependence, between 25 and 75% indicated moderate spatial dependence, and >75% indicated weak spatial dependence. Twelve parameters at Site one, including organic C, total N, pH, and macroaggregation, and four parameters at Site two, including organic C and total N, were strongly spatially dependent. Six parameters at Site one, including biomass C and N, bulk density, and denitrification, and 9 parameters at Site two, including biomass C and N and bulk density, were moderately spatially dependent. Three parameters at Site one, including NO−3 N and ergosterol, and one parameter at Site two, mineral‐associated N, were weakly spatially dependent. Distributions of exchangeable Ca and Mg at Site one were not spatially dependent. Spatial distributions for some soil properties were similar for both field sites. We will be able to exploit these similarities to improve our ability to extrapolate information taken from one field to other fields within similar landscapes.
Energy Balance Model of Spatially Variable Evaporation from Bare Soil
Authors: Evett S. R. , Matthias A. D. , Warrick A. W.
Published: July 2010

Most models of evaporation (E) provide estimates at one rather than many locations and thus cannot be used to describe the spatial variability of evaporation. An energy balance model (EBM) that estimates E at many locations was tested, improved, and validated, using daily evaporation measurements made with microlysimeters, giving an r2 value of 0.82 for regression of actual vs. estimated evaporation. The model is based on the surface energy balances of dry and drying soil. Data needed include only wind speed and soil surface temperature measurements obtained at a suitably small time interval (e.g., 0.5 h) with an automated weather station and reference dry soil at one location, and measurements of predawn and midday soil surface temperature made with a hand‐held infrared thermometer at as many locations as desired for evaporation prediction. The reference dry soil was established in a plastic bucket buried in the soil and protected from rain and irrigation. Model improvements included an easy method of accurately estimating continuous soil surface temperature at many points in a field. Also, an empirically fitted transfer coefficient function for the sensible heat flux from the reference dry soil showed that sensible heat flux from the relatively hot reference dry soil was dominated by free convection. Soil heat flux and reflected shortwave radiation terms are omitted in the EBM and this was shown to reduce model accuracy by as much as 9.2% of the measured evaporation. The model may prove useful for prediction of spatial variability of evaporation based on soil surface temperatures.
Stability of a Calcareous Saline‐Sodic Soil During Reclamation
Authors: Lebron I. , Suarez D. L. , Alberto F.
Published: July 2010

General guidelines to reclaim saline or sodic soils do not adequately consider variables such as pH and the presence of organic matter that are known to affect soil stability. Poor structural stability of sodic, saline‐sodic, and high‐pH soils adversely influences crop yields; promotes piping, tunneling, and slope erosion; and can accelerate the failure of water conveyance systems. We evaluated six soil tests, used to measure physico‐chemical properties, for their suitability to evaluate the structural stability of a calcareous, saline‐sodic soil under reclamation. The stability tests were wilting point, plastic limit, coefficient of linear extensibility (COLErod), water content at 0.03 MPa, liquid limit, and dispersion index. The range of electrical conductivity (EC) studied was 0.5 to 20 dS m−1, sodium adsorption ratio (SAR) 0 to 400 (mmol L−1)0.5, and pH 8.4 to 10.5. The results obtained indicate that the amount of water necessary for a soil to flow under standard conditions for the liquid limit test decreased an average of 25% when the EC decreased from 40 to 2 dS m−1. The liquid limit and EC showed a linear correlation (R2 = 0.785); therefore, the liquid limit was considered to be an appropriate index to evaluate the physical properties of a soil under a leaching process. Plasticity index and available water were more useful in the evaluation of the mechanical properties of the soil when we used amendments. Liquid limit combined with the water content of the soil at 0.03 MPa was the most useful tool for evaluating soil structural stability during reclamation.
Mechanisms and Control of Percolation Losses in Irrigated Puddled Rice Fields
Authors: Tuong T. P. , Wopereis M. C. S. , Marquez J. A. , Kropff M. J.
Published: July 2010

The low water use efficiency of irrigated lowland rice is partly due to water loss by percolation. Mechanisms of percolation losses were studied in a puddled rice field with a permeable subsoil using simulation models and field experiments. Inclusion of small nonpuddled areas (1.5 m2 per 100 m2 of puddled soil) within the field with 5‐cm ponding water depth (PWD) increased field water loss from 2.7 mm d−1 to 15 mm d−1. Under‐bund percolation rate (lateral movement of ponded water into the bunds, then vertically down to the water table) was about 10 mm d−1 in a 25 by 100 m field. A one‐dimensional mechanistic soil‐water balance model, SAWAH, accurately simulated the measured water losses in a homogeneously puddled rice field only when lateral flow toward nonpuddled spots and bunds was prevented. Maintaining shallow PWD did not significantly affect percolation loss through uniformly puddled soil but greatly reduced losses in nonpuddled spots and under‐bund percolation. Sealing the bund walls with puddled soil material will decrease the horizontal conductivity of the bunds and may further reduce under‐bund percolation.
Tracer Characteristics of Brilliant Blue FCF
Authors: Flury Markus , Flühler Hannes
Published: July 2010

