bluuebunny/crossref_metadata_2025 huggingface dataset:
Soil Physical Property Changes during Dung Decomposition in a Tropical Pasture
Authors: Herrick J. E. , Lal R.
Published: July 2010

The contributions of soil macroinvertebrates to the creation and maintenance of soil structure in tropical rangeland ecosystems are poorly understood, in spite of the fact that compaction is frequently cited as a limiting factor in pasture production. Decomposition of cattle dung (fecal material) is associated with high levels of soil macroinvertebrate activity. We hypothesized that bulk density and surface penetrometer resistance would be reduced in soil beneath decomposing cattle dung patches and that soil water infiltration capacity and drainable porosity would be increased. Cattle dung was deposited in patches during the dry and wet seasons of 1991 in a seasonally dry pasture located 5 km north of Cañas, Costa Rica, on a Typic Argiustoll. At the end of the wet season, surface 3‐cm bulk density was <0.93 Mg m−3 under dung patches, compared with 1.05 Mg m−3 in control plots. Drainable porosity, defined as air‐filled porosity at 0.006 MPa, increased from 13% in control plots to >21% under dung. Similar, but smaller, differences were recorded at dung patch edges and at a depth of 4 to 7 cm. At the end of the wet season, equilibrium infiltration rates averaged 71 mm h−1 in the patch plots and 34 mm h−1 in the controls. Changes in penetrometer resistance were transient and no treatment effects persisted to end of the wet season. The results of this study indicate that processes associated with dung decomposition play a role in reducing surface compaction.
Gaseous Exchange and Wetland Plant Response to Soil Redox Intensity and Capacity
Authors: Kludze H. K. , DeLaune R. D.
Published: July 2010

Although reports on plant responses to wetland conditions abound, limited information is available to relate such responses to well‐defined soil redox conditions. Laboratory experiments were conducted to evaluate differences in plant response to soil redox intensity (Eh) and capacity. Mississippi floodplain soil (Mhoon; fine‐silty, mixed, nonacid, thermic Typic Fluvaquent) and rice paddy soil (Crowley; fine, montmorillonitic, thermic Typic Albaqualf) were used to grow saltmeadow cordgrass [Spartina patens (Aiton) Muhlenb.] and rice (Oryza sativa L.), respectively. Plants were grown under controlled Eh levels of 100, 0, −100, and −200 mV to examine the effect of Eh on CO2 fixation. Treatments for the soil redox capacity experiment were imposed by application of different levels of extra energy source while maintaining Eh at ‐200 mV. Redox capacity effects on plant growth, CO2 fixation, root porosity (POR), radial oxygen loss (ROL), and CH4 emissions were also evaluated. In both test plants, CO2 fixation did not respond to soil Eh until −100 mV. Although POR was unaffected, plant growth, CO2 fixation, and CH4 emissions were significantly decreased with increase in soil O2 demand, thus suggesting a complex relationship between soil redox capacity and plant physiological functions. Plant O2 transport to the root environment (ROL) was also governed by soil redox capacity. Results indicate that plants may respond differently in magnitude to soil redox intensity and redox capacity. Evaluating responses, especially ROL, of flood‐tolerant plant species therefore requires proper quantification of the soil redox condition or substrate O2 demand in which the plants are grown.
Brittle Fracture of Fractal Cubic Aggregates
Authors: Perfect E. , Kay B. D.
Published: July 2010

