bluuebunny/crossref_metadata_2025 huggingface dataset:
Analytical Volume Change Model for Swelling Clay Soils
Authors: Tariq Ata‐ur‐Rehman , Durnford Deanna S.
Published: July 2010

Swelling clay soils undergo volume changes as the moisture content varies. This volume change is typically described by a shrinkage characteristic curve. There is a need for a generally acceptable model to describe these curves without requiring extensive soil shrinkage measurements. Models that are presently available in the literature do not represent the shrinkage characteristic curve data adequately and, thus, have limitations to their application. We have developed an analytical volume change model that was derived using the theoretical boundary conditions at the endpoints of the four shrinkage ranges. Thus, the model describes all four shrinkage ranges better without additional input requirements. The proposed model describes a smooth shrinkage characteristic curve with continuous slopes at the endpoints of each shrinkage range. The analytical model was verified against measured shrinkage characteristic curves and was found to represent the data very well.
Reconciliation between Measured and Theoretical Temperature Effects on Soil Water Retention Curves
Authors: Liu Hui Hai , Dane J. H.
Published: July 2010

Because temperature has a considerable effect on water retention curves, it should be accounted for when modeling unsaturated soil water flow under nonisothermal conditions. Changes are commonly attributed to alterations in surface tension at the air‐water interface, to changes in the volume of entrapped air, and sometimes to the presence of contaminants at the air‐water interface. Predicted changes have, however, always fallen short of experimentally determined values reported in the literature. We propose a new theory that considers soil water to be composed of continuous water and isolated packets of water. Measured soil water pressure head values are assumed to be determined by the properties of continuous water only. We hypothesize that, when the temperature increases, water flows from isolated packets to the continuous water phase, thus resulting in an additional temperature effect on the water retention curve. Based on the assumption that the continuous water content changes linearly with the total water content, we derived expressions for the change in soil water pressure head with temperature. Calculated changes in points on the water retention curve were subsequently much closer to measured values than previously reported. The good agreement between measured and predicted results suggests that more definitive experiments be conducted to investigate the role of isolated water packets on soil water retention curves.
Colloid‐Assisted Transport of Trace Metals in Roadside Soils Receiving Deicing Salts
Authors: Amrhein C. , Mosher P. A. , Strong J. E.
Published: July 2010

Calcium magnesium acetate (CMA) is an alternative to NaCl for deicing highways. It has been shown to be an effective, biodegradeable deicing salt that is less corrosive than Cl salts and less toxic to aquatic life. There has been some concern, however, that CMA may increase the mobility of trace metals from roadside soils. A calcareous roadside soil was leached with solutions of the deicing salts NaCl and CMA followed by deionized water to evaluate the potential for trace metal mobilization. Elevated concentrations of Cu, Pb, Ni, Cr, Fe, and organic matter were found in the deionized water leachate following the NaCl leaching. Ultrafiltration and dialysis tubing were used to separate dissolved matter from colloidal (1000 molecular weight cutoff) and to determine if the trace metals were preferentially sorbed to small organic molecules and Fe oxides. Under high salinity, there was little evidence of colloid‐assisted transport. Under low salinity, dispersion and mobilization of colloidal‐sized particles occurred. Effluent Cu, Pb, and Ni concentrations were correlated with organic colloid concentrations, while Cr was correlated with Fe‐oxide concentrations. Evidence for both Fe‐organic matter colloidal complexes and Fe‐oxide mobilization was found. These findings suggest that roadside soils impacted by NaCl from deicing operations could be contributing trace metals to groundwater via colloid‐assisted transport.
Soil Surface Charge Evaluation by Back‐titration: I. Theory and Method Development
Authors: Duquette Martin , Hendershot William
Published: July 2010

Until recently, surface charge chemistry of soils has been measured using two different approaches: ion adsorption and potentiometric titration. The former provides a good measure of both anion‐exchange capacity (AEC) and cation‐exchange capacity (CEC) as a function of pH, although the procedure is time consuming. Potentiometric titration is more rapid, but the resulting curve overestimates the change in surface charge with pH. The error is largely due to consumption of H+ and OH− by dissolution and hydrolysis reactions with dissolved ions instead of being limited to protonation‐deprotonation reactions of surface charge sites. Our objective was to develop the theoretical aspects of the determination of surface charge using a back‐titration method. After suspension in Ca(NO3)2, a subsample of the soil is titrated slowly to pH 3, and then back‐titrated to pH 8 or 10. A second subsample is similarly titrated to pH 3; the soil and the supernatant solution are separated by centrifugation and filtering; only the supernatant solution is back‐titrated. The difference between the two titration curves represents the consumption of OH− by surface reactions corrected for dissolution and other solution reactions. The proposed method provides a relatively rapid means of obtaining pH‐dependent surface charge using potentiometric titration while giving results comparable to those obtained by ion adsorption.
Soil Surface Charge Evaluation by Back‐titration: II. Application
Authors: Duquette Martin , Hendershot William
Published: July 2010

