bluuebunny/crossref_metadata_2025 huggingface dataset:
A simple fracture mechanics approach for assessing ductile crack growth in soil
Authors: Hallett P. D. , Newson T. A.
Published: July 2010

The weak understanding of crack growth mechanisms in ductile soil is addressed by testing a new fracture mechanics approach. Samples are fractured using a deep‐notch (3‐point) bend test, with data on sample bending, crack growth, and crack mouth opening collected to assess the crack opening angle (COA), the crack tip opening angle (CTOA), and plastic energy dissipation rate (Dpl). The test variables are clay and salinity content, with samples formed from mixtures of kaolinite and fine sand. The CTOA and Dpl detect differences in fracture mechanics due to clay, but not salinity. The energy needed to drive crack extension, Dpl, is one order of magnitude higher for samples containing a ratio of sand to kaolinite of 75:25, as compared with 50:50. However, the CTOA due to plasticity was 0.19 and 0.24 for the same samples respectively, indicating that more strain is needed for crack growth in the specimens with more clay.
Time Domain Reflectometry Sensitivity to Lateral Variations in Bulk Soil Electrical Conductivity
Authors: Nissen H. H. , Moldrup P. , Olesen T. , Jensen O. K.
Published: July 2010

It has been assumed, but not proved, that the spatial weighting and sample volume of Time domain reflectometry (TDR) regarding bulk soil electrical conductivity (ECb) equal those determined for the relative dielectric permittivity (K). In this study, two types of experiments were carried out: (i) sensitivity experiments where the cumulative vertical weighting as a function of distance between the probe rods and the soil surface were determined for both K and ECb at two different soil water contents and (ii) solute diffusion experiments to evaluate the discrepancies between actual and TDR‐measured relative ECb in the case where a steep gradient in ECb passes the probe. Three‐rod TDR probes and a sandy loam soil were used in both experiments. The sensitivity experiments confirmed that TDR weights K and ECb equally in the transverse plane. Hence, previously established relationships to calculate the weighting function and sample area of a given TDR‐probe geometry apply for both K and ECb The diffusion experiments showed that if a steep vertical solute gradient passes a horizontally inserted TDR probe, the TDR‐measured ECb profile will be more spread than the actual profile passing the probe. This phenomenon of artificial (TDR‐induced) diffusion is caused by (i) TDR probes not representing a point measurement, and (ii) the nonlinear weighting of K and ECb in the transverse plane. As the steepness of the solute concentration profile diminishes the TDR‐induced diffusion decreases rapidly and becomes negligible for most applications. The effect of TDR‐probe geometry on the discrepancy between actual and TDR‐measured relative concentration was examined theoretically for selected two‐ and three‐rod probe configurations.
Structural Components of Humic Acids as Determined by Chemical Modifications and Carbon‐13 NMR, Pyrolysis‐, and Thermochemolysis‐Gas Chromatography/Mass Spectrometry
Authors: Chefetz Benny , Salloum Myrna J. , Deshmukh Ashish P. , Hatcher Patrick G.
Published: July 2010

The chemical structure of humic acids (HAs) extracted from a grassland surface soil and peat were studied using bleaching (NaClO2 oxidation) and acid hydrolysis (6 M HCl) in combination with advanced analytical techniques: solid‐state 13C nuclear magnetic resonance (NMR), pyrolysis‐gas chromatography/mass spectrometry (GC/MS), and tetramethylammonium hydroxide (TMAH) thermochemolysis‐GC/MS. The purpose of the chemical treatments was to remove known structural fragments from the HA to study the building blocks and components of the macromolecule. Bleaching the peat HA resulted in an attack on the lignin structures leading to a significant reduction in the C‐substituted and O‐substituted aromatic C peaks (128 and 150 ppm) in the 13C NMR spectrum. However, the bleached soil HA still contained residual aromatic C, suggesting that part of its aromaticity had originated from aromatic structures resistant to bleaching, possibly black C (charcoal). The pyrolysates of the bleached HAs contained mainly alkanes and alkenes (C8 to C29), whereas TMAH thermochemolysis yielded a homologous series of long‐chain fatty acid methyl esters (FAMEs) (C8 to C32) and dicarboxylic acid dimethyl esters (DAMES). The data are comparable with those obtained from pyrolysis and thermochemolysis of plant cuticular materials, and therefore suggest that cuticular residues are an integral part of the HA macromolecule. The acid hydrolysis treatment removed esters, amides, carbohydrates, and some of the N‐containing compounds from the HAs. This study demonstrates the effectiveness of bleaching and hydrolysis treatments together with advanced analytical techniques for characterization of aliphatic, lignin‐derived (LG) and nonhydrolyzable N‐containing structures associated with the HA macromolecule.
Organic Amendment and Rotation Crop Effects on the Recovery of Soil Organic Matter and Aggregation in Potato Cropping Systems
Authors: Grandy A. Stuart , Porter Gregory A. , Erich M. Susan
Published: July 2010