The conventional Weibull model for brittle fracture assumes soil aggregates are Euclidean cubes. Recent developments in fractal theory suggest that fractal cubes may provide a more realistic representation of soil aggregates. We modified the Weibull model for fractal cubes, and compared the performance of the modified model with the conventional model using previously published brittle fracture data for air‐dry aggregates, ranging in equivalent cubic length from 1.19 to 25.275 mm, collected from different soils under the same crop, and similar soils under different crops. Goodness‐of‐fit statistics showed that the modified model fit the data much better than the conventional model. The modified model also produced a more accurate estimate of the spread in aggregate strength values. The modified model has an extra parameter. This parameter is the mass fractal dimension (D) of the solid phase. The theoretical range for D is zero to three. All of the estimates of D were less than three. However, 5% of the estimates were significantly less than zero. We attributed these negative estimates to the absence of any relationship between strength and size (within the range of length scales investigated) in the case of the weakest aggregates. The D increased with increasing clay content. We showed that the D for the solid phase is theoretically identical to the D for the pore‐size distribution. Thus, it may be possible to predict the pore‐size distribution of soil aggregates from brittle fracture data.
Surface Aggregates and Macropore Effects on Chemical Transport in Soil under Rainfall
Authors: Heathman G. C. , Timlin D. J. , Ahuja L. R. , Johnsen K. E.
Published: July 2010

Soil surface aggregates and macropores have the potential to greatly influence the transport of surface‐applied agricultural chemicals in soil and to surface and groundwater. We studied the effects of a 1.0‐cm layer of 4.5‐ to 12.5‐mm surface aggregates, a 3‐mm artificial macropore, and their combination on the transport of surface‐applied Br− and Sr2+ tracers (as SrBr2) under simulated rainfall, with the underlying soil initially dry or prewetted to a certain depth (about 7 cm). Air‐dried Kirkland soil (fine, mixed, thermic Udertic Paleustoll), <3.5‐mm size, was packed in duplicate 15‐cm‐diam., 30‐cm‐long, sectionable Plexiglas columns (Cole‐Parmer Instrument Co., Niles, IL) to a bulk density of 1.26 Mg m−3. During the rainfall, surface runoff or macropore flow out of the column bottom was measured and analyzed for Br− and Sr2+. After the rain, the column was sectioned in 1.15‐cm increments to determine soil water, Br−, and Sr2+ content distributions. Without a macropore, the surface aggregates reduced the overall movement of Br− in soil and increased Br− in runoff, the effects being much greater in prewetted columns than in nonprewetted columns. A macropore, by itself, also reduced the movement of the main Br− pulse as it allowed surface solution (runoff) to move down and infiltrate laterally below the main wetting front. Surface aggregates greatly increased Br− content of the macropore flow and of the wetted soil around the macropore below the main wetting front. Aggregates or the initial condition did not change the Sr2+ movement much but increased Sr2+ content of runoff or macropore flow.
Surface Area of Soil Organic Matter Reexamined
Authors: Pennell K. D. , Abriola L. M. , Boyd S. A.
Published: July 2010

The specific surface area of soil organic matter has traditionally been considered to range from 550 × 103 to 800 × 103 m2 kg−1, based on the change in ethylene glycol (EG) retention before and after treatment of soils with H2O2. Recent studies, however, indicate that when N2 is used as the adsorbate, the surface area of soil organic matter is approximately 1 × 103 m2 kg−1. To investigate this large discrepancy, the surface areas of four H2O2‐treated soils were determined by EG retention and the N2‐Brunauer, Emmett, and Teller (BET) method. After exposure of the soils to H2O2, retention of EG decreased, while N2 adsorption increased. The surface area of soil organic matter calculated from the reduction in EG retention ranged from 65 × 103 to 481 × 103 m2 kg−1 and was dependent on the initial organic C content of the soil. In contrast, the N2‐BET data yielded negative estimates of soil organic matter surface area that varied from −13 × 103 to −410 × 103 m2 kg−1. These results demonstrate that techniques based on the removal of organic C fail to provide consistent and reliable values for the surface area of soil organic matter. Due to the complex nature of interactions between soil organic matter and EG, including cation solvation, partitioning, and surface adsorption, it is recommended that EG retention be regarded as an estimate of the total uptake capacity of soils for polar liquids, rather than a measure of surface area.
Effects of Sugarbeet Residues on Soil Redox Potential and Nitrous Oxide Emission
Authors: Flessa H. , Beese F.
Published: July 2010

