bluuebunny/crossref_metadata_2025 huggingface dataset:
Geographic Patterns of Soil Water‐Holding Capacity in the Contiguous United States
Authors: Kern Jeffrey S.
Published: July 2010

The geographic patterns of soil water‐holding capacity are important for studying the response of vegetation and water supply to climate change. This study was conducted to evaluate which dataset to use for national‐scale water balance modeling. Spatial databases of soil properties for the contiguous USA were derived from the USDA national soil database (NATSGO) and the United Nations soil map of the world. The NATSGO data generally resulted in finer textures, greater rock fragment content, and more shallow soils. The organic matter content in the surface was similar, but the NATSGO database did not contain sufficient information to estimate organic matter content of the lower soil. The Rawls model was used to estimate soil water‐holding capacity of the 0‐ to 50‐cm depth from particle‐size distribution, organic matter content, rock fragment content, and soil depth data. The Saxton model was used to estimate the water‐holding capacity of the 50‐ to 150‐cm depth interval, because it didn't require organic matter data. The water‐holding capacity in the surface layer based on the NATSGO data was larger than that based on the soil map of the world despite greater rock fragment contents and shallower soil depths. There were few similarities in the water‐holding capacities of the 50‐ to 150‐cm depth layer because of large differences in rock fragment content and soil depth. The NATSGO dataset is recommended because the map unit composition was based on a statistical framework with a large sample size and it better characterizes rock fragment content and soil depth.
Evaluation of Soil Water Retention Models Based on Basic Soil Physical Properties
Authors: Kern Jeffrey S.
Published: July 2010

Algorithms to model soil water retention are needed to study the response of vegetation and hydrologic systems to climate change. The objective of this study was to evaluate some soil water retention models to identify minimum input data requirements. Six models that function with various combinations of particle‐size distribution, bulk density (ρb), and soil organic matter data were tested using data for nearly 6000 pedons. The Rawls model, which requires particle‐size distribution and organic matter data, had the lowest overall absolute value of the mean error (ME) with 0.020, 0.001, and 0.007 m3 H2O m−3 soil for matric soil water pressures of −10, −33, and −1500 kPa, respectively. The Saxton model, which requires particle‐size distribution data, had small MEs (0.018 and 0.007 m3 H2O m−3 soil) for −10 and −1500 kPa matric soil water pressures, and a moderately small ME (0.017 m3 H2O m−3 soil) at −33 kPa. The Vereecken model, which requires ρb, particle‐size distribution, and organic matter data, had small MEs (0.016 and 0.009 m3 H2O m−3 soil) at matric soil water pressures of −10 and −33 kPa, with a larger ME (0.020 m3 H2O m−3 soil) at −1500 kPa. The remaining three models had relatively large MEs for at least two of the three matric soil water pressures. For estimating water‐holding capacity only, the Saxton model is adequate. The Rawls model is recommended for characterizing the relationship of water content to matric soil water pressure.
Water Content Effect on Soil Salinity Prediction: A Geostatistical Study Using Cokriging
Authors: Vaughan Peter J. , Lesch Scott M. , Corwin Dennis L. , Cone David G.
Published: July 2010

A geostatistical analysis of soil salinity in an agricultural area in the San Joaquin Valley included measurements of electrical conductivity of soil paste extract (ECe) and water content of soil samples supplemented by surface measurements of apparent electrical conductivity (EMH). Prediction of soil salinity at unsampled points by cokriging loge(ECe) and EMH is worthwhile because EMH measurements are quicker than soil sampling. This work studies how patterns of loge(ECe) predicted by cokriging with EMH are influenced by variation in gravimetric water content (W). The data are mean EMH = 1.00 ± 0.13 dS m−1 for 2378 locations, mean loge(ECe) = 1.40 ± 0.29 dS m−1, and mean gravimetric W = 0.260 ± 0.003, both averaged for four samples from 0.3‐m intervals to 1.2‐m depth for 315 locations. The coefficient of determination (R2) for EMH vs. loge(ECe) increased with depth from 0.05 to 0.54 whereas the R2 for EMH vs. W decreased from 0.48 to 0.28. A gray‐scale EMH map contained nine out of 56 quarter‐section boundaries coinciding with step variations in EMH. The t‐statistics for differences in mean W were six of nine significant at 0.001 and nine of nine at 0.05, but mean loge(ECe) had only two of nine at 0.05, implying that W caused EMH steps. Water‐affected EMH impaired prediction of ECe at depth by cokriging, because near‐surface variations in W masked ECe. Two subareas were defined, one where management factors, such as irrigation, controlled EMH, causing steps, and one where near‐surface W varied less, making cokriging predictions more reliable.
Phyllosilicate Distribution and Origin in Aridisols on a Granitic Pediment, Western Mojave Desert
Authors: Boettinger J. L. , Southard R. J.
Published: July 2010