Dyes are widely used to stain the travel paths of water and solutes in soils. Brilliant Blue FCF (C.I. Food Blue 2) is a valuable dye tracer for visualizing water flow patterns. Because this dye is nontoxic, it is particularly suited for field use. Depending on pH, the dye is either neutral or dissociates to a mono‐ or bivalent anion. The water solubility is 200 kg m−3. In this study, properties of the dye that characterize the adsorbing behavior in soil were determined. Batch adsorption experiments were carried out in the laboratory, and a field experiment was conducted to compare the mobility of Brilliant Blue FCF with that of I− and Br−. The batch studies gave distribution coefficients Kd of 0.19, 3.00, and 5.78 dm3 kg−1 for three different soils. The results of the batch studies suggest that the dye forms ion pairs with Ca2+. The field experiment showed that the dye is retarded compared with the conservative tracer Br−. A comparison between Brilliant Blue FCF and I− analyzed with the iodide‐starch method resulted in a relative retardation of 1.2 Brilliant Blue FCF compared with I−.
Near‐Saturated Hydraulic Conductivity in Soils of Contrasting Texture Measured by Tension Infiltrometers
Authors: Jarvis N. J. , Messing I.
Published: July 2010

Soil structure is known to strongly affect water flow and solute transport, yet little information is available concerning soil hydraulic properties close to saturation. This study reports detailed measurements of near‐saturated hydraulic conductivity in undisturbed field soils. Steady‐state infiltration rates from tension infiltrometers were measured at the surface of six tilled soils of contrasting texture ranging from loamy sand to silty clay. Hydraulic conductivities (K) at supply pressure heads (h) in the range from − 100 mm to zero were obtained using Wooding's solution for infiltration from a circular source. The paired K,h data were summarized using the technique of functional normalization with three alternative models of K(h): a single exponential function (Gardner's model), a two‐line exponential model, and the Mualem‐van Genuchten model. The commonly used single exponential model was found to be inappropriate in all cases. Both the two‐line exponential and Mualem‐van Genuchten models satisfactorily described the data, with the former performing marginally better for five of the six soils. In the supply pressure head range − 100 mm to zero, K increased by three to four orders of magnitude in the finer textured soils and by about two orders of magnitude in two sandy soils. Estimates of field‐saturated hydraulic conductivity (Kfs) were largest in the fine‐textured soils, presumably because of the influence of continuous surface‐vented macropores. Spatial variability in measured K(h) was small to moderate for all soils. Predicting unsaturated K from soil water release data using Kfs as a matching point was shown to result in serious overestimations.
Fractal Parameters of Pore Surface Area as Influenced by Simulated Soil Degradation
Authors: Pachepsky Ya. A. , Polubesova T. A. , Hajnos M. , Sokolowska Z. , Józefaciuk G.
Published: July 2010

Fractal parameters of soils become increasingly important in understanding and quantifying transport and adsorption phenomena in soils. It is not known yet how soil degradation affects fractal characteristics of soil pores. We estimated pore surface area fractal parameters from Hg porosimetry data on samples of a Udic Argiboroll, a Typic Haploboroll, and a Ustolic Orthid before and after simulated soil degradation. Three or four distinct intervals with different fractal dimensions were found in the range of pore radii from 4 nm to 5 µm. This was attributed to differences in composition of soil particles of different sizes. The simulated degradation caused an increase in fractal dimensions in one or more fractal intervals, thus manifesting the increased roughness and irregularity of the pore surfaces. The interval of the smallest radii had the highest average fractal dimension, close to 3; some estimated values were >3, probably due to the compressibility of bulk material and air entrapment. Values of the fractal dimension in this interval increased after cyclic wetting‐drying but were not affected by organic matter oxidation. Smaller fractal dimensions were found in the next interval of radii. Here average fractal dimension increased markedly after organic matter oxidation and grew slightly after cyclic wetting‐drying, reflecting the loss of bonds between particles. The range of largest radii included two fractal intervals after cyclic wetting‐drying and one fractal interval for all other samples. Neither organic matter oxidation nor cyclic wetting‐drying significantly affected the boundaries between the fractal intervals.
Time‐Domain Reflectometry Measurements of Surface Soil Water Content
Authors: Nielsen D. C. , Lagae H. J. , Anderson R. L.
Published: July 2010