Soil structural degradation is common in intensively cultivated ecosystems due to the depletion of soil organic matter (SOM). We investigated the mechanisms by which different frequencies of organic amendment application and rotation crops restore C, N, and aggregation in gravelly loam soils used for potato production. A single amendment application [FIRST; 22 Mg ha−1 compost and 45 Mg ha−1 beef cattle (Bos taurus L.) manure] did not affect total C in 1996 and increased it by 28% in 1997 relative to unamended plots (NONE); light fraction (LF) C accounted for 56% of this increase. Plots in which amendment was suspended for 1 yr (SASP) following 4 or 5 yr of annual application had more total C in 1996 (28%) and 1997 (46%) relative to NONE. A green manure crop consisting of oat (Avena sativa ‘Porter’), pea (Pisum sativum ‘Trapper’), and hairy vetch (Vicia villosa Roth) grown in 2‐yr rotation with potatoes (Solanum tuberosum L.) increased soil C in 1997 (25.9 vs. 23.9 g kg−1), LF properties in 1996 and 1997, and water soluble carbohydrates (WSC) on several sample dates relative to an oat rotation crop. Large aggregate (2–6.5 mm) stability in 1996 and 1997 and medium aggregate (1–2 mm) stability in 1997 were increased by FIRST relative to NONE. Total soil C was more strongly related to medium (r = 0.65 in 1997) and large (r = 0.51 in 1997) aggregate stability than LF or water soluble carbohydrate fractions. Compost and manure influences occurred rapidly and were persistent, demonstrating that annual applications are not necessary to reverse soil degradation.
Calibration and Temperature Correction of Heat Dissipation Matric Potential Sensors
Authors: Flint A. L. , Campbell G. S. , Ellett K. M. , Calissendorff C.
Published: July 2010

This paper describes how heat dissipation sensors, used to measure soil water matric potential, were analyzed to develop a normalized calibration equation and a temperature correction method. Inference of soil matric potential depends on a correlation between the variable thermal conductance of the sensor's porous ceramic and matric potential. Although this correlation varies among sensors, we demonstrate a normalizing procedure that produces a single calibration relationship. Using sensors from three sources and different calibration methods, the normalized calibration resulted in a mean absolute error of 23% over a matric potential range of −0.01 to −35 MPa. Because the thermal conductivity of variably saturated porous media is temperature dependent, a temperature correction is required for application of heat dissipation sensors in field soils. A temperature correction procedure is outlined that reduces temperature dependent errors by 10 times, which reduces the matric potential measurement errors by more than 30%. The temperature dependence is well described by a thermal conductivity model that allows for the correction of measurements at any temperature to measurements at the calibration temperature.
Relationships Between Soil Nitrogen Availability Indices, Yield, and Nitrogen Accumulation of Wheat
Authors: Walley Fran , Yates Thomas , van Groenigen Jan-Willem , van Kessel Chris
Published: July 2010

The success of variable rate N fertilizer application rests on our ability to predict the contribution of soil N to growing crops. We assessed relationships between soil N availability indices (SNAIs), yield, and total N accumulation of wheat (Triticum aestivumL.) grown in a typical glacial till landscape in Saskatchewan, Canada. Soil samples were collected at 3‐m intervals along a 300‐m transect comprised of low (LCFS) and high catchment footslopes (HCFS), and low (LCSH) and high catchment shoulders (HCSH). Total soil N and C, organic C, mineral N, depth of A horizon, spring soil moisture, grain yield, and total plant N were measured. Soil N availability indices used in this study included: (i) cumulative N released during a 2‐wk aerobic incubation (NMIN); (ii) potentially mineralizable N estimated using a 16‐wk aerobic incubation (N0); (iii) NO3sorbed on anion‐exchange membranes (NO3AEM); (iv) N extracted with hot KCl (NKCl); and (v) N hydrolyzed with hot KCl (NHYDR). Although all SNAIs were significantly correlated to yield and, with the exception of N0, total plant N when analyzed across the transect, typically <40% of the yield variability was explained. Forward stepwise regression revealed that most SNAIs failed to explain more variability in crop N accumulation than did basic soil properties or relative elevation. Although these results do not invalidate the use of SNAIs for soil testing purposes, it is clear that SNAIs must be combined with additional information about field scale variability for predicting fertilizer N requirements. Without this information, grid sampling as a means of assessing N requirements remains ill‐advised for glacial till semi‐arid landscapes.
Forms and Amounts of Soil Nitrogen and Phosphorus Across a Longleaf Pine–Depressional Wetland Landscape
Authors: Craft Christopher B. , Chiang Connie
Published: July 2010

Forms and amounts of soil N and P were measured across transects from freshwater depressional wetlands into longleaf pine–wiregrass forests of southwestern Georgia to evaluate changes in labile vs. recalcitrant N and P and C:N:P ratios across drainage gradients. Plant‐available NO−3–N (3.7 μg cm−3) and organic N (2000 μg cm−3) were significantly greater in wetland than upland soils (NO3–N = 0.03 μg cm−3, organic N = 890 μg cm−3) and C:N increased from wetlands (17:1) into uplands (43:1). Forms and amounts of P were not as strongly related to landscape position as N. Labile organic P (Po, 2.6 μg cm−3) was significantly greater in wetland than upland soils (0.88 μg cm−3). Recalcitrant organic compounds accounted for 95 to 97% of the N and 50 to 82% of the P stored in wetland and upland soils. Wetland soils stored a disproportionately large share of N as compared with upland soils even though soil organic matter (C) content was uniform across the landscape. Landscape position (drainage, degree of wetness) is an important determinant of nutrient retention in sandy soils of the southeastern coastal plain. Periodic waterlogging favors sequestration of biological (organic) forms of N and P with proportionally greater storage of N relative to P. Soil waterlogging by promoting accumulation of N more than P favors a shift from N limitation in upland soils towards P limitation in wetland soils of the southeastern coastal plain.
Predicting Molybdenum Adsorption by Soils Using Soil Chemical Parameters in the Constant Capacitance Model
Authors: Goldberg Sabine , Lesch Scott M. , Suarez Donald L.
Published: July 2010