There is considerable uncertainty about the extent of mineral weathering and neosynthesis in arid soils. Although researchers have speculated that smectite neosynthesis can occur in soils containing opaline silica cement, no evidence has been presented to support this hypothesis. We investigated the mineralogy of two Aridisols in the western Mojave Desert, California, to study phyllosilicate distribution and origin in soils with opaline silica. The clay fraction of both soils is dominated by Al‐rich, dioctahedral smectite, characterized by a Mg‐saturated d(001) spacing of 1.52 nm and a d(060) spacing of 0.149 to 0.150 nm. This smectite is also present in silt and sand fractions of deeper horizons, where it exists mainly as microagglomerates of clay‐sized crystals. Biotite is most abundant in silt and sand fractions of near‐surface horizons of both pedons, where physical weathering is greatest. Deep, alkaline, silica‐rich horizons of both pedons contain more silt‐ and sand‐sized vermiculite than biotite, probably due to rapid chemical weathering of biotite to trioctahedral vermiculite. Clay‐ and silt‐sized hydroxy‐interlayered 2:1 minerals are present in the upper horizons of these soils. Given the lack of gibbsite and substantial amounts of kaolinite and Al hydroxy‐interlayered 2:1 phyllosilicates, we propose that neosynthetic dioctahedral smectite was the dominant sink for Al released by feldspar weathering in these Aridisols. Silica not consumed in smectite neosynthesis can cement microagglomerates. We further speculate that minimal physical weathering in the alkaline, high‐silica environment of deep horizons favors aggregation of smectites to silt‐ and sand‐sized microagglomerates.
Measurement of Divalent Lead Activity in Lead Arsenate Contaminated Soils
Authors: Kalbasi M. , Peryea F. J. , Lindsay W. L. , Drake S. R.
Published: July 2010

Many soils used for tree fruit production before 1968 contain elevated Pb concentrations resulting from use of lead arsenate pesticides. The chemical form of Pb in these soils may influence its bioavailability and the need for and type of remediation actions. The competitive chelation method was used to determine the single‐ion activity of Pb2+ in aqueous suspensions of 11 lead arsenate contaminated soils and, for comparison, three Pb‐enriched roadside soils. The activity of Pb2+ ranged from 10−9.37 to 10−7.21, consistent with values published elsewhere, and was inversely related to pH. The soils appeared to fall into two solubility groups, one where Pb2+ activity was consistent with control by a Pb‐phosphate mineral, possibly Pb5(PO4)3OH or Pb3(PO4)2, and a second group where Pb2+ activity was consistent with control by Pb(OH)2. Both groups contained calcareous soils. Adding monocalcium phosphate to two of the soils from the higher Pb solubility groups shifted one soil to the lower solubility group but had no effect on the second soil. The experimental results suggest that Pb solubility in lead arsenate contaminated soils is not appreciably higher than in soils contaminated by Pb from gasoline and that phosphate amendment may not always reduce soil Pb solubility.
Effect of Adsorbed Phosphate on Transport of Arsenate through an Oxisol
Authors: Melamed R. , Jurinak J. J. , Dudley L. M.
Published: July 2010

Competitive oxyanion adsorption can increase the mobility of toxic trace elements in soils leading to groundwater contamination and a potential threat to the environment. We conducted a study to determine how phosphate fertilization might affect arsenate mobility in soil. The competitive nature of sorbed phosphate on arsenate mobility was investigated using laboratory soil columns constructed from an aggregated Oxisol. The soil was incubated with four levels of phosphate (0–48.6 mmol P kg−1) before being leached with 0.66 mmol KH2AsO4 L−1 in 0.05 M KCl. Arsenate mobility was greatly enhanced by treatment with increasing amounts of phosphate. The As breakthrough curves (BTCs) were divided into two phases. The initial phases of arsenate interaction was consistent with ligand exchange of Me‐OH+2 groups, where Me is the coordinating metal atom at the oxide surface. This phase was characterized by the constant pH of the effluent. The second phase was consistent with ligand exchange of As with Me‐OH groups and surface‐completed P. The asymmetry of the As BTCs suggested that the physical structure of the matrix influenced the convective flow path of the As and subsequently its reaction and movement through the soil.
Chamber Measurement of Soil‐Atmosphere Gas Exchange: Linear vs. Diffusion‐Based Flux Models
Authors: Anthony W. H. , Hutchinson G. L. , Livingston G. P.
Published: July 2010

We compared linear regression with a diffusion‐based model for N2O flux estimation using non‐steady‐state chamber gas concentrations from a long‐term study of N cycling in a managed grass pasture on sandy soil in southern Texas. Of 2224 chamber deployments, 449 met criteria established for using the diffusion‐based model, which yielded flux estimates that averaged 54% larger than linear regression (n = 3). Although they represented only about 20% of all chamber deployments, this group included most of the data with greatest influence on the magnitude and dynamics of total N2O exchange at our site; e.g., of the 263 fluxes >10 g N ha−1 d−1, 192 (or 73%) were included. Apparently, application of a linear model to nonlinear chamber concentration data represents a potentially serious source of measurement bias that may influence not only summary statistics for the experiment, but also larger scale budgets based partially or wholly on those data.
Semicontinuous Measurement of Soil Atmosphere Gases with Gas‐Flow Soil Core Method
Authors: Swerts M. , Uytterhoeven G. , Merckx R. , Vlassak K.
Published: July 2010

To study denitrification, a method was developed for simultaneous measurement of CO2, N2, N2O, and NO production in soil cores. The technique does not interfere with biochemical processes in the soil and is fully automated to enable highly time‐resolved measurements for prolonged periods. Soil cores (intact or repacked) are connected to individual gas‐tight circuits (up to nine). Gas is continuously circulated through the soil cores to avoid diffusional problems. Each individual circuit can be automatically sampled for chromatographic gas analysis. The N2O production rates can be measured accurately down to 0.04 µg N kg−1 measuring interval−1 (MI). Determination of N2, CO2, and O2 production rates is less sensitive: 0.09 mg N kg−1 MI−1, 0.04 mg C kg−1 MI−1, and 0.02 mg O kg−1 MI−1, respectively. Dynamically changing gas production rates are monitored accurately, and total N gas production under anaerobiosis is balanced by NO3− disappearance.
Differential Effects of Soil Water Content and Temperature on Nitrification and Aeration
Authors: Grundmann G. L. , Renault P. , Rosso L. , Bardin R.
Published: July 2010