Surface soil water content is an important parameter influencing such processes as residue decomposition, nutrient cycling, microbial activity, and weed seed germination and emergence. This experiment was conducted to determine if time‐domain reflectometry (TDR) could be used to measure water content in the 0‐ to 50‐mm soil layer. Wave guides were installed horizontally at a depth of 25 mm below the soil surface in field plots differing in current crop‐tillage history. The TDR system was interrogated hourly by a data logger. Daily average soil water contents from TDR were computed and compared with water contents obtained from gravimetric soil sampling. The TDR values were linearly related (r2 = 0.84) with gravimetric samples of soil water content when wave guides were at a depth of 25 mm, but not when wave guides were at a depth of 13 mm. Continuous measurements of surface soil water content can be obtained from an automated TDR system.
Nitrogen and Water Interactions in Trickle‐Irrigated Watermelon
Authors: Pier J. W. , Doerge T. A.
Published: July 2010

Rising water costs and concern for groundwater contamination by NO3 and other agricultural chemicals are forcing growers in arid regions to improve irrigation and N fertilization efficiency. Applying N and water through a subsurface drip irrigation system in conjunction with in‐season monitoring techniques for assessing crop water and N status has the potential to greatly improve water and N use efficiency. The objectives of this research were to: (i) investigate the interactive effects of water and N applied through a subsurface trickle irrigation system on watermelon [Citrullus lanatus (Thumb.) Matsum and Nakai var. lanatus] fruit yield and on the potential for leaching losses of NO3, and (ii) determine the optimum range of soil water tension for subsurface trickle‐irrigated watermelon. Field experiments were conducted during 1990 and 1991 on a reclaimed Casa Grande soil (fine‐loamy, mixed, hyperthermic Typic Natrargid) in southern Arizona. Levels of target soil water tensions and fertilizer N were arranged in a factorial design to determine fruit yield response surfaces. Tensiometers at 0.3‐ and 0.6‐m depth were used to monitor soil water tension throughout the growing season. Marketable watermelon yield showed a pronounced positive water × N interaction in both years of the study. Predicted marketable yields were 90 Mg ha−1 when mean soil water tension was 6 kPa and applied N was between 200 and 270 kg ha−1 in 1990 and a predicted yield maximum of 102 Mg ha−1 at 7.2 kPa and 336 kg N ha−1 in 1991. Scheduling of irrigations using feedback from tensiometers proved very useful in providing optimum amounts of water while avoiding conditions that favor N loss through leaching or denitrification.
Tillage and Traffic Influences on Water and Solute Transport in Corn‐Soybean Systems
Authors: Wu L. , Swan J. B. , Allmaras R. R. , Logsdon S. D.
Published: July 2010

Tillage management many influence water and solute movement throughout the root zone. To evaluate tillage effects on solute transport, parameters were estimated from breakthrough curves and directly measured macropores in no‐till (NT) and moldboard‐plowing (MBD) systems with traffic control on a Nicollet clay loam (fine‐loamy, mixed, mesic Aquic Hapludoll) and a Rozetta silt loam (fine‐silty, mixed, mesic Typic Hapludalf). Ridge tillage recently imposed on long‐term NT (RT/NT) was compared with chisel tillage (CHT) on a Seaton silt loam (fine‐silty, mixed, mesic Typic Hapludalf) where manure was injected without traffic control. Breakthrough curves (Br− tracer) were measured in undisturbed soil cores from the Ap horizon (0–250 mm) and adjacent subsoil (250–500 mm) and were fitted to a physical, nonequilibrium two‐domain model to estimate mean pore‐water velocity of the mobile domain (Vm) and the fraction of water (v) participating in Br− transport. The Vm ranged from 0.10 to 10 mm s−1, while v ranged from 0.02 to 0.50. Means did not differ between the Nicollet and Rozetta soils in 1988, yet significant tillage × horizon interactions indicated more solute bypass in NT than in MBD. More macropores and a larger macropore conductivity (Km) were observed in NT than in MBD. Greater Vm and smaller v occurred (P < 0.05) in CHT than in RT/NT because RT/NT lacked secondary cultivation to remove compaction from manure injection. Traffic control in NT on the Nicollet and Rozetta soils produced high Vm and low v compared with RT/NT on the Seaton soil. Both Vm and v can be estimated from the more easily measured Km.
Calcium Salts and Mechanical Stress Effects on Water‐Dispersible Clay of Oxisols
Authors: Fontes M. P. F. , Gjorup G. B. , Alvarenga R. C. , Nascif P. G. S.
Published: July 2010