The constant capacitance model, a chemical surface complexation model, was applied to Mo adsorption on 36 samples from 27 soil series selected for variation in soil properties. A general regression model was developed for predicting soil Mo surface complexation constants from four independently measured soil chemical characteristics: cation exchange capacity, organic carbon content, inorganic carbon content, and iron oxide content. The constant capacitance model was well able to predict Mo adsorption on all 36 soil samples. Incorporation of these regression prediction equations into chemical speciation–transport models will allow simulation of soil solution Mo concentrations under diverse environmental and agricultural management conditions without requiring soil specific adsorption data and subsequent parameter optimization.
Estimation of Soil Organic Matter from Red and Near‐Infrared Remotely Sensed Data Using a Soil Line Euclidean Distance Technique
Authors: Fox Garey A. , Sabbagh George J.
Published: July 2010

The soil line is a well‐known linear relationship between the near‐infrared and red reflectance or image intensity of bare soil images. Remotely sensed estimations of soil surface properties can lead to improved representation of spatial heterogeneity. The objectives of this research are to develop an approach based on image soil lines that incorporates image intensity in the red and near‐infrared bands for mapping surface organic matter (OM) and to provide guidance for soil sampling. The soil line concept is used to develop predictive relationships between the amount of OM within the surface horizon of the soil profile and intensity in the red and near‐infrared bands. The soil line Euclidean distance (SLED) technique is based on relating a pixel's Euclidean distance of the red and near‐infrared intensity value to the red and near‐infrared intensity value for the bottom‐most point on the soil line. The technique is evaluated for two fields in the U.S. Midwest. The technique performs as well as or better than a recently proposed technique, while at the same time relating to the parent material (i.e., soil series) within the field. A technique for significantly reducing the number of soil samples required in grid sampling is also introduced and evaluated. This technique utilizes pixels at various percentile locations along the soil line to characterize the predictive relationship between percentage surface OM and image intensity.
Measuring Water‐Extractable Phosphorus in Manure as an Indicator of Phosphorus in Runoff
Authors: Kleinman Peter J. A. , Sharpley Andrew N. , Wolf Ann M. , Beegle Douglas B. , Moore Philip A.
Published: July 2010

Water‐extractable P (WEP) in manure is correlated with P concentration in runoff from soils amended with manure and is, thus, an effective indicator of environmental P loss. This study sought to elucidate methodological factors affecting WEP measurement in manure and to quantify errors related to two established methods of manure WEP measurement. Dairy cow (Bos taurus) manure, poultry (Gallus gallus domesticus L.) (layer) manure, and swine (Sus scrofa domestica L.) slurry were used. Varying dry matter/distilled water ratios (1 to 20:200) revealed that greater dilution of manure dry matter increased WEP (mean 1.8–5.4 g kg−1), likely because of the dissolution of calcium phosphates. Increasing shaking time from 1 min to 24 h, increased manure WEP concentration (average 3.7–8.2 g kg−1). Filtration with Whatman 1 paper filters resulted in significantly higher WEP measurements in dairy and poultry manure (4.1 g kg−1) than with a 0.45‐μm filtration (3.7 g kg−1). No significant difference was observed in the swine slurry. A rainfall‐runoff experiment using simulated rainfall was conducted to determine the effect of the individual factors on predicting dissolved‐reactive P (DRP) concentration in runoff. Comparison of regression coefficients relating manure WEP to runoff DRP concentration revealed an optimum shaking time between 30 min and 2 h, but did not support any single manure/distilled water ratio or filtration method. Replication of two established methods of manure WEP measurement resulted in coefficients of variation of 0.01 to 0.12. Results of this study support the use of a single method with a fixed manure/distilled water ratio for liquid and dry manures.
Scale‐ and Rate‐Dependent Solute Transport within an Unsaturated Sandy Monolith
Authors: Javaux M. , Vanclooster M.
Published: July 2010

Solute transport in unsaturated soil is a scale‐ and flow rate‐dependent process, and high quality experimental studies are needed to introduce scaling relationships in the governing flow and transport models. Within this paper, we present an experimental study to characterize the solute transport for an unsaturated and undisturbed subsoil monolith sampled in a sandy aquifer. Leaching experiments were performed at different flow rates, and solute breakthrough at different positions in the monolith was measured by time domain reflectometry (TDR) probes. At TDR probe scale, the transport is assessed as being a convective‐dispersive process with mixing lengths <0.3 m. At the scale of the monolith, that is, the meso‐scale, transport is rather a stochastic‐convective process with mixing lengths >1 m. At the bottom of the monolith, the meso‐scale dispersivity is at least two times larger than the local‐scale dispersivity. In addition, this meso‐scale dispersivity can be successfully predicted using a stream tube model in combination with a local scale convective‐dispersive process. Important scale effects are also observed in the relationship between the square coefficient of variation of the local velocities and the dispersivity. In contrast to previous studies, we do not observe a significant difference in transport properties for different flow rates (from 1.05 to 54.16 cm d−1). We attribute this insignificance either to a balance between the decrease of the tortuosity and the increase of the coefficient of variation of the velocity with the flow rate or a non‐dependency of both terms on the flow rate.
Estimating Ammonia Volatilization from Swine‐Effluent Droplets in Sprinkle Irrigation
Authors: Wu J. , Nofziger D. L. , Warren J. , Hattey J.
Published: July 2010