Environmental concerns have stimulated increased interest in NO−3 accumulation in soils. The aeration status of the soil, which is mainly governed by the water content and temperature, is a central factor. The biological process responsible for NO−3 accumulation, nitrification, was measured to estimate the combined effects of water content and temperature and determine their joint effect on soil aeration. The effects of temperatures of 15, 20, 25, 30, and 35°C and water contents equivalent to 0.35, 0.42, 0.50, 0.52, 0.57, and 0.60 relative water content (volumetric water content/total porosity) on the nitrification activity of soil samples containing 2‐mm sieved soils taken at 0 to 20 or 20 to 40‐cm depth were determined by measuring NO−3 accumulation for 17 h. A descriptive model including three biological parameters, maximum nitrification rate (Nrmax), optimal relative water content (Θopt), and temperature, was developed. Maximum Nrmax occurred at 25.5°C in 0‐ to 20‐cm soil and at 20°C in 20‐ to 40‐cm soil, suggesting an adaptation of soil nitrifying populations to the temperature regime of the soil. The Nrmax value was negatively related to Θopt, and Θopt was dependent on temperature (T). This Θopt (T) relationship was parabolic in nature, with Θopt being at a minimum between 20 and 25°C. It could be simulated using O2 diffusion and respiration rates, inferring that these processes influenced Θopt and T correlation. The ranges of O2 concentrations favorable to maximum nitrification within an aggregate volume fraction were estimated for different temperatures. Nitrification was generally maximum when the intraaggregate pore spaces were saturated with water, with no water in the interaggregate pore space (i.e., 0.44 relative water content at 25°C and 0.36 at 20°C at 0‐ to 20‐ and 20‐ to 40‐cm depths).
Physical Property Variation of a Soil and Saprolite Continuum at Three Geomorphic Positions
Authors: Schoeneberger P. J. , Amoozegar A. , Buol S. W.
Published: July 2010

Effective land use planning for waste disposal purposes requires knowledge of water movement in the vadose zone in and below the soil solum. This study was conducted to assess the changes in the saturated hydraulic conductivity (Ksat), water retention, particle‐size distribution, and bulk density of a Typic Kanhapludult soil and its underlying gneiss‐schist saprolite across a typical landscape in the Piedmont region of North Carolina. Intact cores (6.6‐cm diam., 8.0‐cm length) were collected from Bt, B/C, and C (saprolite) horizons at the center of 15 by 30 m areas located at three adjacent geomorphic positions (ridge top, shoulder, and ridge nose). Large L‐shaped observation pits were then excavated over the sample locations for profile description and other data collection. In situ Ksat of the three horizons was also determined by the constant‐head well permeameter technique around each of the observation pits. Regression analysis was employed to assess the relationship between Ksat and depth at each geomorphic position, and an analysis of variance was used to compare Ksat values between horizons and geomorphic positions. At all three geomorphic positions, the laboratory‐determined Ksat was consistently the highest in the clayey Bt horizon (arithmetic mean values 1.56 × 10−6 to 2.19 × 10−6 m s−1), diminished with depth until reaching a minimum value in the transitional, less clayey B/C horizon (arithmetic mean values 7 × 10−8 to 2.3 × 10−7 m s−1), and then increased with depth into the upper part of the massive, low clay content saprolite (arithmetic mean values 8.1 × 10−7 to 1.57 × 10−6 m s−1). The patterns of Ksat with depth were consistently associated with major horizons and not strongly influenced by texture or bulk density.
Analysis and Field Evaluation of the Ceres Models Water Balance Component
Authors: Gabrielle Benoît , Menasseri Safya , Houot Sabine
Published: July 2010

The soil water status partly determines the N losses from soilcrop systems. With the ultimate objective of estimating N losses, the capacity‐based water balance module of the Ceres models was tested against field data collected from various pedoclimatic regimes in France. A process‐oriented analysis of initial simulation results for a loamy soil prompted introduction of Darcy's law in the drainage and capillary rise parts of the model. As a result, a more accurate prediction of the soil water storage and surface water content was achieved. This was confirmed by comparing model output against independent data from bare or maize (Zea mays L.)‐cropped conditions and for silt loam or sandy loam soils. For a 1‐yr period, the mean square error between modeled and measured water storages was in the range 1.9 to 3 cm2 water for the modified model, in contrast with 4 to 12 cm2 using the original model (which performed best on well‐drained soils). A unidimensional sensitivity analysis was conducted with regard to the three new parameters introduced in the revised model: the saturated hydraulic conductivity and two texture‐dependent constants used in simple analytical representations of the moisture retention and hydraulic conductivity curves. The sensitivity analysis proved that this more physical approach in capacity‐based models required less rigorous parameterization than mechanistic models. Moreover, the accuracy of the simulations performed with the modified model fell within the experimental error in the measurements.
Soil Organic Carbon in a Mountainous, Forested Region: Relation to Site Characteristics
Authors: Homann P. S. , Sollins P. , Chappell H. N. , Stangenberger A. G.
Published: July 2010