It can be hypothesized that clay dispersion as a function of lime application and mechanical stresses is an important factor in the deterioration of physical conditions of some Brazilian Oxisols. Therefore, a laboratory study was conducted to test the influence of these varaibles on clay dispersion. Different water‐dispersible clay (WDC) contents and incubation times varying from 0 to 50 d were tested. The Ca salts used were CaCl2, CaSO4, and CaCO3. The methodologies utilized to measure WDC and vary mechanical stresses were: (i) manual soil sample shaking for 1 min with a glass rod (low‐energy stirring system, LESS); and (ii) shaking in a reciprocating shaker for 3 h (high‐energy shaking system, HESS). The LESS did not yield significant amounts of WDC regardless of the Ca salt used. The HESS yielded approximately 120 to 311 g kg−1 of WDC by itself or when combined with CaCO3. Calcium chloride and CaSO4 yielded very low amounts of WDC in the range of 1 to 20 g kg−1.
Temperature Effects on Kinetics of Microbial Respiration and Net Nitrogen and Sulfur Mineralization
Authors: MacDonald Neil W. , Zak Donald R. , Pregitzer Kurt S.
Published: July 2010

Global climate change may impact the cycling of C, N, and S in forest ecosystems because increased soil temperatures could alter rates of microbially mediated processes. We studied the effects of temperature on microbial respiration and net N and S mineralization in surface soils from four northern hardwood forests in the Great Lakes region. Soil samples were incubated in the laboratory at five temperatures (5, 10, 15, 20, and 25°C) for 32 wk. Headspace gas was analyzed for CO2‐C at 2‐wk intervals, and soils were extracted to determine inorganic N and S. Cumulative respired C and mineralized N and S increased with temperature at all sites and were strongly related (r2 = 0.67 to 0.90, significant at P = 0.001) to an interaction between temperature and soil organic C. Production of respired C and mineralized N was closely fit by first‐order kinetic models (r2 ≥ 0.94, P = 0.001), whereas mineralized S was best described by zero‐order kinetics. Contrary to common assumptions, rate constants estimated from the first‐order models were not consistently related to temperature, but apparent pool sizes of C and N were highly temperature dependent. Temperature effects on microbial respiration could not be accurately predicted using temperature‐adjusted rate constants combined with a constant pool size of labile C. Results suggest that rates of microbial respiration and the mineralization of N and S may be related to a temperature‐dependent constraint on microbial access to substrate pools. Simulation models should rely on a thorough understanding of the biological basis underlying microbially mediated C, N, and S transformations in soil.
Modeling Transport Kinetics in Clinoptilolite‐Phosphate Rock Systems
Authors: Allen E. R. , Ming D. W. , Hossner L. R. , Henninger D. L.
Published: July 2010

Nutrient release in clinoptilolite‐phosphate rock (Cp‐PR) systems occurs through dissolution and cation‐exchange reactions. Investigating the kinetics of these reactions expands our understanding of nutrient release processes. Research was conducted to model transport kinetics of nutrient release in Cp‐PR systems. The objectives were to identify empirical models that best describe NH4, K, and P release and define diffusion‐controlling processes. Materials included a Texas clinoptilolite (Cp) and North Carolina phosphate rock (PR). A continuous‐flow thin‐disk technique was used. Models evaluated included zero order, first order, second order, parabolic diffusion, simplified Elovich, Elovich, and power function. The power‐function, Elovich, and parabolic‐diffusion models adequately described NH4, K, and P release. The power‐function model was preferred because of its simplicity. Models indicated nutrient release was diffusion controlled. Primary transport processes controlling nutrient release for the time span observed were probably the result of a combination of several interacting transport mechanisms.
Effect of Water‐Soluble Phosphorus on the Effectiveness of Triple Superphosphate Fertilizers
Authors: Mullins G. L. , Sikora F. J. , Williams J. C.
Published: July 2010