Measurement of net surface charge by potentiometric titration and ion adsorption methods often gives different estimates of pH‐dependent exchange capacity. One of the major problems has been that potentiometric titration measures the H+ or OH− consumption due to surface reactions as well as those associated with dissolution of the soil and hydrolysis of ions in solution. In general, potentiometric titration has overestimated pH‐dependent exchange capacity, compared with ion adsorption methods. The backtitration procedure presented here corrects for these errors by performing two titrations. After slowly titrating the soil sample, suspended in 0.1 M Ca(NO3)2, to pH 3, the suspension is either titrated with 0.005 M Ca(OH)2 or centrifuged and the supernatant alone titrated. By subtracting the amount of OH− required to titrate the supernatant from the amount required to titrate the soil‐solution suspension, the net OH− consumption is calculated. If a value for pH‐independent CEC is known, then the pH‐dependent surface charge curve can be adjusted to give results that correspond closely to those obtained by ion adsorption. The proposed method is simpler and more rapid than the ion adsorption procedures. The inflection points on the titration curve can also be correlated with the chemical composition of the soil. Preliminary results suggest that the pH‐dependent CEC curve can be predicted from knowledge of organic matter content and various forms of amorphous Fe and Al.
Labile and Nonlabile Aqueous Silica in Acid Solutions: Relation to the Colloidal Fraction
Authors: Xu Shihe , Harsh James B.
Published: July 2010

The significance of polymeric Al‐Si complexes and Si associated with colloids on Si speciation in acidic solutions is unknown. To characterize Si solutions, the kinetics of the color reaction of Mo with Si in various synthetic Al‐Si solutions was determined and a method to quantify the labile fraction of Si was developed. We determined labile Si, dialyzable Si, and nonpositively charged Si in both synthetic and natural solutions, and total Si in solutions passing through 0.45‐, 0.2‐, and 0.1‐µm filter membranes. Silica in various solutions was divided into labile and nonlabile Si according to the rates of the reaction of Si species with Mo in 0.05 M H2SO4. Silica species in the labile fraction were <0.001 µm in diameter and were either nonpositively charged or were converted to nonpositively charged species during passage through a cation‐exchange column. The major portion of nonlabile Si was associated with colloidal particles, some of which could pass through a 0.45‐µm filter membrane. At pH 4 to 5.5, the quantity of polymeric Al‐Si complexes with sizes between 0.001 to 0.1 µm was not significant in solutions in contact with soils or soil clays except for those from a Spodosol that contained allophane and imogolite. Therefore, ignoring soluble polymeric Al‐Si complexes will not introduce a significant error in Al and Si speciation in this pH range in the absence of allophane and imogolite. Incomplete removal of colloidal particles may, however, cause significant overestimation of soluble Si and Al, if nonlabile fractions are determined.
Flocculation of Kaolinitic Soil Clays: Effects of Humic Substances and Iron Oxides
Authors: Kretzschmar Ruben , Robarge Wayne P. , Weed Sterling B.
Published: July 2010

Dispersion of clay affects soil physical properties and can result in clay transport with surface runoff or with percolating water moving through soil macropores. The water‐dispersihle fine clay (<0.2 µm) is probably the most mobile fraction with percolating water. This study was conducted to characterize the mineralogy and flocculation behavior of water‐dispersible fine clays isolated from surface horizons (Ap) of three Ultisols in North Carolina, and to study the effects of treating the clays for removal of organic substances and Fe oxides. Critical coagulation concentrations (CCC) of dilute suspensions (0.5 g clay L−1) in CaCl2 and KCl solutions were determined for: (i) untreated clays; (ii) clays treated with NaOCl for removal of organic substances; and (iii) clays treated with NaOCl and dithionite‐citrate‐bicarbonate for removal of organic substances and Fe oxides. Suspension pH was varied between 4.5 and 9.0. Analysis by x‐ray diffraction (XRD), thermogravimetry, and Mössbauer spectroscopy showed that the mineralogy of the clays was dominated by kaolinite and hydroxy‐Al‐interlayered 2:1 clay minerals (HIM), with smaller amounts of gibbsite and poorly crystalline, Al‐substituted Fe oxides (hematite and goethite). The untreated clays contained between 17 and 32 g total C kg−1, and difference infrared spectra between the untreated and NaOCl‐treated clays were typical for humic substances. Treatment of the clays for removal of organic substances markedly decreased the CCC values in both CaCl2 and KCl solutions in the entire pH range studied. Treatment for removal of Fe oxides had little effect on CCC values. All fine clays exhibited high electrophoretic mobilities (u) at pH 6 (±0.6) in 0.001 M CaCl2, with u ranging between ‐2.95 × 10−8 and −5.11 × 10−8 m2 s−1 V−1. For two of the soil clays, u was not clearly affected by either of the clay treatments, indicating that there was little change in surface charge density. The results of this study strongly support the hypothesis that naturally occurring humic substances increase the colloidal stability of kaolinitic soil fine clays in aqueous suspensions. The data suggest that a combination of electrostatic stabilization and steric stabilization is responsible for this effect, with steric stabilization possibly being the more important mechanism.
Soluble Anthrone‐Reactive Carbon in Soils: Effect of Carbon and Nitrogen Amendments
Authors: DeLuca T. H. , Keeney D. R.
Published: July 2010