Soil organic C content (SOC, kilograms C per square meter) and its relation to site characteristics are important in evaluating current regional, continental, and global soil C stores and projecting future changes. Data were compiled for 499 pedons in the largely forested, mountainous western Oregon region. The SOC of mineral soil ranged from 0.9 to 24 kg C m−2 (mean = 6.5) for 0‐ to 20‐cm depth and 2.3 to 88 kg C m−2 (mean = 15.8) for 0‐ to 100‐cm depth. Variability in each of the three terms that determine SOC − C concentration, bulk density, and rock volume ‐ contributed substantially to SOC variation. Regression analysis of 134 forest pedons indicated that combinations of site characteristics explained up to 50% of the SOC variability. The SOC increased with annual temperature, annual precipitation, actual evapotranspiration, clay, and available water‐holding capacity and decreased with slope. Relations for western Oregon differed qualitatively and quantitatively from those for other regions and contrasted with the decrease in SOC associated with increased temperature in Great Plains grasslands. Of the variability not explained by regression analysis, one‐half may be due to the combined uncertainty associated with measurements of C concentrations, bulk density, and rock volume; natural within‐site variability; and site‐characteristic measurements. Other unexplained variability is probably due to potentially important but poorly documented site characteristics, such as recent vegetation composition, geomorphic disturbance regime, and fire history.
Colloid Chemistry of Kaolinitic Tropical Soils
Authors: Chorover Jon , Sposito Garrison
Published: July 2010

Highly weathered soils of the humid tropics may exhibit loss of aggregate stability and dispersion of hydroxyl‐bearing soil constituents as a result of management‐induced changes in soil solution chemistry. The effects of pH and 1:1 “indifferent” electrolyte concentration on the surface charge and flocculation of colloids extracted from four representative kaolinitic tropical soils were examined by microelectrophoresis and light scattering methods. Electrophoretic mobilities (u) of clay‐sized particles (0.1‐1.0‐µm diameter) were predominantly negative between pH 2 and 6 for all soils, but decreased in magnitude with increasing solution proton concentration. Measured u values were between −3.0 × 10−8 and +0.5 × 10−8 m2s−1V−1, with isoelectric points (i.e.p.) ≤ 2.5. Photon correlation spectroscopy revealed an abrupt change from small to large particle size with increasing proton concentration in the pH range 2.5 to 3.5. Light scattering by whole‐soil suspensions indicated a proton‐induced transition from kinetically stable to kinetically unstable suspensions in the same pH range. Rapid flocculation occurred in the electrophoretic mobility range −1.2 to −1.8 × 10−8 m2 s−1 V−1. The pH and ionic strength dependence of colloidal stability observed for the soils is consistent with an electrostatic stabilization mechanism among their clay‐sized particles.
Stability Constants of Metal‐Humate Complexes: Titration Data Analyzed by Bimodal Gaussian Distribution
Authors: Manunza Bruno , Deiana Salvatore , Maddau Vittoria , Gessa Carlo , Seeber Renato
Published: July 2010

A continuous multiligand model was applied to the study of the systems Cu(II)‐, Pb(II)‐, Mn(II)‐, and Cd(II)‐humate. The model is essentially based on a bimodal Gaussian distribution of the binding sites. It allows the calculation of pH‐independent metal‐binding thermodynamic constants which are more representative than simple empirical curve‐fitting parameters. This model was applied to the study of the potentiometric titration data of an humic acid extracted from an Andosol of Macomer (Sardinia, Italy) in order to calculate the formation constants of the complexes of humic acids with four metal ions. Its application to the potentiometric titration experiments showed that different classes of sites are involved in the binding of the metal ions considered. The affinity to the humic matrix followed the order: Cu(II) > Pb(II) > Mn(II) >> Cd(II); phenolic groups play a relevant role in Cu binding while Cd appears to bind exclusively to carboxylic groups.
DNA Probe Analysis of Microbial Community Selected in Field by Long‐Term 2,4‐D Application
Authors: Ka J. O. , Burauel P. , Bronson J. A. , Holben W. E. , Tiedje J. M.
Published: July 2010

The 2,4‐D (2,4‐dichlorophenoxyacetic acid)‐degrading microbial populations selected by 2,4‐D amendments to soil were analyzed by DNA probes during a 4‐yr field study. Two were control subplots not treated with 2,4‐D, and the other six were treated with 2,4‐D in replicates at three different concentrations. Total soil bacterial DNA was regularly extracted from each subplot soil and analyzed on slot and Southern blots using the gene probes tfdA and Spa that detect two different 2,4‐D‐degrading populations. Slot blot analyses revealed that the hybridization signal of the probes to total soil bacterial DNA substantially increased with time and continued 2,4‐D application, indicating a shift from a more diverse 2,4‐D‐degrading microflora to populations that shared greater homology to the two probes. More hybridization signal was obtained with the Spa probe, suggesting that microorganisms with this gene sequence were more dominant than degraders with tfdA‐like genes. The Southern blot band patterns indicated that the dominant Spa‐hybridizing strain was Sphingomonas paucimobilis strain 1443 and the dominant tfdA hybridizing strain was Burkholderia (formerly Pseudomonas) pickettii strain 712, similar to the results of a previous laboratory microcosm study. At the highest 2,4‐D application rate (100 mg kg−1), Southern blots probed by 16S rRNA genes showed a limited number of bands, suggesting that the total soil microbial community had been shifted to one or two dominant strains, presumably 2,4‐D degraders.
Crop Residue Effects on Nitrogen Mineralization, Microbial Biomass, and Rice Yield in Submerged Soils
Authors: Clément A. , Chalifour F.‐P. , Ladha J. K.
Published: July 2010