Impure phosphate rock results in the formation of P‐containing impurities having low water solubility in commercial triple superphosphate (TSP) fertilizers, which may influence their effectiveness. A greenhouse study was conducted to determine the level of water‐soluble P needed in TSP to maximize plant P uptake and yield when applied to a slightly acid soil (pH = 6.4). Fertilizer impurities were isolated from four commercial TSP fertilizers as a water‐washed fraction by washing the whole fertilizers with deionized water. Fertilizers with varying water‐soluble P were simulated by mixing reagent‐grade monocalcium phosphate (MCP; ≈ 100% water soluble) with water‐washed fertilizer fractions or reagent‐grade dicalcium phosphate (DCP; ≈ 1.2% water‐soluble P). The mixtures contained 0, 20, 40, 60, and 80% of the available P (available = water‐soluble + ammonium citrate soluble P) as MCP. An additional treatment received 100% water‐soluble P as MCP. Mixtures were applied to supply 50 mg available P kg−1 soil. Sorghum‐sudangrass [Sorghum bicolor (L.) Moench] was harvested 21 and 42 d after planting. Forage yield and P uptake were affected by P source and water‐soluble P content. Compared with using a 100% water‐soluble P source (MCP), individual TSP fertilizer materials required 52 to 66% water‐soluble P to attain 90% of the dry matter yield and 41 to 51% water‐soluble P to attain 90% of P uptake. Results of this study demonstrate that current levels of “water‐insoluble P” in commercial TSP sources are not affecting their performance when added to slightly acid soils.
Correspondence of Hydraulic Functions for Unsaturated Soils
Authors: Warrick A. W.
Published: July 2010

This study investigated the correspondence of hydraulic functions for unsaturated soils. The overall idea was to identify general properties and explore conditions for which alternative functions give the same or similar responses. Five commonly used empirical forms were examined in this context. The objective was to extend solutions based on particular forms of hydraulic models to more general conditions. For example, the commonly applied formula for steady‐state flow of a disk infiltrometer assumes a quasi‐linear form of Richards' equation. Another area of application is parameter identification and the choice of alternative models. In order for two sets of models to correspond, the zeroth and first moments of the soil water diffusivity, D, should be equal for the respective models. The zeroth moment is simply the integral of D with respect to water content across the range of interest and is closely related to sorptivity and capillary length. The first moment characterizes the shape of D.
Mobility of Anion Spin Probes in Hectorite Gels: Viscosity of Surficial Water
Authors: McBride M. B. , Baveye P.
Published: July 2010

The mobility of water adjacent to layer silicate clay surfaces and the spatial range of influence of these surfaces on the physical properties of water have been subjects of some controversy. In an attempt to further characterize the surficial water of clays, electron spin resonance (ESR) spectroscopy was used to determine the rotational mobility of an anionic nitroxide spin probe (3‐carboxy‐2,2,5,5‐tetramethyl‐1‐pyrrolidinyloxy) in hectorite gels of variable water content. There was a measurable decrease in average mobility of the anion probe only when the clay/water ratio exceeded 1 kg L−1. Because “negative adsorption” tended to exclude the anion from the near‐surface regions except at quite low water content, mobility of the anion probe was unaffected by adsorption processes and could be used to estimate the viscosity of water within 100 Å of the surface. The spin probe results indicate that, at low water contents, the addition or removal of water changes the rigidity of all the water, so that the viscosity of the interlamellar water cannot be estimated by averaging that of the adsorbed and added free water. The results place the maximum range of silicate surface influence on dynamic water properties at about five molecular diameters, with up to two monolayers of water at each surface being one to two orders of magnitude more rigid than free water.
Chemical Relaxation and Double Layer Model Analysis of Boron Adsorption on Alumina
Authors: Toner C. V. , Sparks D. L.
Published: July 2010

Boron is a plant nutrient essential for adequate plant growth, yet the range between B deficiency and toxicity levels is rather narrow. Boron adsorption reactions with soil components, particularly sesquioxides, most often regulate the amount of B in the soil solution. The reaction mechanisms of B adsorption on oxides have not been fully characterized, however. Pressure‐jump relaxation experiments were conducted to measure the rates and determine the reaction mechanism for B adsorption on an alumina (γ‐Al2O3) surface from B(OH)3‐B (OH)−4 solutions. Relaxation times (τ) were measured from pH 7.0 to 9.7 in alumina suspensions with 0.012 mol L−1 total B. A plot of τ−1 vs. B(OH)−4 plus surface site concentration obtained from the triple layer model (TLM) assuming inner sphere B(OH)−4 adsorption yielded an adsorption rate constant (kintf) of 3.3 × 105 L mol−1 s−1 and a desorption rate constant (kintr) of 1.8 × 10−3 L mol−1 s−1. The ratio kintf/kintr yielded an equilibrium constant (log KintKIN) of 8.26, in agreement with the intrinsic equilibrium constant for B(OH)−4 adsorption (log KintBIS = 7.69) obtained from adsorption isotherms. Four additional surface complexation models were tested for their ability to model both the equilibrium and kinetic data simultaneously: the constant capacitance model, the diffuse layer model, a Stern model variant, and the TLM assuming outer sphere B(OH)−4 adsorption. Only the TLM, assuming both B(OH)3 and B(OH)−4 were adsorbed via ligand exchange on neutral surface sites, was successful. The TLM indicated that B(OH)−4 is the predominant adsorbed species throughout the pH range 7.0 to 10.8.
Laboratory Determination of Interrill Soil Erodibility
Authors: Truman C. C. , Bradford J. M.
Published: July 2010