Irrigation of swine effluent is one of the most economical ways to dispose of swine manure. The determination of appropriate application rates requires the quantification of volatilization losses of ammoniacal N. In this research, a mechanistic model was developed to estimate droplet volatilization loss (excluding drift losses) of ammoniacal N from swine effluent in sprinkler irrigation. Field experiments were conducted to validate the model. The model combined energy balance, mass balances of ammoniacal N, and water to predict equilibrium temperature of a droplet, and losses of ammoniacal N and water from a droplet. Trajectory analysis was employed in the model to estimate a droplet's exposure time in air. Mass and heat transfer coefficients developed in boundary layer theory were incorporated in the mass and energy balance equations to predict fluxes of heat, ammoniacal N, and water. Equilibrium constants of transformation reactions were used in the model to establish relationships among component species of ammoniacal N. Field experiments measured concentrations of ammoniacal N in swine‐effluent samples collected at the nozzles and ground surface to determine the loss of ammoniacal N caused by droplet volatilization. The measured concentration decreases were combined with water loss percentages to estimate percentage losses of ammoniacal N. Results from both field experiment and mathematical modeling indicated that loss of ammoniacal N from droplet volatilization was only a few percentages which was considered insignificant compared with common soil surface volatilization loss of 20 to 50% at the end of 1 wk after application.
Spatio‐Temporal Simulation of the Field‐Scale Evolution of Organic Carbon over the Landscape
Authors: Walter C. , Rossel R. A. Viscarra , McBratney A. B.
Published: July 2010

The spatial or temporal variability of soil has been extensively considered in the literature using either experimental or modeling approaches. However, only a few studies integrate both spatial and temporal dimensions. The aim of this paper is to present a method for field‐scale simulations of the spatio‐temporal evolution of topsoil organic C (OC) at the landscape scale over a few decades and under different management strategies. A virtual landscape with characteristics matching part of Brittany (France) was considered for the study. Stochastic simulations and regression analysis were used to simulate spatial fields with known spatial structures: short‐range, medium‐range, and long‐range variability. These were then combined using an additive model of regionalization. Agricultural land use was simulated considering four different land uses: permanent pasture, temporary pasture, annual cereal crops, and maize (Zea maysL.). Land use evolution over time was simulated using transition matrices. Evolution of soil organic matter was estimated each year for each pixel through a rudimentary balance model that accounts for land use and the influence of soil waterlogging on mineralization rates. This spatio‐temporal simulation approach at the landscape level allowed the simulation of several scales of soil variability including within‐field variability. Spatial variability decreased drastically over time when only the influence of land use was considered. This effect on soil variability over the landscape may have implications for site‐specific soil management and precision agriculture. The presence of redoximorphic conditions was found to maintain soil spatial variability.
Evaluation of the Fluoride Retardation Factor in Unsaturated and Undisturbed Soil Columns
Authors: Bégin Louis , Fortin Josée , Caron Jean
Published: July 2010

Fluorides deposited on the soil surface near aluminum smelters represent a risk for ground water contamination. This study was conducted to evaluate F retention in soils sampled near an aluminum smelter. The F retardation factor (R) was evaluated at different depths in four coarse‐textured soils by percolating a Br and F solution under steady‐state conditions through 26 unsaturated and undisturbed soil columns. The convective‐dispersive equation (CDE) and convective‐lognormal transfer function (CLT) models were used to evaluateR, using Br and F breakthrough curves (BTCs) and F accumulation profiles evaluated with oxalate‐extractable F (Fox). Following percolation of 4.0 to 13.7 pore volumes of F solution through the soil columns, F BTCs were obtained in 5 out of 26 columns. In the other columns, all F remained in the soil profiles, indicating higher F retention. WhenRwas evaluated using F accumulation profiles, the CDE‐evaluated retardation factor (RCDE) was consistently higher than the one evaluated with the CLT (RCLT), by as much as 111%. This was explained by a difference in the models' behavior near the soil surface. The retardation factor varied among soils and depths by a 20‐fold factor, from 4.4 to 91. The highest retardation factors were obtained in slightly acid and neutral soils. The retardation factor was lower in strongly acid and alkaline soils. The large range ofRvalues obtained and its influence on the velocity of F movement in the soil show thatRis one of the critical factors controlling the risk of ground water contamination by F.
ON THE VANISHING OF SOLUTE DIFFUSION IN POROUS MEDIA AT A THRESHOLD MOISTURE CONTENT
Authors: Hunt A. G. , Ewing R. P.
Published: July 2010

It was recently shown that the solute diffusion coefficient tends to zero at some threshold water content. We examine this phenomenon in light of recent simulation results from a network diffusion model. The experimental and simulation results agree, inviting an explanation from continuum percolation theory.
Insight into the Active Organic Nitrogen Pool Estimated by Isotopic Equilibrium Approaches
Authors: Wang W. J. , Smith C. J. , Chen D.
Published: July 2010