Urea, crop residues, and green manure are all suitable sources of N for lowland rice (Oryza sativa L.). However, N sources undergo mineralization at different rates, affecting rice N uptake and utilization. Little is known about the effect of the chemical composition of crop residues on N mineralization and rice performance. Two greenhouse experiments were conducted to determine the dynamics of N mineralization, microbial biomass, and rice N uptake. Twelve treatments representing a wide range of crop residue chemical composition and two controls (with and without urea) were compared. The N mineralization pattern ranged from rapid (Sesbania rostata Brem. & Oberm.) to immobilization at the beginning of the season (Cassia velosa L.). Immediately after incorporation, N mineralization was positively correlated to crop residue N concentration (r2 = 0.64, significant at P < 0.01), and negatively correlated to tannin concentration. However, at tillering, the tannin/N ratio was best correlated to the rate of N release (r2 = 0.86, significant at P < 0.01). Grain yield was best predicted by the (lignin + polyphenol)/N ratio (r2 = 0.67, significant at P < 0.01). Incorporation of residue into soil generally increased microbial biomass ninhydrin‐reactive N compared with control treatments at the tillering stage, but not at maturity. Microbial biomass N was highly correlated to soil NH+4‐N at rice tillering (r2 = 0.76, significant at P < 0.01). The N derived from fertilizer in rice was 35.4% on average for residue treatments, which was comparable with that of the urea control (33.5%). This study emphasizes the importance of considering the interactions among chemical constituents of crop residues to understand the dynamics of N release and uptake by rice.
Decomposition Rate Constants of Size and Density Fractions of Soil Organic Matter
Authors: Hassink Jan
Published: July 2010

One of the main drawbacks of models describing soil organic matter turnover is that most pools cannot be experimentally identified. The aim of this study was to define meaningful soil organic matter fractions that can be physically isolated and to determine their decomposition rate constants so that they can be incorporated into models. This would be a major step toward verification of models. In this study, soil organic matter was separated into five fractions: the light (density <1.13 g cm−3), intermediate (density 1.13–1.37 g cm−3), and heavy (density >1.37 g cm−3) fractions of macroorganic matter (>150 µm) and the microaggregate fractions with particle sizes between 20 and 150 µm and smaller than 20 µm. The decomposition rate constants of the fractions decreased in the order light, intermediate, and heavy macroorganic matter (23.9, 9.8, and 3.9 × 10−4 d−1, respectively) and were lowest for C in the microaggregate fractions <20 and 20 to 50 µm (0.5–0.7 × 10−4 d−1). Since the rate constants of the fractions seemed not to depend on soil texture, they can probably be applied widely. We propose to use these fractions as the principal pools in future soil organic matter models.
Spectroscopy of Aqueous Extracts of Forest Litter. I: Suitability of Methods
Authors: Gressel N. , McGrath A. E. , McColl J. G. , Powers R. F.
Published: July 2010

Structural characteristics of dissolved organic matter (DOM) have not been closely examined until recent analytical techniques became available. We examined the applicability of ultraviolet‐visible (UV‐VIS), Fourier‐transform infrared (FTIR), and electron spin resonance (ESR) spectroscopies using aqueous extracts of pine and broadleaf understory leaf litter. The ratio of UV‐VIS absorbances at 465 nm and 665 nm (E4/E6) were 7.8 to 8.5 for extracts of pine and 6.3 to 7.7 for the understory between pH 3.0 and 6.0. Using the standard method for humic substances (in 0.05 M NaHCO3 solution, pH 8.0), E4/E6 measurement produced varying results, and is not recommended for DOM. Well‐resolved FTIR spectra had features similar to those of humic substances; peak‐height ratios were used for semiquantitative analysis of DOM functional groups. Two FTIR sample‐preparation procedures were also compared: dried extracts on AgCl disks, and freeze‐dried DOM in KBr pressed pellets. The first allowed better analysis of COOH functional group deprotonation; the second preparation procedure produced spectra with identical absorbance ranges as those of AgCl disks, with similar peak‐height ratio trends as a function of pH, except for those associated with COOH/COO−. Contact between DOM and the salt in pressed pellets may have aided deprotonation of COOH, even at low pH. Interference from water occurred with KBr pressed pellets. Free radicals identified by ESR increased with pH, and shifts from 2.0036 to 2.0050 in ESR g values indicated that oxidation also increased with pH. We conclude that these spectroscopic techniques, modified from those developed for analysis of humic substances, are complementary and together provide useful information about the structure of the DOM of forest litter.
Comparison of Available Soil Nitrogen Assays in Control and Burned Forested Sites
Authors: Knoepp Jennifer Donaldson , Swank Wayne T.
Published: July 2010