Studies were undertaken to determine the effect of plant residues on levels of soluble sugars measured as anthrone‐reactive carbon (ARC) in soils. Three different soils were amended with 10 g kg−1 of grain sorghum [Sorghum bicolor (L.) Moench], soybean [Glycine max (L.) Merr.] straw, or soybean straw plus 100 mg N kg−1 as glycine. The treated soils were maintained at 40% water‐holding capacity and 30 °C and observed for changes in soluble ARC, microbial biomass C, and extractable NO−3 and NH+4 at 0, 5, 10, 20, and 30 d. Changes in soluble ARC also were observed in soil treated with 2 g kg−1 cellulose with or without N added as glycine or as (NH4)2SO4. Soluble ARC in soil is initially increased by plant residue addition to soil as a result of the free sugar associated with the plant tissue. Levels of soluble ARC decline rapidly, but grain sorghum and soybean straw plus glycine treatments maintained soluble ARC at levels above the background. Soybean straw alone resulted in an N‐limited environment in which soluble ARC declined to background levels after 10 d. Microbial biomass C increased in all treated soils. Cellulose additions to soil increased soluble ARC only in the presence of added N, demonstrating the relation of free sugars to microbial activity. Soluble ARC reflects the level of free sugars associated with plant residues and that made available by microbial activity.
Properties of Iron—Manganese Nodules from a Sequence of Eroded Fragipan Soils
Authors: Rhoton F. E. , Bigham J. M. , Schulze D. G.
Published: July 2010

Iron‐manganese nodule concentrations increase in Ap horizons of Fragiudalfs in the lower Mississippi River Valley as profile thickness is reduced by erosion. This study was conducted to determine how nodule mineralogy and chemistry change with profile depth and after incorporation into the plow layer. Nodules larger than 1.0 mm were separated from each horizon of uneroded, slightly, moderately, and severely eroded pedons. Nodules were ground to <45 µm and analyzed for organic C, in addition to Fe, Mn, and P following extraction with dithionite‐citrate‐bicarbonate (DCB) and acid ammonium oxalate (AAO). Nodules from the E/Bx horizons of the least eroded pedons had the highest DCB‐extractable Fe (Fed) contents. Distributions of AAO‐extractable Fe (Feo) fluctuated with depth; however, Feo/Fed ratios reached a minimum in E/Bx horizons. In A horizon nodules, Fed increased with degree of erosion, ranging from 70 (uneroded) to 99 g kg−1 (severely eroded). Conversely, Feo and Feo/Fed decreased in Ap horizon nodules as erosion increased. Phosphorus contents in DCB extracts followed the Fed depth distribution curve. Manganese extracted by DCB decreased with profile depth and severity of erosion. Although Feo/Fed ratios suggested the presence of ferrihydrite, goethite was the only Fe‐oxide mineral identified from differential x‐ray diffraction (DXRD) scans. Goethite XRD peak intensities tended to reflect Fed contents, reaching a maximum in the E/Bx horizons and A horizons of the more severely eroded pedons. The color of the crushed nodules from the A horizons became progressively lighter as degree of erosion increased, due to increases in Fed and decreases in Mnd and organic C. Manganese had the greatest effect on color, accounting for 93 and 85% of the variability in value and chroma, respectively. However, no crystalline Mn‐oxide minerals were identified. Incorporation of E/Bx nodules in Ap horizons by gradual erosion and removal of finer materials is probably responsible for the observed changes in nodule color, chemistry, and mineralogy.
Laboratory Method for Studying Pesticide Dissipation in the Vadose Zone
Authors: Rambow Jürgen , Lennartz Bernd
Published: July 2010

A laboratory apparatus was constructed to study the leaching behavior of soil‐applied herbicides in disturbed and undisturbed soil columns under vadose zone conditions. The setup was built in such a manner that a broad variety of field conditions regarding rainfall characteristics and soil water tension could be simulated. Long‐term transport experiments up to 3 mo required an automatic withdrawal of the soil percolate and a continuous control and recording of the soil water tension, the applied vacuum, and the amount of precipitation. During the sampling of the leachate, a newly designed lock guaranteed a constant vacuum at the bottom end of the soil column. First experiments with the new apparatus showed an earlier break‐through of terbuthylazine (2‐chloro‐4‐ethylamino‐6‐terbuthylamino‐s‐triazine) and a larger solute dispersion under transient water flow than under steady‐state water flow in a sandy soil.
A Simple Method for Predicting the Spatial Distribution of Soil Hydraulic Properties
Authors: Lascano R. J. , Stroosnijder Leo
Published: July 2010

We developed a method to calculate the statistical distribution of a scaling factor from soil‐water retention curves. The method consists of measuring the particle‐size distribution by laser scattering, computing soil water retention curves using the Arya and Paris model, and calculating a scale factor for each function. The method was tested by comparing measured and predicted soil‐water retention curves. The advantage of this method is in its simplicity and ease of use. The applicability of the method to a given soil texture depends on the verification of the calculated soil‐water retention functions.
Competing Metal Ion Influences on Chromium(III) Oxidation by Birnessite
Authors: Fendorf Scott E. , Zasoski Robert J. , Burau Richard G.
Published: July 2010