Laboratory soil pans and rainfall simulators have been used to study fundamental erosion processes affecting interrill soil loss. Problems exist when laboratory results are extended to those determined under field conditions. Experimental methodology influences a soil's interrill erodibility and interrill erodibility ranking among soils. We compared interrill soil loss and erodibility data from two erosion pans with field data from identical soils. Three plot size‐rainfall simulator methodologies were used: (1) 0.14‐m2 lab pan under a constant drop size (4.6 mm) rainfall simulator, (2) 0.32‐m2 lab pan with soil border areas under an oscillating nozzle (2.3 mm median drop diameter) rainfall simulator, and (3) 1‐m2 field plots under the same rainfall simulator as used in Method 2. Soil loss was measured at 5‐min intervals. Interrill erodibility (Ki) was calculated from two equations (E = KiiI2 and E = KiqIq) using measured soil loss (E), rainfall intensity (I), and flow discharge (q) values. The interrill erosion equation, methodology, time, and initial water content influenced calculated Ki values. Soil loss and Ki values from Method 1 did not correlate with and were greater than corresponding values from Methods 2 and 3. Soil loss and Ki values from Method 2 were correlated (r = 0.56 to 0.79) with corresponding values from Method 3. The Kiq values decreased with time and were a function of soil properties related to soil detachment and sediment transport. The Kii values (i) increased with time, (ii) were primarily a function of soil properties related to soil detachment only, and (iii) did not account for infiltration and runoff differences among soils.
Erosion‐Crop Productivity Relationships for Soils of Africa
Authors: Lal Rattan
Published: July 2010

Soil erosion and land degradation are severe problems in Africa. Some quantitative information on the magnitude of soil erosion for different soils and ecoregions is available. However, quantitative data on effects of erosion on crop yields under different systems of management are available only for a few sites. The objective of this review is to use existing data on erosion rates and productivity relationships to estimate losses in crop yield due to past erosion at the continental level. The available information is also used to predict losses during the next 30 yr. Analyses of the available data indicate that yield reductions due to past erosion may range from 2 to 40%, with a mean of 8.2% for the continent and 6.2% for sub‐Saharan Africa (SSA). If accelerated erosion continues unabated, yield reductions by the year 2020 may be 16.5% for the continent and 14.5% for SSA. Annual reduction in total production for 1989 due to accelerated erosion in the continent was estimated at 8.2 million Mg for cereals, 9.2 million Mg for roots and tubers, and 0.6 million Mg for pulses. The reduction in total production in 1989 for SSA was estimated at 3.6 million Mg for cereals, 6.5 million Mg for roots and tubers, and 0.36 million Mg for pulses.
Soil Losses by Wind Erosion
Authors: Fryrear D. W.
Published: July 2010

Measurements of wind erosion from fields are limited, but with recently developed erosion samplers, it is possible to determine soil losses from natural winds. Erosion of soils by wind was measured in five states. The soil loss was dependent on wind velocity, storm duration, and soil surface characteristics. Average annual rainfall varied from 356 mm at Eads, CO, to 915 mm at Crown Point, IN. Field soil varied from 29% sand at Eads, CO, to 83% sand at Big Spring, TX. Soil losses from individual erosion events in 3.1‐ha fields varied from 0.050 kg m−2 at Big Spring, TX, to 7.007 kg m−2 at Elkhart, KS. Soil surface roughness, percentage of wind‐erodible aggregates on the soil surface, and percentage of the soil surface covered with nonerodible material such as crop residues are extremely important in controlling wind erosion. A ground cover of 4% reduced wind erosion losses 15% compared with bare soil. Ridging a smooth soil with 50‐ to 70‐mm ridges reduced soil erosion 98% on a sandy loam soil.
Method to Estimate Water Diffusivity and Hydraulic Conductivity of Moderately Dry Soil
Authors: Globus A. M. , Gee G. W.
Published: July 2010