The existence of several different methods for measuring net N mineralization and nitrification rates and indexing N availability has raised questions about the comparability of these methods. We compared in situ covered cores, in situ buried bags, aerobic laboratory incubations, and tension lysimetry on control and treated plots of a prescribed burn experiment in the southern Appalachians. Environmental influences were examined with soil moisture and temperature measurements. All methods detected significant differences in net N mineralization rates between treated and control plots; P = 0.04, 0.007, 0.001, and 0.07 for covered cores, buried bags, lab incubations, and lysimeters, respectively. Mean rates of N mineralization during the growing season were similar for the three soil incubation methods. The laboratory incubation of soil from treated plots produced significantly greater potential nitrification rates than the in situ methods. The four methods were not well correlated; this indicates the need for caution when comparing N transformation data derived from different methods. We conclude that the in situ covered core method is superior because it best incorporates site‐specific soil temperature and moisture changes into N transformation measurements.
Anion Transport through Columns of Highly Weathered Acid Soil: Adsorption and Retardation
Authors: Bellini Giampaola , Sumner Malcolm E. , Radcliffe David E. , Qafoku Nikolla P.
Published: July 2010

Subsoils of highly weathered, acid soils usually contain appreciable quantities of anion‐exchange capacity (AEC), which retards the passage of anions through the profile. This AEC varies in magnitude with ambient soil solution parameters such as pH and ionic strength. In order to be able to predict NO‐3 movement in such soils, estimates of the effect of subsoil charge characteristics on retardation are required. The effect of subsoil liming and ionic strength on the movement of NO‐3 in repacked Kanhapludult soil columns was studied for this purpose. Columns comprising topsoil (untreated) over subsoil limed to different levels to give a range of AEC were used. The soil was first leached with a solution of 10 mmolc Ca(NO3)2 L‐1 followed by a 10 mmolc CaCl2 L‐1 solution. Convection‐dispersion equation (CDE) parameters for Cl‐ were determined using CXTFIT. Liming reduced the retardation coefficient (R) from 2.39 in the unamended subsoil to 1.12 in the treatment that received 2.08 g Ca(OH)2 kg‐1. Both subsoil pH (r2 = 0.96) and AEC (r2 = 0.98) were correlated with R for Cl‐. To test the effect of electrolyte concentration on retardation, columns of untreated subsoil were leached with solutions of Ca(NO3)2 and CaCl2 at four input concentrations: 5, 10, 20, and 30 mmolc L‐1. Retardation coefficients for Cl‐ at these concentrations were 2.81, 2.25, 1.76, and 1.51, respectively. Anion retardation needs to be taken into account in modeling NO‐3 movement through acid subsoils.
Nitrogen and Water Interactions in Subsurface Trickle‐Irrigated Leaf Lettuce: I. Plant Response
Authors: Thompson Thomas L. , Doerge Thomas A.
Published: July 2010

Rising water costs and concerns about groundwater contamination by NO‐3 are forcing growers in arid regions to adopt practices to improve water and N use efficiency. Subsurface trickle irrigation offers the ability to precisely deliver appropriate amounts of water and N to the crop root zone. The objectives of this research were to: (i) determine the optimum level (with respect to marketable yield) of soil water tension for subsurface trickle‐irrigated leaf lettuce (Lactuca sativa L. cv. Waldmann's Green), (ii) determine N by water interactions on crop yield and quality, and (iii) determine seasonal patterns of crop N uptake. Three field experiments were conducted during the 1990 to 1993 winter growing seasons on a Casa Grande soil (fine‐loamy, mixed, hyperthermic, Typic Natriargid [reclaimed]) in southern Arizona. Deficient to excessive N (35–300 kg ha‐1) and target soil water tension (SWT) treatments (12.0–4.0 kPa) were applied in factorial combinations each year. Maximum marketable leaf lettuce yields observed were 54.0 Mg ha‐1. Nitrogen rates associated with maximum yields ranged from 120 to 300 kg ha‐1, and depended on average SWT. Excessive irrigation (5.3 kPa average SWT) resulted in lower yields, head weight, and head length. Marketable yields showed a pronounced water × N interaction, but yield quality (head length and weight) did not. Maximum N flux was 4.3 kg ha‐1 d‐1, and an average of 75% of total N accumulation occurred in the 38 d prior to harvest. Results suggest that the soil water tension at 0.3‐m depth should be maintained at 6 to 7 kPa for optimum yields of trickle‐irrigated leaf lettuce.
Phosphorus in Urban Sewage Sludges as Assessed by Isotopic Exchange
Authors: Frossard E. , Sinaj S. , Dufour P.
Published: July 2010

Sewage sludges have been used for years as fertilizers but little information exists on the availability of P in these complex materials. Isotopically exchangeable P was measured in 12 urban sewage sludges using the isotopic exchange kinetics method. The results obtained with this method were then interpreted according to the multicompartmental model recently proposed. The two activated sludges and the composted sludge had the highest quantities of P isotopically exchangeable within 1 min (E1min higher than 0.77 g P kg‐1 sludge). The flocculated and heat‐treated sludges contained low quantities of rapidly exchangeable P but high quantities of slowly exchangeable P (E >12wk higher than 9 g P kg‐1 sludge). The quantity of P exchangeable within 1 d was positively correlated to the sum of water‐ and bicarbonate‐extractable P while slowly exchangeable P was positively correlated to the quantity of P present in precipitated forms deduced from a sequential extraction (i.e., HCl plus NaOH‐extractable P for FeSO4‐flocculated sludges and HCl‐extractable P for the other sludges). Results of this study demonstrate that the isotopic exchange kinetics method can give valuable information on inorganic P forms and exchangeability in urban sewage sludges.
Mineralogical Investigation of Soils Formed in Calcareous Gravelly Alluvium, Eastern Crete, Greece
Authors: Timpson M. E. , Lee S. Y. , Ammons J. T. , Foss J. E.
Published: July 2010