The influence of the incorporation of fresh sugarbeet (Beta vulgaris L.) plant residues (green weight, 36.5 t ha−1; N content, 113 kg ha−1) on microbial activity, soil redox potential, and N2O emissions was studied using soil microcosms with an automated registration of N2O and CO2 emissions. The experiment was conducted in well‐drained soil columns (water‐filled pore space 63%) and in soil columns with temporary water‐logging (water‐filled pore space continuously increased to 81‐89% for 5 to 7 d). The cumulative CO2 losses during 45 d were equal for the well‐drained and the temporarily water‐logged soils and amounted to ≈30% of the total residue C added. For the well drained soils, the total N2O‐N emission during 45 d was 35‐fold higher than for the control without added plant residues and amounted to 0.37% of the residue‐bound N. Water logging increased the N2O‐N losses to ≈1% of the N added with the sugarbeet residues. The period of increasing N2O release coincided with the starting net mineralization of the residue N and decreasing redox potentials in the vicinity of the plant material. The greater availability of NO−3 combined with enhanced metabolic activity led to the formation of anaerobic microenvironments within and around the plant residues, providing favorable conditions for denitrification. The results emphasize the importance of anoxic zones in the soil caused by respiratory O2 consumption for N2O production in well‐aerated soils and indicate that N2O emission from N‐rich plant residues could be in the same order of magnitude as reported for mineral N fertilizers.
Wheat Responses to Vesicular‐Arbuscular Mycorrhizal Fungal Inoculation of Soils from Eroded Toposequence
Authors: Mohammad M. J. , Pan W. L. , Kennedy A. C.
Published: July 2010

Low fertility of eroded soils is often attributed to the loss of topsoil, an important source of nutrients for microorganisms and plants. Vesicular‐arbuscular mycorrhizal (VAM) fungi are known to improve nutrient uptake by plants from infertile soil. The objective of this study was to determine whether wheat (Triticum aestivum L.) responses to VAM fungal inoculation differ among eroded and noneroded soils from the same toposequence. Spring wheat plants were grown under growth chamber conditions in soils collected from three landscape positions of a typical toposequence in eastern Washington, with and without VAM fungal inoculation. Noninoculated soils exhibited 13 to 26% VAM root colonization, while inoculation increased VAM colonization up to 49 to 55% by 5 wk after planting. The VAM fungal inoculation of eroded soils increased shoot dry weight and tiller number, advanced the leaf stage, and increased root length compared with the noninoculated controls. Shoot concentrations of P and Zn were increased by VAM inoculation. In contrast, shoot concentrations of Cu and Mn were unchanged, while Fe was decreased. While VAM fungal inoculation also increased P, Zn, and Cu concentrations in water‐stressed plants, Mn and Fe concentrations decreased. No responses to VAM fungal inoculation were observed in soil from a noneroded toeslope, although VAM colonization was similar to that observed in the eroded soils. These results indicate the potential role of VAM fungi for improving dryland wheat productivity on eroded soils.
Stochastic Analysis of Soil Water Monitoring for Drip Irrigation Management in Heterogeneous Soils
Authors: Or Dani
Published: July 2010

Many drip irrigation management schemes rely on frequent monitoring of soil water content and matric potential, using various sensors (e.g., tensiometers or time domain reflectometry probes). The soil water information is used either for irrigation scheduling or for adjusting schedules based on evapotranspiration measurements. Spatial variations in soil properties induce variations in wetting patterns about the drippers, which complicate the acquisition and interpretation of information on soil water status. The objective of this study was to quantify the effects of mild spatial variation in soil hydraulic properties on wetting patterns and the consequences on soil water sensor placement and interpretation. The soil hydraulic properties (saturated hydraulic conductivity, Ks, and the exponent α of the exponential unsaturated hydraulic conductivity function) were considered as random space functions. Small perturbation expansions were applied to analytical solutions of steady state flow from point sources. The resulting analytical expressions relate the variability of Ks and α to the expected variability in soil water pressure head (h) and relative saturation (S) about a point source. Comparisons of the analytical predictions with Monte Carlo simulations (for surface and buried sources) resulted in excellent agreement for coefficients of variation of ln(Ks) and α ≤ 0.3. The expressions may be used as first approximations to define regions with small uncertainty that may be most suitable for soil water monitoring and control in drip‐irrigated fields. Another application is for determining the minimum number of sensors needed to obtain estimates with a prescribed estimation error. The derived spatial covariance functions of S and h may be used to identify regions in the field with different wetting patterns or simply to interpolate measurements to unmeasured portions of the field.
Coprecipitation of Sodium, Magnesium, and Silicon with Calcium Carbonate
Authors: Levy D. B. , Amrhein C. , Anderson M. A. , Daoud A. M.
Published: July 2010