The oxidation of Cr(III) to Cr(VI) presents a significant environmental hazard due to the much greater mobility and toxicity of Cr(VI). Despite the importance of Cr(III) oxidation, many factors influencing this important chemical process in soils and waters remain unknown. In this study we investigated the effects of competing metal sorbates on Cr(III) oxidation by δ‐MnO2. Aluminum, La, Co(II), Mn(II), and Cr(VI) were reacted with δ‐MnO2 prior to the addition of Cr(III). Only Al affected the extent of Cr(III) oxidation at pH ≥ 4, and at pH 5 the presence of Al, with 2.84 µmol Cr(III) initially present, decreased oxidation from 2.80 to 0.28 µmol Cr(VI). None of the competing ions influenced the degree of oxidation at pH 3. Neither of the oxidation reaction products, Mn(II) or Cr(VI), affected the extent of oxidation. Electrokinetic investigations revealed that both Al and La induced a charge reversal on δ‐MnO2. As pH increased from 3 to 5, the electrophoretic mobility (EM) of δ‐MnO2 sharply increased at pH 4 in the presence of 2.90 × 10−4 M Al. The sorption behavior and influences on EM of Al indicate that a surface alteration occurs whereby the oxidizing ability of δ‐MnO2 (birnessite) is inhibited. High‐resolution transmission electron microscopy revealed that an Al‐hydroxide surface precipitate formed on δ‐MnO2, thus accounting for the sorption, EM trends, and Cr(III) oxidation inhibition.
Nitrate Dynamics during the Aerobic Soil Phase in Lowland Rice‐Based Cropping Systems
Authors: George T. , Ladha J. K. , Buresh R. J. , Garrity D. P.
Published: July 2010

In tropical rice (Oryza sativa L.) lowlands, soil NO3 is lost during the transition from the dry to the wet season. To understand how soil and crop management influences NO3 loss, we examined NO3 dynamics during a 2‐yr period in an Alfisol in the Philippines: weedy, weed‐free, and frequently tilled main plots during the February to May dry season (DS), and Sesbania rostrata (Brem. & Oberm), mungbean [Vigna radiata (L.) R. Wilczek var. radiata], weedy, and weed‐free subplots during the May to July dry‐to‐wet transition (DWT). Weed‐free plots were maintained by removing weeds as they emerged. Soil NH4 (0–60 cm), which was not affected by management, averaged only 9 kg N ha−1. While soil NO3 increased under frequent tillage and weed‐free fallowing, it decreased rapidly under weedy fallowing. On most sampling dates, NO3 was the highest in DS tilled main plots. The widest range of NO3 during the DS or DWT was 14 to 110 kg N ha−1 in the first year, and 12 to 155 kg N ha−1 in the second. During the second half of the DWT, NO3 declined in all plots, but more markedly when plants were present than when not, indicating plant N uptake. Above‐ground plant N prior to permanent flooding ranged widely from 31 kg N ha−1 in weeds to 222 kg N ha−1 in N2‐fixing S. rostrata plants in the first year, and 37 to 193 kg N ha−1 in the second. The data also indicate NO3 leaching following heavy rains. Further, the high water‐filled pore space, exceeding 0.7 L L−1 in the second half of the DWT and approaching 1 L L−1 with permanent flooding, is presumed to have favored denitrification. Regardless of DS management or DWT plant N accumulation, the soil was virtually depleted of NO3 soon after permanent flooding; NO3 rarely exceeded 10 kg N ha−1 when measured after 9 d (first year) and 11 d (second year) of permanent flooding. Our data indicate the immense capacity of this lowland soil to accumulate NO3 and the marked effect of DS and DWT management on the amount of NO3 that actually accumulates. In tropical rice lowlands, soil and crop management during the DS should be designed to limit NO3 buildup so as to reduce NO3 that is prone to loss during the DWT.
Tillage and Winter Wheat Residue Management Effects on Water Infiltration and Storage
Authors: Dao Thanh H.
Published: July 2010

In traditional winter wheat (Triticum aestivum L.) cropping regions of the southern Great Plains, production constraints exist because of extreme variability in precipitation during planting periods. Intensive tillage aggravates dry early fall conditions to further hamper wheat emergence and growth. From 1983 through 1991, we determined the effects of three tillage and residue management practices on soil water storage in Bethany (fine, mixed, thermic Pachic Paleustoll) and Renfrow (fine, mixed, thermic Udertic Paleustoll) silt loams on a 2% slope near El Reno, OK. We measured soil management cumulative effects on soil bulk density and water infiltration during and after the 1990–1991 growing season. Every season, no‐till soil consistently had higher volumetric water content in the 0‐ to 1.2‐m depth, except in late fall or early spring when root‐zone recharge took place for both soils. The gain was corroborated by increased water‐holding capacity and decreased bulk density of no‐till soils. Seasonal variability of field infiltration was more evident in plowed soil than in no‐till soil. Such temporal dependence would affect surface distribution and flow of precipitation to explain differences in runoff, soil erosion, and environmental impacts of tillage systems between October and June. Water infiltration into no‐till soil was significantly higher than into plowed soil at similar water contents. Under ponding, recharge occurred through macropores, directly wetting depths of 0.4 to 0.6 m of no‐till soil, in contrast to a layered pattern in plowed soil. The phenomenon has many implications on profile recharge rate and solute transport in the field. Therefore, elimination of inversion tillage enhanced precipitation storage, thereby alleviating detrimental effects of climate variability in annual winter wheat cultivation.
Environmental Tracers for Water Movement in Desert Soils of the American Southwest
Authors: Phillips Fred M.
Published: July 2010