The mineralogical composition of soils along a toposequence formed on Quaternary‐aged alluvium from the northeastern portion of the island of Crete, Greece, and the examination of their mineral weathering characteristics should help provide an insight into the stage of weathering and relative age of the soils. Four soils were described and sampled from three landscape positions along the toposequence and the clay and silt fractions from all horizons of each soil were examined using a combination of x‐ray diffraction and electron microprobe techniques. Coarse and fine silt fractions from each soil contained dolomite, calcite, quartz, feldspars, mica, and chlorite. Feldspar content generally decreased with depth, whereas calcite increased with depth. Minerals identified in the silt fractions resulted from a combination of physical and chemical weathering and possible eolian additions. Clay fractions were dominated by trioctahedral ferrous chlorite and dioctahedral mica (muscovite), with minor amounts of kaolinite, quartz, dolomite, and calcite. Goethite was also identified in the clay fraction of some soil horizons. The source of the phyllosilicates was inheritance from phyllite gravels in the alluvium and chemical dissolution of the dolostone releasing entrained clay minerals. Some portion of the kaolinite in surface horizons was probably the result of eolian additions. Goethite formation resulted from release of Fe from the chlorite. The extent of clay mineral inheritance, limited expansion of the chlorites, and lack of a chlorite or mica weathering product (vermiculite), as well as the presence of carbonates in the clay fractions, all suggest that these soils are still in the initial stages of weathering.
Oxidation States and Fractionation of Plaque Iron on Roots of Common Reeds
Authors: Wang Tiangen , Peverly John H.
Published: July 2010

Characterization of iron plaque stability on roots of wetland plants is of value in consideration of several wetland issues. Active and dormant roots of common reed [Phragmites australis (Cav.) Trin. ex Steudel] sampled from a constructed wetland were treated sequentially with 0.5 M MgCl2 (salt extraction), HCl solution at pH 3, 2, 1.6, 0.6, 0.3, and 0 (acid extraction), and 0.385 M Na2S2O4 (reductant extraction). Iron was measured with an orthophenanthrolene method. Results indicated that dormant roots contained 6, 890, 180, and 140 µg Fe g‐1 fresh root weight in salt extractable, acid extractable, reductant extractable, and residual fractions, respectively, and active roots had a higher proportion of acid extractable Fe and lower proportions of reductant extractable and residual Fe than dormant roots. Following acid extraction, substantial Fe2+ was detected. Ferric iron reduction after extraction was also found. The models of Fe extraction dynamics at different pH levels were fit to standard equations. According to the comparison of the constants in these models, the Fe release rate and potential for extraction in the acid solutions tended to increase as the pH decreased, but the effect on Fe extraction became saturated when pH approached zero. The oxidation state of Fe in root plaque was examined with Mossbauer spectroscopy after the roots were dried at 80°C in a regular oven and were powdered. The results indicated that the Fe(II) proportion is 33% in powdered dormant roots, and 27% in active roots. Based on these results, we suggest that the previously accepted concept of plaque composed entirely of Fe3+ needs to be further examined.
An Extended Transfer Function Model of Field‐Scale Solute Transport: Model Development
Authors: Liu H. H. , Dane J. H.
Published: July 2010

Commonly used one‐dimensional models for predicting solute transport in the field are based on either a deterministic convection‐dispersion equation or a stochastic‐convective principle. These models, however, are limited to describing solute transport characterized by either a linear or a quadratic increase in the travel time variance with depth in a steady‐state flow field. Based on an existing transfer function model, we propose a solute transport model in which the travel time variance increases proportionally with depth to the power 2a, where a is an adjustable constant representing the degree of horizontal solute mixing. The model based on the deterministic convection‐dispersion equation is well approximated by the proposed model for a = 0.5, while the model based on a stochastic‐convective principle is a special case of our model for a = 1. Effects of different a values on solute transport and relevant problems are briefly discussed.
Field Measurement of Air‐Entry and Water‐Entry Soil Water Pressure Heads
Authors: Fallow D. J. , Elrick D. E.
Published: July 2010

An effective field measurement of the air‐entry and water‐entry soil water pressure heads gives an improved description of the hydraulic properties of soils. One study obtained measurements of air‐entry soil water pressure heads using a modified falling head infiltrometer. This study then estimated the value of the water‐entry soils water pressure head from this value. Now direct field measurements of both the air‐entry and water‐entry soils water pressure heads can be easily obtained using an external tension bottle attached to the Guelph Pressure Infiltrometer at the conclusion of quasi‐steady state infiltration readings.
A Parametric Model for Two‐Dimensional Water Uptake Intensity by Corn Roots under Drip Irrigation
Authors: Eugenio Coelho F. , Or Dani
Published: July 2010