Calcite oversaturation in soil solutions is often attributed to overestimations of Ca2+ (aq) and CO2−3 (aq) activities, organic matter mineralization, and the presence of metastable forms of CaCO3. Earlier research, however, has indicated that these explanations are often insufficient to explain the levels of calcite oversaturation observed in soil suspensions. This study was conducted to evaluate the solubilities of precipitates formed during evapoconcentration of synthetic irrigation waters. Solids precipitated from waters of various Mg and Si concentrations were redissolved in deionized water under controlled CO2 conditions. Ion activities were calculated from the solution composition using the geochemical speciation model MINTEQA2. X‐ray diffraction (XRD) and infrared absorption spectroscopy provided evidence for precipitation of poorly crystalline Mg silicates and hydrous Na‐Mg silicates (sepiolite [Mg2Si3O7.5(OH)·3H2O], loughlinite [Na2Mg3Si6 (OH)16·8H2O], stevensite [Na0.33Mg2.87Si4O10(OH)2], or magadiite [Na Si7O13(OH)3·3H2O]). Calcite, aragonite, gypsum, and amorphous SiO2 were also identified. The solubility of the solid phase was proportional to the amount of Si and Mg in the solid, and dissolution of the solids in deionized water under atmospheric CO2 yielded an average CaCO3 ion activity product value of 10−8.0. The maximum mole fraction of Mg (XMg) in the calcite determined by XRD was <0.03, while XMg values calculated from mass balance were up to 0.53. The high solubility of the solid phases and the high Ca2+ (aq) and CO2−3 (aq) activities are attributed to poorly crystalline Mg silicates and Na silicates similar in composition to sepiolite, loughlinite, stevensite, and magadiite.
Extraction of Manganese from Soils with Potassium Salts
Authors: Tu S. , Racz G. J. , Cho C. M.
Published: July 2010

Application of KCl usually increases the solubility of soil Mn and its bioavailability, yet the mechanisms of the phenomenon are not fully understood. The purpose of this study was to investigate the effects of KCl and salts such as KBr, KI, KNO3, K2SO4, K2CO3, K2HPO4, KH2PO4, Ca(NO3)2, Ca diethylenetriamine pentaacetic acid, and Cu(NO3)2 on the amount of Mn extracted from soils as affected by ionic strength, acidity of soil or extractant, and extraction time. The formation of anionic complexes of Mn and Cl in aqueous solution, using cation‐anion‐exchange resins, was also investigated. Amounts of Mn extracted by KCl from soils increased with increases in ionic strength and extraction time but decreased with increases in pH. The salts usually extracted more Mn from the noncalcareous than from the calcareous soils. The KI was capable of reducing MnO2 at normal soil pH, whereas Mn extracted by KCl was similar to Mn extracted by other neutral salts. The KCl‐extractable Mn was highly correlated with readily soluble [Ca(NO3)2‐extractable] Mn and weakly adsorbed (CaDTPA‐extractable) Mn fractions, but not to the specifically adsorbed [Cu(NO3)2‐extractable] Mn fraction. The Mn2+ in KCl solutions was shown to form anionic Mn‐Cl complexes. These complexes, along with ionic strength effects, were most likely responsible for the increases in solubility of soil Mn.
Mechanistic Simulation of Vertical Distribution of Carbon Concentrations and Residence Times in Soils
Authors: Elzein Abbas , Balesdent Jérôme
Published: July 2010