A method that reliably estimates the size of theactive organic N poolhas been desired for predicting N mineralization dynamics. Among the methods proposed to date, the isotopic equilibrium approaches characteristically use N mineralization‐immobilization turnover processes to label the biologically labile organic N. In the present study, the implications and usefulness of the active organic N pool estimated with this approach were assessed in a long‐term incubation.15N‐labeled NH+4and glucose were added to soil, and the dynamics of the labeled N in the organic and inorganic phases were monitored by periodic sampling during the incubation. After appropriate revisions, two equations from the literature were used to estimate the active organic N in soil at Time 0 (OT#) and at each sampling during incubation (OT*), respectively. BothOT#andOT* tended to increase with incubation time and were affected by the amounts of labeled N and available C addition. Considerable amounts of mineralized N came from other thanOT* fraction of soil organic N, but N mineralization from theOT* pool was much faster than from the bulk soil organic matter. The results supported the concept that soil organic N exists in a continuum of degradability. If the operational procedure is standardized, however,OT#can be used as a relative measure of the active organic N in soil.
Argillic Horizons in Stratified Drift
Authors: Hopkins David G. , Franzen David W.
Published: July 2010

Although pedogenesis and landscape evolution have occurred on the eastern North Dakota till plain since the inception of the Holocene, soils with argillic horizon are recognized on a very limited basis. Argillic soils are not mapped in well‐drained positions on the youngest drift prairie landscapes. Roughly half of 47 pedons examined at a precision farming research site in Barnes County, North Dakota show evidence of genetic argillic horizons based on soil structure, hand texturing, and cutan presence. This study applies physical and micromorphological evidence to verify that diagnostic argillic horizons have formed in well‐drained settings on this eastern North Dakota landscape. Total clay distributions and fine/total clay ratios show marked increases in genetic argillic horizons compared with surface horizons and parent materials. The argillic horizons average about 28 cm in thickness; only two pedons were thinner than 7.5 cm. Consequently, the increase in total clay and fine clay distributions, and the thickness criteria required to meet diagnostic horizon status as established bySoil Taxonomyare satisfied. The upper argillic horizon boundary of seven profiles was 50 cm or more below the surface, possibly indicating relict argillic horizons. Micromorphologic evidence shows the presence of illuvial clay in the three soils sampled for microscopy. Porosity variations caused by the argillic horizons likely affect subsurface flow and may be responsible for variable N levels measured for these soils. These soils formed in well‐drained landscape settings, where stratified drift may predispose the soils to illuviation.
Drying and Wetting Effects on Carbon Dioxide Release from Organic Horizons
Authors: Borken W. , Davidson E. A. , Savage K. , Gaudinski J. , Trumbore S. E.
Published: July 2010

Drying and wetting cycles of O horizon in forest soils have not received much attention, partly due to methodological limitations for nondestructive monitoring of the O horizon water content. The objective of this study was to determine the importance of moisture limitations in the O horizon of a temperate forest on summertime soil respiration. We measured soil respiration in three replicated plots in a mixed deciduous forest at Harvard Forest, Massachusetts, weekly from May to October 2001. Direct Current (DC) half‐bridge sensors that had been calibrated using destructive samples of the Oi and Oe/Oa horizons were placed in the Oi and Oe/Oa horizons to record hourly changes of gravimetric water contents. Soil temperature explained 47% of the variation in soil respiration using the Arrhenius equation. The residuals of the temperature model were linearly correlated with gravimetric water content of the Oi horizon (r2 = 0.72, P < 0.0001) and Oe/Oa horizon (r2 = 0.56, P < 0.001), indicating that temporal variation in soil respiration can be partly explained by water content of the O horizon. Additionally, a laboratory study was performed to evaluate drying/wetting cycles of the O horizon at constant temperature. Even small simulated rainfall amounts of 0.5 mm significantly increase CO2 flux from dry O horizon within a few minutes. The duration of CO2 pulses increased with the amount of applied water, lasting from a few hours to days. A strong correlation between CO2 release and water content of the O horizons demonstrates a clear regulatory role of litter water content on decomposition within the O horizons.
Laboratory Validation of a Resource Quality‐Based Conceptual Framework for Organic Matter Management
Authors: Vanlauwe B. , Gachengo C. , Shepherd K. , Barrios E. , Cadisch G. , Palm C. A.
Published: July 2010

Organic resources (ORs) are essential inputs in tropical farming systems and their decomposition dynamics are related to their quality. A Decision Support System (DSS) for organic N management has been proposed earlier that subdivides ORs in four classes depending on their N, lignin, and soluble polyphenol contents. To validate this DSS, a 28‐d aerobic incubation experiment was initiated with 32 ORs, mostly crop and tree residues, applied to a sandy loam soil. The ORs contained 1.4 to 53.2 g kg−1of N, 25 to 295 g kg−1of lignin, and 4 to 148 g kg−1of soluble polyphenols. In vitro dry matter digestibility (IVDMD) ranged from 70 to 820 g kg−1After 28 d, CO2–C production varied between 199 and 905 mg CO2–C kg−1soil, and mineral N contents ranged from 5 to 109 mg N kg−1soil. Based on N mineralization data, three classes of ORs were evident: Class A with N release > 0, Class B with N release approximately 0, and Class C with N release < 0 (N immobilization). Criteria to separate those classes were based on the OR N and polyphenol content and cut‐off values between the classes agreed well with those proposed in the original DSS. For Class A ORs, N mineralization was negatively related to their lignin/N ratio (except forGliricidaresidues) and for Class C ORs, N immobilization was positively related to their N content. Short‐term mineralization data supported the existence of three classes of ORs instead of four originally proposed by the DSS. However, ORs also govern other functions, operating in the medium to long term, and for these functions, the original four‐class concept may be proven valid.
Soil Salinity Using Saturated Paste and 1:1 Soil to Water Extracts
Authors: Zhang H. , Schroder J. L. , Pittman J. J. , Wang J. J. , Payton M. E.
Published: July 2010