Modeling of water distribution for drip irrigation management under cropped conditions requires information on water uptake patterns by plant roots. Based on experimental data, we propose using Bivariate Gaussian distribution density functions (normal, semilognormal, and lognormal) as parametric models for two‐dimensional water uptake intensity patterns under drip irrigation. These models offer (i) a concise representation of large amounts of uptake information; (ii) parameters that may be useful for modeling and comparisons between crops, locations, and management scenarios; (iii) physical and statistical insight into spatial and temporal changes in uptake patterns that may be gained by applying moment analyses for the center of activity location and the spread in uptake intensity about it. Different water uptake patterns associated with four basic plant‐dripper configurations (surface and subsurface source, within and between crop rows) were identified and measured in field and greenhouse experiments. Uptake intensity pattern was influenced by the relative positions of the plant and the source and by the presence of no‐uptake boundaries (e.g., soil surface) requiring different models based on various forms of the Bivariate Gaussian model (normal, semi‐log, and log‐log). The proposed models were in good agreement with corn (Zea mays L.) uptake data measured by dense grids of time domain reflectometry probes in large containers and in the field.
Solid‐Phase Manganese Fractionation Changes in Saturated Arid‐Zone Soils: Pathways and Kinetics
Authors: Xiang Han Feng , Banin Amos
Published: July 2010

Short‐ and intermediate‐term transformations of Mn among the solid‐phase components of two arid‐zone soils were studied under saturated paste incubation. Manganese was fractionated by a selective sequential dissolution procedure into several empirically defined fractions. Before incubation, Mn was predominantly in the easily reducible oxide (ERO) fraction, followed by the carbonate (CARB) fraction and the residual (RES) fraction, with almost negligible amounts present in the exchangeable (EXC) fraction. During saturation incubation, Mn in both soils was transformed from the ERO fraction to the EXC and CARB fractions. However, the pathways differed between the two soils. The rates of the transformations were initially very high: major changes occurred during the first 6 d in the sandy soil and the first 12 d in the loessial soil. Following the initial rapid changes, both soils showed only subtle changes for the rest of the period (1 yr). The transformation (reduction) of Mn from the main source, the ERO fraction, was controlled by, and synchronized with, changes of soil pe + pH. The initially measured rates of reduction in ERO Mn were 1.43 × 10−4 and 0.828 × 10−4 mg kg−1 s−1 in the sandy and loessial soils, respectively. A parameter, Uts, measuring the departure from the initial state of equilibrium in the soil showed that Mn initially departed rapidly from the native state of quasi‐equilibrium in the oxidized soils, but tended to return toward it during prolonged incubation. The flux of Fe to the EXC and CARB fractions during the early stages of incubation was very small compared with that of Mn, suggesting increased Mn availability.
Acetate Fate in Organic and Conventionally Managed Soils
Authors: Wander M. M. , Dudley R. B. , Traina S. J. , Kaufman D. , Stinner B. R. , Sims G. K.
Published: July 2010

Organic management practices and mixed‐crop rotations have beneficial impacts on biologically active soil organic matter (SOM); however, the specific impacts of these practices on SOM composition and turnover characteristics are not known. The effect of organic and conventional management on soil C cycling was assessed using soils collected from Rodale's Farming Systems Trial experiment, which included organic‐manure amended, organic‐legume cover cropped, and conventionally managed‐inorganically fertilized farming systems. Soil respiration rates, atom % 13C, and cross polarization 13C nuclear magnetic resonance (CP‐MAS 13C NMR) were used to trace the fate of 13C‐labeled acetate added to soils incubated for 2 and 13 wk. Microbial activity and C turnover rates were initially greater in the manure‐amended soil than in the other two soils. Acetate mineralization lagged in the cover‐cropped soil during the first 2 wk. Low atom % 13C in the conventional soil showed that, even though its soil respiration rates, and therefore overall microbial activity, equaled rates in the cover‐cropped soil, microorganisms present in this soil metabolized significantly more of the applied acetate. By the end of the experiment, C cycling characteristics of the cover‐cropped soil were similar to the manure‐amended soil; NMR spectra indicated 13C was incorporated into new SOM in both of these soils. Soil respiration and 13C assimilation rates in the conventional soil remained relatively low. Microorganisms in the organic soils mineralized and respired more native C and assimilated more 13C than the conventional soil. Conversely, the conventional soil mineralized more and assimilated less 13C than the organic soils. Differences observed in this laboratory experiment may or may not reflect on C cycling occurring in the field.
Hardsetting Soils in Southeast Australia: Landscape and Profile Processes
Authors: Franzmeier D. P. , Chartres C. J. , Wood J. T.
Published: July 2010

Hardsetting soil horizons are relatively soft when moist but become unusually hard when dry. Soils with these horizons are common in Australia and elsewhere. They have been studied mainly in exposures such as road cuts where their development is most marked. The main objective of this study was to elucidate the relationships between landscape position and hardsetting processes. Soils were sampled along two transects that start in bedrock hills, cross broad eolian‐fluvial plains, and terminate in a local flood plain or lacustrine basin. They are mainly Dystrochrepts, Palexeralfs, and Natrixeralfs, and many soils are aquic intergrades. Samples were analyzed using standard characterization methods, and extracted with water, oxalate solution, and citrate‐bicarbonate‐dithionite solution; Fe, Al, and Si contents were determined from the extracts. Soil strength was characterized by the modulus of rupture test. Strength was partly related to clay content, but several E and BE horizons were very strong relative to their clay content, thus hardsetting. They were in the zone of the pedon in which exchangeable Na and pH increased sharply with depth. This sodicity was also cyclic in the landscape. Strength was correlated positively with extractable Si and negatively with extractable Al. In hardsetting soils, which tend to have impeded drainage, Fe‐oxide minerals periodically precipitate and dissolve. We postulate that silica is dissolved and dispersed in upper horizons with a large pH gradient, moves down the profile, and bonds with Fe‐oxide minerals on clay surfaces. The bonds strengthen on drying to form hardsetting horizons.