Tritium and 36Cl from nuclear weapons testing fallout can serve as tracers for water and solute movement in desert soils on the time scale of decades. Chloride deposited from the atmosphere can provide a tracer for much longer periods. We reviewed and compared studies using these tracers at locations across the southwestern USA in order to assess the controls on water and solute movement across time scales ranging from decades to millennia. The displacements of all the tracers were quite similar despite variations in location, climate, and soil properties. The time‐averaged 36Cl fallout transport rate velocity is 23 ± 17 mm yr−1. The Cl concentration profiles indicate that, prior to about 15 000 yr ago, soil‐water fluxes were approximately 20 times the magnitude of modern fluxes, and that the soil water shifted rapidly (on a geological time scale) to the dry regime between 16 000 and 13 000 yr ago. The remarkable uniformity of the water and solute transport processes across the region may be dictated by the water extraction efficiency of the vegetation.
The Feasibility of Recharge Rate Determinations Using the Steady‐State Centrifuge Method
Authors: Nimmo John R. , Stonestrom David A. , Akstin Katherine C.
Published: July 2010

The establishment of steady unsaturated flow in a centrifuge permits accurate measurement of small values of hydraulic conductivity (K). This method can provide a recharge determination if it is applied to an unsaturated core sample from a depth at which gravity alone drives the flow. A K value determined at the in situ water content indicates the long‐term average recharge rate at a point. Tests of this approach have been made at two sites. Unsaturated K values were measured easily for sandy core samples from a site in the San Joaquin Valley of California. The results indicate that a better knowledge of the matric pressure profiles is required before a recharge rate can be determined. Fine‐textured cores from a site in southeastern Washington required new developments of apparatus and procedures, especially for making centrifuge measurements with minimal compaction of the samples. Measured K values led to preliminary recharge rate determinations that are reasonable considering the known hydrology and topography of the site.
Accumulation of Water in Soils under Gravel and Sand Mulches
Authors: Kemper W. D. , Nicks A. D. , Corey A. T.
Published: July 2010

Opportunities to influence the portion of the precipitation that percolates through the soil to aquifers lie primarily in the management of vegetation to control transpiration and of the surface to control infiltration and evaporation. Rates of percolation and recharge are of major interest to those concerned with the quantity and quality of water in our aquifers. Evaporative loss, precipitation, and water accumulation were measured on 0.9‐m‐long columns containing soil topped by sand and gravel mulches in the field for 388 d. Results show evaporation reductions due to gravel and sand mulches of differing thickness, color, and particle size on soil surfaces. These data show effects of mulch thickness, particle size, and color on the steady and transient components of evaporation and provide coefficients for equations to help predict effects of such mulches on evaporation and accumulation of water in soils and aquifers. Gravel mulches 5 cm thick resulted in accumulation of 80 to 85% of the annual precipitation.
Groundwater Mounding below a Surface Line Source in a Typic Fragiudalf
Authors: Daniels M. B. , Fritton D. D.
Published: July 2010

Nearly one‐third of the soils in Pennsylvania contain fragipans. Onsite sewage disposal systems on these soils historically have been failure prone. This leads to a public health hazard due to contamination of surface and groundwaters with unrenovated effluent. In order to gain a better understanding of water flow in a soil that contains a fragipan, a field study was conducted to determine in situ, two‐dimensional, steady‐state water flow patterns from a surface line source. Tensiometers were used to determine the soil water pressure head distribution for five application rates ranging from 125 to 625 mm d−1 in 125 mm d−1 increments. Field results indicated that steady‐state groundwater mounds (mound‐shaped regions of saturation) developed for all application rates because of the reduced permeability of the fragipan and the presence of small lateral hydraulic gradients on either side of the saturated region. Field results were simulated with a finite difference solution of the steady‐state Richards' equation. Simulated results agreed well with field measurements within the region of saturation for the three highest application rates. However, the model underpredicted the height of the mounds for the two lowest application rates. The application rate both experimentally and theoretically seemed to have little influence on the hydraulic head beyond 2 m laterally in either direction from the source.
Profile Nitrous Oxide and Carbon Dioxide Concentrations in a Soil Subject to Freezing
Authors: Burton D. L. , Beauchamp E. G.
Published: July 2010

Soil is an important source for the production of greenhouse gases such as CO2 and N2O. Significant temporal trends exist in the production and concentration of these gases in temperate climates where soils are subject to freezing. We examined the effect freeze‐thaw cycles had on soil profile CO2 and N2O concentrations. A multilevel sampling probe was designed to allow sampling the atmosphere composition of soil profiles. Changes in CO2 and N2O gas concentrations in the profile of a loamy sand were studied during two winter periods. Fluxes of N2O were strongly influenced by the formation of a frozen layer that segregated the soil profile into two distinct regions. The surface region was characterized by brief, intense events, which contribute to highly variable CO2 and N2O concentrations near the surface. The subsurface region beneath the ice layer allowed N2O accumulation. The highest concentrations of N2O were observed in the subsurface region following ice‐layer formation, indicating that N2O production is not restricted to surface horizons in this soil. Thawing of the frozen layer in the spring resulted in the release of N2O from the subsurface region. The formation of a frozen layer influenced subsurface CO2 accumulation through its influence on water percolation.
Comparison of Lysimeter Types in Collecting Microbial Constituents from Sewage Effluent
Authors: Krejsl Jana , Harrison Rob , Henry Chuck , Turner Nedda , Tone Dennis
Published: July 2010