Saturated paste (SP) and 1:1 soil/water extractions (1:1) are commonly used to assess soil salinity for field remediation. Correlation of electrical conductivity (EC) and other analytes between the SP and 1:1 extraction methods have been documented, except the relationships were based on limited soil types and require further examination to be adequately evaluated. This study examined these relationships using 170 soils from petroleum and agriculture production sites. Saturated pastes and 1:1 extracts were prepared and analyzed for EC, major cations (Na+, K+, Mg2+, Ca2+), and major anions Cl−, SO42− Relationships of all analytes were established between the two methods using linear regression. Saturated paste extract EC (ECSP) was highly correlated with that of 1:1 extract EC (EC1:1) (r2 = 0.85, P < 0.001). Significant relationships also existed (r2 > 0.73, P < 0.001) between different ions in SP and 1:1 extracts. An independent validation set of 22 soils showed that the slopes of the regressions between predicted EC, Na+, and Cl− of SP equivalents from 1:1 extract measurements and direct SP extract measurements were very close to 1.0 suggesting that the regressions developed can accurately assess soil salinity in salt affected soils using 1:1 extract analysis instead of using the more expensive and time‐consuming SP extraction.
Element Redistribution along Hydraulic and Redox Gradients of Low‐Centered Polygons, Lena Delta, Northern Siberia
Authors: Fiedler S. , Wagner D. , Kutzbach L. , Pfeiffer E.-M.
Published: July 2010

Wetland soils affected by permafrost are extensive in subarctic and arctic tundra. However, this fact does not imply these soils have been sufficiently investigated. In particular, studies of element translocation processes are scarce. This study was conducted (i) to determine the relationship between water and redox regimes in wetland soils in the Siberian tundra, and (ii) to investigate their influence on the distribution of redox sensitive and associate elements (Mn, Fe, P). Major geomorphic units were chosen (microhigh, polygon rim and slope; microlow, polygon center) from two low‐centered polygons in the Lena Delta. Within polygons, redox potential, permafrost, and water level were measured during summer in 1999 and 2000 and (related) compared with element distribution. Manganese, Fe, and P accumulations were preferentially observed in aerobic microhighs. Anaerobic conditions in the microlows lead to a mobilization of Mn, Fe, and P. The elements migrate via water and are immobilized at the microhigh, which acts as an oxidative barrier. The element pattern, indicating an upward flux via water along redox gradients, is explained by higher evapotranspiration from soils and vegetation of the microhighs (Typic Aquiturbel) compared with soils and vegetation of the microlows (Typic Historthel). However, in further research this upward transport should be validated using labeled elements.
Use of Bulk Density Profiles from X‐Radiography to Examine Structural Crust Models
Authors: Bresson L. M. , Moran C. J. , Assouline S.
Published: July 2010

Exponential and sigmoidal functions have been suggested to describe the bulk density profiles of crusts. The present work aims to evaluate these conceptual models using high resolution X‐radiography. Repacked seedbeds from two soil materials, air‐dried or prewetted by capillary rise, were subjected to simulated rain, which resulted in three types of structural crusts, namely, slaking, infilling, and coalescing. Bulk density distributions with depth were generated using high‐resolution (70 μm), calibrated X‐ray images of slices from the resin‐impregnated crusted seedbeds. The bulk density decreased progressively with depth, which supports the suggestion that a crust should be considered as a nonuniform layer. For the slaking and the coalescing crusts, the exponential function underestimated the strong change in bulk density across the morphologically defined transition between the crust and the underlying material; the sigmoidal function provided a better description. Neither of these crust models effectively described the shape of the bulk density profiles through the whole seedbed. Below the infilling and slaking crusts, bulk density increased linearly with depth as a result of slumping. In the coalescing crusted seedbed, the whole seedbed uniformly collapsed and most of the bulk density change within the crust could be ascribed to slumping (0.33 g cm−3) rather than to crusting (0.12 g cm−3). Finally, (i) X‐radiography appears as a unique tool to generate high resolution bulk density profiles and (ii) in structural crusts, bulk density profiles could be modeled using the existing exponential and sigmoidal crusting models, provided a slumping model would be coupled.
Miscible Displacement of Initial Solute Distributions in Laboratory Columns
Authors: Wang H. , Persaud N.
Published: July 2010