Water diffusivity and unsaturated hydraulic conductivity of moderately dry soils are seldom measured directly, yet are often needed by numerical models to simulate flow processes. We propose a relatively simple method to obtain water diffusivity or hydraulic conductivity in moderately dry soils. Steady‐state profiles of water content and temperature in a closed, non‐isothermal soil column are used to determine Sw = DT/D(θ) = − dθ/dT where Sw is the thermogradient coefficient, DT is the thermal water diffusivity, D(θ) is the isothermal water diffusivity, and dθ/dT is the rate of change of water content with temperature. The relative constancy of DT allows D(θ) to be estimated as D(θ) = ‐DT(dT/dθ) where dT/dθ is measured at various locations along the soil column. Using water retention data, the steady water content profile can be transformed into a suction head profile and the above procedures for estimating D(θ) can then be applied to estimate hydraulic conductivity, K(θ). Soil columns ranging in length from 5 to 10 cm, exposed to thermal gradients of 1°C/cm for periods as short as 7 d, at initial suction values ranging from 0.1 to 1.5 MPa, can be used to estimate water diffusivity and hydraulic conductivity with uncertainties of a factor of five or less in the suction range from 0.03 to 3 MPa.
Assessing Temporal Variations in Soil Water Composition with Time Domain Reflectometry
Authors: Heimovaara T. J. , Focke A. G. , Bouten W. , Verstraten J. M.
Published: July 2010

Time domain reflectometry (TDR) can be used to study temporal variations in volumetric soil water content (θ) and bulk soil electrical conductivity (σa). The variations in σa are associated with changes in θ and the soil water composition. Laboratory and field experiments were conducted to verify if TDR can be used to monitor the temporal variation in the soil water composition between solution sampling occasions. Effects of cable length and temperature on the σa measurement were evaluated. Including the series resistance of the cable and connectors in the analysis improves measurements at high electrical conductivity levels. The temperature factor of the bulk soil appears to be similar to the temperature factor of soil extracts. Laboratory experiments showed that the theoretical model giving σa as function of θ and the electrical conductivity of the soil solution (σw) combined with the water retention function was capable of describing σw measured on soil solution extracted with ceramic cup solution samplers under static water flow conditions. After optimization of a single parameter, the model was able to describe σw values of the soil solution obtained in the laboratory, whereas literature values were sufficient for field data. Concentrations of a number of solutes in a field data set spanning 3 yr were positively correlated with σw. Site‐specific regressions between solute concentration and σw combined with automated TDR measurements of σa and θ enable a more meaningful interpretation of the temporal variation of the concentration of major solutes present in the soil solution between sampling occasions.
Error Analysis of Heat Pulse Method for Measuring Soil Heat Capacity, Diffusivity, and Conductivity
Authors: Kluitenberg G. J. , Bristow K. L. , Das B. S.
Published: July 2010

A dual‐probe heat pulse (DPHP) method was developed recently that allows for the simultaneous, automated measurement of soil thermal diffusivity (κ), volumetric heat capacity (ρc), and thermal conductivity (λ). Estimation of thermal properties is based on theory for the conduction of heat away from an infinite line source (ILS) that is heated for a short period of time. In this study, we examined possible sources of error in the use of the ILS theory by comparing it with other models that explicitly account for finite length and cylindrical shape of the actual heater. For probe geometry and heating times typical of our experimental work, the analysis of model error showed that assuming an infinite length for a heat source of finite length caused errors <2% in the estimated thermal properties. Assuming the cylindrically shaped heater to be a line heat source caused errors of <0.6% in the estimated thermal properties. Thus, the ILS theory appears to be appropriate for use in the DPHP method if probe geometry is considered carefully. However, small changes in probe geometry can lead to large model errors. First‐order error analysis also was used to predict how thermal property estimates will be affected by experimental errors in the measured inputs to the ILS model. The analysis shows that κ and ρc estimates are sensitive to measurement error in probe spacing (r), but λ is unaffected by error in r. Estimates of κ and λ were shown to be sensitive to measurement error in the time to the temperature maximum (tm), whereas ρc was affected only slightly by such error.
Kinetics of Cadmium and Cobalt Desorption from Iron and Manganese Oxides
Authors: Backes Clare A. , McLaren Ronald G. , Rate Andrew W. , Swift Roger S.
Published: July 2010

Oxides of Fe and Mn in soils are capable of sorbing large amounts of trace metal ions and can therefore be important in controlling trace metal concentrations in soil solution, and hence trace metal bioavailability in soils. There is, however, relatively little information on the rates of desorption of trace metals from oxide materials or on the factors affecting desorption rates. The objective of this study was to examine the kinetics of desorption of Cd and Co from two Fe oxides, goethite and ferrihydrite, and from two Mn oxides, hausmannite and cryptomelane. The concentrations of Cd and Co specifically sorbed by the oxides at pH 6.0 were greater for the Mn oxides than for the Fe oxides. The metals were also much less readily desorbed from the Mn than the Fe oxides and, in general, Cd was more readily desorbed than Co from all four oxides. Increasing the initial sorption period from 1 to 15 wk substantially decreased the proportion of sorbed Cd or Co subsequently desorbed from goethite, with a similar but much smaller effect also observed with the Mn oxides. Desorption kinetics for both Cd and Co were found to be described well by assuming either the occurrence of two simultaneous first‐order desorption reactions, or by a continuous distribution of reaction sites, distributed lognormally with respect to desorption first‐order rate constant. With increasing initial sorption period, the parameters obtained from fitting either type of kinetic equation to the experimental data could be interpreted as indicating a movement of metal ions to sites with slower desorption reactions.
Formation and Activity of Urease‐Tannate Complexes Affected by Aluminum, Iron, and Manganese
Authors: Gianfreda Liliana , Rao Maria A. , Violante Antonio
Published: July 2010