There are no standard methods of determining the impact of septic systems on contamination of groundwater aquifers. Sampling of water for microbiological properties in unsaturated soil is particularly difficult. Successful water sampling devices should collect large amounts of percolating water with minimal change in chemical and biological properties of the water. This laboratory study compared the effectiveness of four types of soil water sampling devices for qualitative sampling of water, using total coliform, fecal coliform, and fecal streptococcus as the primary indicators of water quality. The devices tested were: (i) high‐flow ceramic tension lysimeter, (ii) high‐flow fritted glass tension lysimeter, (iii) sand‐filled lysimeters, and (iv) perforatedtube wells. A constant source of vacuum (0.01 MPa) was applied to all devices to collect water samples. High‐flow ceramic tension lysimeters collected adequate volumes of water (an average of 0.340 L after 6 h) but excessive filtering led to underestimation of microbial numbers (only 6% of total coliform, 2.2% of fecal coliform, and no fecal streptococcus were collected). Perforated‐tube wells did not collect any water under partially saturated soil conditions; collection occurred only below the water table. Sand‐filled lysimeters showed the best combination of volume collection (an average of 0.660 L after 6 h) with good estimation of microbial numbers (90% for total coliform, 83% for fecal coliform, and 45% for fecal streptococcus).
Map Unit Development for Sand‐ and Gravel‐Pit Soils in New Hampshire
Authors: Strain M. R. , Evans C. V.
Published: July 2010

Current growth rates of human population have increased the areal extent of anthropogenic soils. One of the most important aspects of this disturbance is the creation of new soil surfaces and landscapes, most of which are not currently characterized, classified, or mapped in sufficient detail to develop adequate interpretations for subsequent uses. We characterized 20 pedons in four abandoned sand and gravel pits in order to determine which physical, chemical, and morphological properties were most useful in the development of soil map units and interpretations. Organic C and bulk density values significantly differentiated groups of pedons, and we expanded on concepts presented by others by using bulk density and organic C to define great groups and subgroups. The resulting classes reflect soil characteristics that are important for use and management of these soils, and are also more connotative of soil genesis than the names “pits”, “gravel”, or “Udorthents”, which are used in most soil survey legends.
Use of Theissen Areas in Models of Nutrient Uptake in Forested Ecosystems
Authors: Porter P. S. , Comerford N. B. , Escamilla J. A.
Published: July 2010

Nutrient uptake models have generally incorporated simplifying assumptions, one of which is that roots are regularly arrayed in space. This allowed an evaluation of an average root to be extrapolated to a whole plant by simply multiplying the single root result by the total root length. In low‐fertility soils, particularly those with very low buffer powers, this regular root assumption causes nutrient uptake to be overestimated. Nutrient uptake models would be more realistic if Theissen areas were used as the area of influence of roots. A computer program for computing Theissen maps for a hydrological application has been adapted, tested, and applied to nutrient uptake by roots in a forested ecosystem. Nutrient uptake using Theissen areas is compared with uptake based on the assumption of regularly arrayed roots and with worst‐case situations of uptake influenced by nearest‐neighbor roots. For slash pine (Pinus elliottii Engelm. var. elliottii) growing on a Spodosol of the lower Coastal Plain of the USA, root patterns of the A horizon have been found to be random, not regular. Computer simulations of nutrient uptake showed that the average root procedure was appropriate for K under the conditions investigated, but suggested that P uptake would be overestimated unless the spatial pattern of roots was included. Ratios of Theissen radii to 1/2 nearest neighbor distances show that root areas of influence tend to be noncircular or that roots are eccentrically positioned, either of which are violations of assumptions used in nutrient uptake models.
Downed Bole‐Wood Decomposition in Lodgepole Pine Forests of Central Oregon
Authors: Busse M. D.
Published: July 2010

Input of decaying wood from natural disturbances may benefit nutrient cycling processes in lodgepole pine [Pinus contorta var. murrayana (Grev. & Balf.) Engelm.] forests of central Oregon by supplementing detrital C and nutrient pools. This study was conducted to determine the decomposition rate and nutrient retention of downed lodgepole pine boles, and to characterize their effect on soil biological and chemical properties. Three sites, in stands recently damaged by insects, had an average downed wood biomass of 38.7 Mg ha−1. Bole‐wood decay was rapid once in contact with the soil surface. The decomposition rate constant based on the single‐exponential decay model was 0.027 yr−1 and the turnover time of C was 37 yr. Nitrogen content per unit volume was 75% greater for downed boles after 38 yr of decay compared witb their initial content. A net release of Ca (38%), Mg (54%), and K (42%) was found, but P content remained constant during decomposition. With the exception of Mg, however, downed boles comprised <3% of the combined mineral soil, O horizons, and downed wood nutrient pool. In comparison, downed boles comprised 23.5% of the detrital C. Microbial biomass C (CM) and the ratio of microbial C to total organic C (CM/CT) were significantly greater in the surface 0 to 4 cm of soil beneath rapidly decaying boles than in soil without a bole‐wood component. The CM/CT ratio remained elevated throughout the advanced stages of wood decomposition, indicating a long‐term change in the efficiency of C utilization by soil microbial communities associated with decaying wood.
Effect of Tensiometer Cup Size on Field Soil Water Tension Variability
Authors: Hendrickx J. M. H. , Nieber J. L. , Siccama P. D.
Published: July 2010