Traditional laboratory column systems that were designed to apply tracer solution at one end of the column and collect the effluent at the other end cannot be used to study miscible displacement of initial solute distributions created by multiple inputs of the same (or different) solutes at several locations in the column. Rather, these experimental scenarios require a column system in which the tracer can be injected directly into the flow field. Miscible displacement in a laboratory column system designed for this purpose was investigated. Specifically, the influence of initial solute distributions on the breakthrough curve (BTC) was analyzed using the one‐dimensional (1‐D) convection‐dispersion equation (CDE). Numerical and experimental tests were performed to evaluate the conditions for which one can use a 1‐D Dirac delta source solution to model the initial value problem posed by miscible displacement tests in the column system. For a given outlet boundary condition, if the injected solute was distributed over a distance no larger than 5% of the distance (L) between the injection point and the column outlet, and the column Peclet number (Pe) was not too large (<200), the solute distribution after injection can be assumed as a Dirac delta function for solving the initial value problem.
Transport of Heavy Metals in Surface Runoff from Vegetable and Citrus Fields
Authors: He Z. L. , Zhang M. K. , Calvert D. V. , Stoffella P. J. , Yang X. E. , Yu S.
Published: July 2010

Heavy metal accumulation in agricultural soils increases export potential of the metals to the environment. The concentrations of dissolved heavy metals (Cd, Co, Cr, Cu, Fe, Ni, Pb, Zn, Mn, and Mo) in surface runoff were monitored over a 2‐yr period at 11 sites of vegetable farms and citrus groves in Florida. A total of 1277 surface runoff samples were analyzed for dissolved metals and extractable metals in the surface soils of each field site were determined. Concentrations of the metals in the runoff ranged widely from nondetectable level to 2.80, 18.5, 14.1, 1475, 9227, 39.3, 30.4, 1401, 2118, and 15.0 μg L−1for Cd, Co, Cr, Cu, Fe, Ni, Pb, Zn, Mn, and Mo, respectively. Spatial and temporal variations in the concentrations of heavy metals and runoff discharge were noted among the different sites. Ninety‐four, 96, 55, 32, 93, and 61% of the samples had metal concentrations below the detection limits for Cd, Co, Cr, Ni, Pb, and Mo, respectively, whereas 0.62, 30, and 23% of the samples had Cu, Fe, and Mn higher than their drinking water standards. Annual loads of dissolved metals in the runoff varied widely among monitoring sites and were different between the year 2001 and 2002. The concentrations of heavy metals in the surface runoff were associated with the accumulation of the metals in the soils. The 0.01MCaCl2extractable Cu, Fe, Zn, and Mn in soil were found to significantly correlate with Cu, Fe, Zn, and Mn concentrations in the surface runoff.
Hydraulic Properties in a Silt Loam Soil under Natural Prairie, Conventional Till, and No‐Till
Authors: Fuentes Juan P. , Flury Markus , Bezdicek David F.
Published: July 2010

Tillage in the Palouse region of Washington State over the past 100 yr has influenced the soil physical and biological properties. In particular, hydraulic properties are significantly affected by soil cultivation. The objectives of this study were to assess the temporal patterns of soil hydraulic properties under three management systems, natural prairie (NP), conventional till (CT), and no‐till (NT), and to compare hydraulic properties between these three systems. Saturated and near‐saturated hydraulic conductivities (up to −15 cm‐H2O hydraulic head), and soil water retention curves were determined using intact soil cores taken from the top 10 cm of soil. Soils were sampled at six different times during a period of 1.5 yr from a NP, a long‐term (>100 yr) CT, and a 27‐yr‐old NT system. The NP represented the original soil and natural vegetation of the area. Significant temporal variation in hydraulic conductivity was found. Temporal variation was most evident in the NP soil, where organic matter content was twice as large as under the CT and NT soils. Hydraulic conductivities in the NP were about one order of magnitude larger than in the cultivated soils. In NT, saturated hydraulic conductivities in the top 5 cm of soils were significantly larger than in CT. No‐till and CT soils had similar near‐saturated hydraulic conductivities, indicating that even 27 yr of continuous NT could not restore the original hydraulic properties of the soil. Restoration of original hydraulic properties in cultivated former prairie soils may take considerably longer.
Carbon and Nitrogen Pools of Southern High Plains Cropland and Grassland Soils
Authors: Bronson Kevin F. , Zobeck Ted M. , Chua Teresita T. , Acosta-Martinez Veronica , van Pelt R. Scott , Booker J. D.
Published: July 2010

Soil C and N have long been recognized as important indicators of soil productivity. The current low levels of soil C and N of cropland soils have led to interest in sequestering C with reduced tillage cropping systems and the Conservation Reserve Program (CRP). Our objective was to assess agroecosystem effects on soil C and N pools in the Southern High Plains. The agroecosystems included three cotton (Gossypium hirsutumL.) cropping systems, CRP land, and native rangeland (NR). We sampled 0‐ to 5‐, 5‐ to 10‐, 10‐ to 15‐, and 15‐ to 30‐cm soil depths at 12 farm sites in five counties in West Texas. Total soil C and N, particulate organic matter (POM) C and N, natural abundance of carbon‐13 isotope (δ13C) of POM and of whole soil, potentially mineralizable C and N, water‐extractable carbon (WEC), and extractable ammonium (NH+4) and nitrate (NO−3) were determined. Total C and N in the 0‐ to 30‐cm soil profile were 34 Mg C ha−1and 2.5 Mg N ha−1for NR, and 23 Mg C ha−1and 1.9 Mg N ha−1for cropland systems, respectively. Total soil C and N in CRP land were greater in cropland soils only in the 0‐ to 5‐cm layer, and were 24 Mg C ha−1and 2.1 Mg N ha−1in 0 to 30 cm. Labile C and N pools were positively correlated with each other and with total soil C and N. Low soil test P may have limited C and N sequestration in CRP land and NR. Improved management practices are needed to sequester C and N in CRP and conservation‐tillage cotton systems in the Southern High Plains.
Nitrogen Mineralization Following Fertilization of Douglas‐fir Forests with Urea in Western Washington
Authors: Fox Thomas R.
Published: July 2010