The formation of active enzymatic complexes, obtained by the interaction of urease, tannic acid, and soluble and insoluble species of Al, Fe, or Mn, was studied. Tannic acid strongly inhibited urease activity (inhibition constant, Ki = 0.040 mM). The dependence of kinetic parameters (Vmax and Km) on tannic acid concentration suggests a pure uncompetitive inhibition mechanism. Soluble and insoluble urease‐tannate complexes were formed by increasing both the contact time and the tannic acid/urease ratios. Soluble (FeCl3, MnCl2, and OH‐Al polymers) and insoluble (Fe2O3 and MnO2) inorganic components differently influenced the interaction of urease with tannic acid molecules and the residual activity of the immobilized enzyme. An increased removal of urease molecules from solution as well as an enhanced residual enzymatic activity of urease complexes was observed in the presence of all the additives. Soluble species (FeCl3 and OH‐Al polymers) probably acted as flocculating agents or promoted the formation of more active enzymatic complexes. On the contrary, MnO2 showed a predominant polymerizing action on tannate molecules that gave rise to insoluble aggregates occluding urease molecules. Adsorption, as opposed to polymerization, probably occurred in the presence of Fe2O3.
Evaluation of Methods for Estimating Net Nitrogen Mineralization in a Semiarid Grassland
Authors: Hook P. B. , Burke I. C.
Published: July 2010

Methods for estimating net N mineralization in semiarid grasslands are not well tested. We compared three in situ incubation methods to evaluate (i) effects of root density and length of incubations on methodological artifacts involving N immobilization, (ii) effects of sieving on estimates of net N mineralization, and (iii) effects of containment on soil water dynamics. We incubated intact soils and sieved soils with or without roots for 15 to 90 d in uncovered tubes and determined changes in NO3 and NH4. Net N mineralization was transient and usually declined after 15 to 30 d. Nitrogen immobilization may have contributed to decreases in extractable N, but root abundance was not a dominant control of net N mineralization. Greater root mass under blue grama grass [Bouteloua gracilis (Willd. ex Kunth) Lagasca ex Griffiths] plants was expected to promote immobilization, but net N mineralization was higher under plants than in bare areas. Removing roots from sieved soil enhanced net mineralization, but not consistently. Sieving soil caused problematic changes in N dynamics. Under plants, for example, N concentrations were higher in sieved than intact soil after 15 d, but lower after 60 d. Effects of sieving may reflect accelerated depletion of a small, biologically active organic matter pool during prolonged incubations. Soil in tubes was slightly wetter than bulk soil during moist periods, but water content in tubes tracked ambient conditions well. We recommend 15‐ to 30‐d incubations of intact cores to avoid artificially high rates of net mineralization caused by soil disturbance and to capture effects of environmental variation.
Spatial and Temporal Variations in the Chemical Weathering of Basaltic Pyroclastic Materials
Authors: Okuda Itaru , Okazaki Masanori , Hashitani Takusei
Published: July 2010

The spatial and temporal distributions of 17 elements in basaltic pyroclastic materials originating from Mt. Fuji, Japan, were investigated. The relatedness of the profiles was established tephrochronologically, which then allowed a comparison of the chemical compositions of the various weathered pyroclastic materials in terms of their spatial (distance away from the volcano) and temporal (older to younger deposits) relations. The distributions of mobile elements (e.g., Ca, Mg, Na, and Sr) and resistant elements (e.g., Ti) are strongly affected by the extent of weathering, which increases with time and increases with distance from the volcano (the effects of the thickness and specific surface area of parent material). The application of a few weathering indices based on chemical composition was successful. Furthermore, a principal component analysis statistically extracted the expected dominance of the element mobility‐resistance, which accounts for 43% of the field‐scale variability of the distribution of the 17 elements. The second principal component explained 21% of variability, and discriminated soils developed from similar parent materials but different weathering‐soil‐formation factors (agricultural activities and the impact of a river).