Reliable estimates of soil water tension in field soils using few tensiometers are often difficult to obtain because of the large spatial variability of soil water tensions. Therefore, the objective of this study was to evaluate the effect of tensiometer cup size on soil water tension variability. We installed three sets of tensiometers with different effective outside cup surface areas (4.8, 42.3, and 88.3 cm2) in a silty clay loam that was flood‐irrigated for sorghum [Sorghum bicolor (L.) Moench] production. During a period of 10 mo, soil water tensions were measured with a tensiometer under fallow and cropped conditions on 71 nonconsecutive days. We found that an increase in tensiometer cup size reduced the variability of soil water tension measurements. This reduction in variability suggests that the medium cup size (42.3 cm2) extensively used for irrigation scheduling and environmental monitoring, is rather small and consequently causes a relatively large variability. Therefore, the medium cup should be replaced by a larger cup size. We also demonstrate that a logarithmic transformation of the water tensions is necessary to stabilize their variance and to allow pooling of the variances. The use of larger tensiometer cup sizes and the pooling of variances of log‐transformed soil water tension measurements will lead to a lower number of tensiometers per irrigated field or environmental site.
Organic Matter Comparison of Wettable and Nonwettable Soils from Bentgrass Sand Greens
Authors: Hudson R. A. , Traina S. J. , Shane W. W.
Published: July 2010

Previous studies have attributed hydrophobicity in sandy soils to alkaline extractable macromolecular complexes, as well as lipid fractions, but have not included wettable soils for comparison. Our study was conducted to compare the alkaline‐extractable and lipid fractions of a nonwettable soil to a wettable soil from creeping bentgrass (Agrostis stolonifera L. var. stolonifera) sand greens. Samples were collected from two sites and several extraction sequences performed on each. Humates were extracted in 1.25 mol L−1 NaOH. Alkaline‐extractable fractions were separated by precipitation with 6 mol L−1 HCl. Acid precipitated and nonprecipitated fractions were purified, lyophilized, and analyzed by Fourier transform infrared spectroscopy. In addition, the acid nonprecipitated fractions, which included fulvic acids, were analyzed by solid‐phase 13C nuclear magnetic resonance spectroscopy. Lipid fractions were extracted with a soxhlet apparatus using either hexane or methanol as solvent. Lipid fractions were analyzed by gas chromatography‐mass spectroscopy. No qualitative differences were observed with Site 1 extracts, and it appeared that hydrophobicity may have been at least partly due to physical or structural differences. Qualitative differences were observed with Site 2 extracts when initially extracted with methanol.
Stability Constants of Pseudobactin Complexes with Transition Metals
Authors: Chen Yona , Jurkevitch Edouard , Bar‐Ness Eli , Hadar Yitzhak
Published: July 2010

The stability constants of the fluorescent siderophore pseudobactin St3 (PSB3) produced by Pseudomonas putida strain 3 with Fe3, Cu2+, Mn2+, Zn2+, and Fe2+ were determined using fluorescence spectroscopy and potentiometric titrations. Stability constants for PSB3, with Fe3+, Fe2+, Mn2+, and Zn2+ were determined based on titration to saturation of a PSB3 ligand solution with either of the metals, then back‐titrating the solution with an EDTA solution along with simultaneous measurements of fluorescence and visible spectra. A nonlinear curve fitting between the fluorescence intensity (Y) and the concentration (C) of the metal or EDTA was used for the determination of the concentrations of the free and complexed ligands. The overall stability constant (log KML) obtained for Fe3+ by this method was the same as that obtained by visible spectrosopy. The log KML. values obtained by fluorescence spectroscopy for Mn2+ and Zn2+ (17.5 and 19.4, respectively) were in good agreement with log KML values obtained by potentiometric titration (17.3 and 19.8, respectively). It was found that two chromatography bands obtained from PSB3 solutions isolated and purified from cultures grown under different conditions chelated Fe3+ with practically the same affinity. The log KML of PSB3 with Fe3+, Zn2+, Cu2+, Mn2+, and Fe2+ were calculated using the protonation constants determined by potentiometry (pKa1 = 11.28, pKa2 = 7.74, pKa3 = 5.35) and were found to be 29.6, 19.8, 22.3, 17.3, and 8.3, respectively.
Soil Phosphorus Desorption Kinetics and Its Relationship with Plant Growth
Authors: Raven K. P. , Hossner L. R.
Published: July 2010

The kinetics of P desorption in soils is important to consider from a plant nutrition perspective. This study was designed to compare the ability of several equations to describe P desorption kinetics from soils and to relate the information derived from desorption kinetic curves to plant growth and P uptake. Samples of five soils (Psammentic Paleudult, Typic Calciustoll, Udic Pellustert, Vertic Albaqualf, and Aridic Calciustoll) were treated with five P levels (0, 25, 50 75, and 100 mg kg−1) and incubated at field capacity and 24 °C for 31 d. Phosphorus desorption kinetic data for each treatment were generated by equilibrating suspensions of the incubated soil samples with mesh bags containing HCO−3‐saturated anion‐exchange resin for periods up to 96 h. Desorbed P was then determined by extraction of resin P with 0.5 M NaC1. Initial (after 0.25 h) and final (after 72 h) amounts of desorbed P and desorption rates were determined. The kinetic data were best described by the expanded form of the Elovich equation. The initial and final amounts of desorbed P were closely related to relative plant growth and P uptake. The final desorbed soil P was the best soil P availability index. Soil P desorption rates generally had soil‐specific relationships with growth of corn (Zea mays L.) and P uptake and did not seem to limit the P nutrition of plants. The parameters of the expanded Elovich equation were not satisfactory indicators of the P‐supplying capacity of the soil.
Analyzing Root Competition with Dirichlet Tessellation for Wheat on Three Landscape Positions
Authors: Van Rees K. C. J. , Hoskins J. A. , Hoskins W. D.
Published: July 2010