Nitrogen mineralization following repeated applications of urea fertilizer was determined in the A horizon soil from two stands of Douglas‐fir [Pseudotsuga menziesii(Mirb.) Franco] in the Cascade Mountains of Washington. Repeated applications of urea at rates ranging from 0 to 600 kg N ha−1were made at annual and 5‐yr intervals over a 6‐yr period. Nitrogen fertilization increased N mineralization potential in these soils. However, soil N mineralization followed a quadratic relationship with the total amount of N applied in fertilizer over the 6‐yr treatment period, increasing up to total application rates of 450 kg N ha−1and then declining at higher rates. The decrease in N mineralization rates at the high N fertilization rates may be due to changes in the quality of soil organic matter, which reduced the effectiveness of extracellular enzymes and decreases the rate of decomposition and mineralization. Soil pH dropped following urea fertilization, with greater declines observed in the highest rates of urea fertilizer. Decreases in extractable Ca and Mg levels in the soil accompanied the decline in soil pH. These results suggest that high rates of nitrification occurred and that nitrate leaching was stripping Ca and Mg from the cation‐exchange complex in these soils. It appears that repeated applications of urea fertilizer at low to intermediate rates may increase long‐term N availability and thus improve soil quality. However, annual applications of high rates of urea may decrease soil quality because under these circumstances N mineralization did not increase and there was a loss of cations from the soil.
Investigation into the Origin of Magnetic Soils on the Oak Ridge Reservation, Tennessee
Authors: Rivers John M. , Nyquist Jonathan E. , Roh Yul , Terry Dennis O. , Doll William E.
Published: July 2010

In 1993–1994, researchers at Oak Ridge National Laboratory collected high‐resolution airborne geophysical data on the Oak Ridge Reservation, Oak Ridge, TN. The data were collected in part to address concerns about possible undocumented hazardous waste sites. Interpretation of the aeromagnetic data was complicated, however, by the discovery in remote areas of numerous small magnetic anomalies of natural origin. Magnetic susceptibility measurements of core showed that the underlying Copper Ridge Dolomite was non‐magnetic. We attribute the magnetic anomalies to the presence of the ferromagnetic mineral maghemite (γ‐Fe2O3), which formed during pedogenesis of Fe‐enriched colluvium that had infilled low‐lying areas, including dolines. We discuss explanations offered in the literature for the formation of magnetic soils, and present evidence based on profile descriptions, thin sections, x‐ray diffraction (XRD), and scanning electron microscopy (SEM), that in this case, maghemite formed either by anaerobic microbial Fe reduction followed by the formation of single‐domain maghemite, or by abiological weathering and reduction of an Fe‐bearing mineral followed by oxidation. Naturally occurring magnetic soils may produce magnetic anomalies similar to those characteristics of anthropogenic objects, such as buried waste drums, and complicate interpretation of airborne geophysical surveys.
Calculation of the Dielectric Properties of Temperate and Tropical Soil Minerals from Ion Polarizabilities using the Clausius–Mosotti Equation
Authors: Robinson D. A.
Published: July 2010

Soil dielectric properties are commonly used to estimate soil water content, however, the relative permittivity of common soil minerals is often unknown or estimated to be 5, similar to quartz (4.6). In this study, the Clausius–Mosotti model is used to estimate mineral relative permittivity from atomic polarizability for common soil minerals. Atomic polarizabilities are used to calculate the average mineral polarizability using the oxide additivity rule. Results of these calculations are compared with measurements found in the literature for rock and mineral samples. Predictions correspond well with measurements on single crystals or obtained using the immersion method to determine relative permittivity (better than 10%). The application of this simple calculation technique allows estimates of relative permittivity to be obtained for minerals for which it is difficult to measure relative permittivity directly, such as soil clay minerals.
An Approach for Estimating the Shrinkage Geometry Factor at a Moisture Content
Authors: Chertkov V. Y. , Ravina I. , Zadoenko V.
Published: July 2010

Soil shrinkage is characterized, along with the shrinkage curve, by a partition of the volume change of the soil matrix between contributions of cracks and soil subsidence. This partition is determined by the shrinkage geometry factor (rs). Knowledge of the value of rs is important for the consideration of water and solute transport in swelling and cracking soils. The rs concept was recently used for the generalization of flow equations in the case of the axially symmetric two‐dimensional deformation of shrink‐swell soils. Sufficient accuracy of the rs value is very essential for all these applications. However, the theoretical definition and available measurement method of the rs factor include some implicit assumptions that are disturbed in real conditions. These disturbances, which are not accounted for in rs measurements, can lead to distortion of the rs value. The objectives of the work are: to explicitly formulate the assumptions; to introduce a new presentation of the rs concept based on a comparison between different shrinkage curves of a soil; to suggest an approach for estimating the rs values corrected by taking into account the disturbance of one of the assumptions; and to experimentally illustrate the approach using the simplest case of pure‐clay paste samples when they dry, shrink, and crack. The results show the necessity and practical possibility of considering the rs factor as a function of soil moisture and introducing to the factor the multiplicative correction that is connected with accounting for possible macrocracks in soil samples to be used for experimental estimation of the rs factor.