We developed a numerical simulation model of the decay and vertical transport of soil organic matter. Soil organic matter is divided into a number of compartments each with different decay rates but with similar migration parameters. Decay has been represented with first‐order processes. Two elementary forms of migration were explored: diffusive equations represent transport through soil mixing whereas translational convection simulates movement within the liquid phase. Coefficients of decay, diffusion, and convection were taken to be uniform with depth. Carbon input is both through the surface in the form of litter fall and directly underground from roots. Profiles of total C content and 14C content were measured for a temperate forest silt loam soil. Similar data for other forest soils, available in the literature, were also assembled. Statistical adjustment, through nonlinear regression, of decay, migration, compartment configuration, and plant production parameters, accurately simulated profiles of C and 14C. The rapidly decaying compartment was successfully likened to the 50‐ to 2000‐µm particle‐size fraction. Convection rates of ≈0.3 mm yr−1 and diffusion rates ranging from 1 to 15 cm2 yr−1 were obtained. Diffusive processes appear to be preponderant relative to convection. The model has proven sensitive to differences between temperate and tropical soils as well as textural variations.
Total and Light‐Fraction Carbon Dynamics during Four Decades after Cropping Changes
Authors: Bremer E. , Ellert B. H. , Janzen H. H.
Published: July 2010

The dynamics of soil organic C after a change in cropping practice are important in defining the potential impact of the new practice on soil behavior and net CO2 exchange. Our objective was to determine the duration and magnitude of a shift in soil organic C after a change in summer fallow frequency in spring wheat (Triticum aestivum L.) systems. Total organic and light‐fraction C in continuous wheat (W), fallow‐wheat‐wheat (FWW), and fallow‐wheat (FW) treatments were determined in soil samples collected periodically from 1954 to 1992 from a long‐term spring wheat study located in the northern Great Plains. Organic C concentrations in the 0‐ to 15‐cm depth were 11% lower in the FW and FWW treatments than in the W treatment from 1967 to 1992. Organic C concentrations in the 15‐ to 30‐cm depth were not affected by treatment, but varied among sampling years due to differences in effective sampling depth. Light‐fraction C concentrations were 38% lower in the FW and FWW treatments than in the W treatment from 1967 to 1992. Reduced light‐fraction C accounted for about one‐third of the decrease in total organic C due to inclusion of fallow in the crop rotation. The negative influence of fallow was primarily due to enhanced decomposition in the fallow year. We conclude that most losses of soil organic C due to introduction of a fallow system occur within two decades, and that further loss of organic C is negligible provided soil erosion is minimal.
Soil Acidification from Long‐Term Use of Anhydrous Ammonia and Urea
Authors: Bouman O. T. , Curtin D. , Campbell C. A. , Biederbeck V. O. , Ukrainetz H.
Published: July 2010

Acidity generated by N fertilizers depends on factors such as the composition of the fertilizer, climatic and soil conditions, and the crops grown. Our objective was to quantify the acidifying effects of urea and anhydrous NH3 when used as fertilizers for cereal production in Saskatchewan, Canada. The fertilizers were injected annually (at 10‐cm depth) into a medium‐textured, moderately acid (pH ≈5.5) Typic Haploboroll, at rates of 0, 45, 90, and 180 kg N ha−1 for 9 yr. Soil acidity increased as N application rate increased, with anhydrous NH3 causing greater acidification than urea. Although pH values as low as 4.3 were recorded in soil treated with anhydrous NH3, KCl‐exchangeable acidity remained low. The major effect of acidification was a depletion of exchangeable Ca and Mg. The solubility of Mn (but not Al) increased substantially as pH decreased, with solution concentrations of almost 30 mg Mn L−1 being recorded 6 d after injection of NH3. Acidity generated by anhydrous NH3 compared well with values predicted assuming that all of the applied NH3 was oxidized to NO3− (with the production of 1 mol H+ mol−1 of N) and that these protons were partly neutralized by OH− released when NO3− was taken up and assimilated by plants. Acidification due to export of bases in grain was insignificant because wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) remove only a slight excess of cations over anions. Urea failed to realize its full acidification potential because of an apparent loss of urea‐N from the soil by NH3 volatilization.
An Alkaline Oxidation Method for Determining Total Arsenic and Selenium in Soils
Authors: Zhu Baojin , Tabatabai M. A.
Published: July 2010