The underlying mechanisms for root competition are not completely understood; yet, it is important to accurately define the zone of influence around individual roots for nutrient or water uptake, particularly when modeling nutrient uptake processes. The objectives of this study were to investigate the use of the Dirichlet tessellation for quantifying soil polygons around roots of spring wheat (Triticum aestivum L. cv. Katepwa) grown on three landscape positions and to quantify the root spatial patterns. Roots were exposed on a horizontal soil plane at 2‐, 5‐, and 10‐cm depths and mapped on acetate sheets at 21, 45, and 88 days after planting (DAP) for shoulder, backslope, and footslope positions. Tessellation of scanned root maps showed that soil polygons decreased in size during the sampling period; however, polygon areas increased with soil depth regardless of slope position and ranged from 0.08 to 52.77 cm2. Eccentricity averaged 0.74 ± 0.01, indicating that polygons were more circular than elliptical and root displacement averaged one‐third the distance from the polygon center to the polygon edge. Root spatial patterns using R‐index values, showed that roots on shoulder positions had clumped to random patterns while roots at the footslope had random to regular patterns. Root spatial patterns and displacement for wheat violated the assumptions used in nutrient uptake modeling and should be incorporated into future modeling efforts. Dirichlet tessellation is an effective procedure for analyzing root competition and has promise for multispecies competition if different root systems can be weighted by morphological or physiological characteristics.
Spatial Patterns of Soil Organic Carbon in the Contiguous United States
Authors: Kern Jeffrey S.
Published: July 2010

Spatial patterns and total amounts of soil organic C (SOC) are important data for studies of soil productivity, soil hydraulic properties, and the cycling of C‐based greenhouse gases. This study evaluated several approaches for characterizing SOC to determine their relative merits. The first approach entailed grouping data from a global pedon SOC database by type of ecosystem, resulting in a total of 78.0 Pg of C (Pg = 1015 g) to 1‐m depth for the contiguous USA. In a second approach, a pedon database was aggregated using soil taxonomy, resulting in a total for the contiguous USA of 80.7 ± 18.6 Pg of C when the great group SOC was spatially distributed with Major Land Resource Areas (MLRAs) using the 1982 National Resource Inventory (NRI) and the Soil Interpretation Record databases. The third approach used pedon and spatial data from a global soil map grouped by soil unit that resulted in 84.5 Pg of C for the contiguous USA. Although the ecosystem and soil taxonomic approaches resulted in similar totals, the taxonomic approaches are recommended because they gave more realistic results in areas of Histosols, shallow soils, and soils with high rock fragment content. The ecosystem approach did not give reliable spatial patterns and is only useful for very broad‐scale work where precisely georeferenced data are not needed. Grouping data by great group provided more information than grouping by order or suborder. The approach based on soil taxonomy is very useful because it is based on the NRI statistical framework and it allows stratification by other NRI items, such as land use and vegetation.
Inheritance of 2:1 Phyllosilicates in Costa Rican Andisols
Authors: Jongmans A. G. , van Oort F. , Nieuwenhuyse A. , Buurman P. , Jaunet A. M. , van Doesburg J. D. J.
Published: July 2010

The occurrence of 2:1 phyllosilicates in Andisols is variously ascribed to in situ pedogenic origin, aeolian addition, or the presence of hydrothermally altered rock fragments. We studied the origin of 2:1 phyllosilicates that occur in Holocene Hapludands on andesitic, sandy beach ridges in Costa Rica by micromorphological, mineralogical, and submicroscopical techniques. The 2:1 phyllosilicates also occur as pseudomorphs after primary minerals in fresh rock of the inland volcanoes, from which the parent material of the beach ridges was mainly derived. Hydrothermal processes are most likely responsible for the formation of such pseudomorphs. Rock weathering produces sand‐sized rock fragments with clay pseudomorphs and also liberates individual pseudomorphs. Subsequent erosion and alluvial transport affect their shape, but not their internal fabric. In the beach ridges, clay pseudomorphs appear as individual, sand‐sized clay bodies, and inside sand‐sized andesitic rock fragments. Submicroscopical analyses of these individual clay bodies and andesitic rock fragments with clay pseudomorphs indicate a predominance of 2:1 phyllosilicates. This implies that they are inherited from the parent material and are not due to postdepositional soil formation in the beach ridges. Weathering and biological activity affect the clay bodies and rock fragments with clay pseudomorphs, leading to the formation of clay‐sized particles consisting of 2:1 phyllosilicates. Toward the soil surface, these particles are incorporated into the allophanic groundmass resulting from actual soil formation. The geographically extensive occurrence of 2:1 phyllosilicates in Andisols suggests that the genetic processes described here may have more than regional validity.