A simple, accurate, and precise method for determination of total As and Se in soils was developed. In this method, a mixture of soil and NaOBr solution was boiled to dryness in a sand bath (260–280°C), and the total amounts of As and Se were extracted with 1.25 M H2SO4 and determined by an atomic absorption spectrophotometer, equipped with a hydride generation system, after reducing As(V) to As (III) and Se(VI) to Se(IV). With 12 soils analyzed, the As values ranged from 5.0 to 14.3 mg kg−1 soil and the Se values ranged from 0.5 to 5.5 mg kg−1 soil. The average results by the proposed NaOBr oxidation method agreed closely with those obtained by the acid digestion methods of USEPA and USGS proposed for determination of As and Se in soils and geologic materials. The method gives quantitative recovery (96–103%) of As and Se in certified standard reference materials and of these elements added to soils (99–101%). Digestion of soil samples with NaOBr solution and determination of the extracted As and Se by hydridge‐generation atomic absorption spectrometry permit rapid analysis of a large number of samples at one time. Tests with 13 metals indicated that, at the concentrations normally expected in soils, none of those metals interfered with determination of As and Se by the proposed method. A single operator can complete the oxidation and analysis of one element in 40 samples in a normal working day.
Rapid Method for Accurate Determination of Colorless Siderophores and Synthetic Chelates
Authors: Shenker M. , Chen Y. , Hadar Y.
Published: July 2010

A rapid, simple, and accurate method of determining concentrations of Fe‐chelating agents in solution was developed. The assay employs Cu‐CAS (chrome azurol S) complex as a testing agent and measures the equilibrium concentration of this complex in the presence of other Fe chelators. The Cu‐CAS was found to be advantageous in comparison to other compounds due to: (i) its high extinction coefficient (17 000 M−1 at pH 5.7) when complexed with Cu, and (ii) the rapid rate at which equilibrium was achieved. The color of the Cu‐CAS was measured at a wavelength of 582 nm, and the decrease in absorbance when the Cu is lost to other chelates was measured. Using this method, we successfully measured 50‐ to 100‐nmol quantities of the following chelates: ethylenediaminetetraacetic acid disodium salt (EDTA), ethylenediamine‐di(o‐hydroxyphenylacetic acid) (EDDHA), desferriferrioxamine‐B (DFOB), rhizoferrin, and mugineic acid (MA). We found the method to be accurate and useful in determining concentrations of chelates (not absorbing intensely at 582 nm) including phytosiderophores and microbial siderophores. This method is of particular importance for colorless chelates, which are very difficult to determine otherwise.
Rate of Nitrogen Mineralized from Incorporated Crop Residues as Influenced by Temperature
Authors: Vigil M. F. , Kissel D. E.
Published: July 2010

Methods are needed to predict seasonal mineralization of N when crop residues are added to soils in order to assess N availability to crops. The objective of this investigation was to evaluate and develop methods for predicting the effect of temperature on the rate of N mineralization (Nmin) from crop‐residue‐amended soils maintained at near‐optimum soil water content. Unamended and crop‐residueamended soils were incubated at temperatures between 5 and 35°C to obtain Nmin rates for calculation of Q10 values, defined as the ratio of reaction rates at a temperature interval of 10°C. Measured Q10 depended on the C/N ratio of the residue and incubation time, indicating that for predictive purposes a single Q10 value is inadequate for describing the effect of temperature on crop‐residue Nmin. Temperature functions were developed from Nmin data collected from unamended soils incubated at four temperatures under laboratory conditions. These functions were then tested in the MINIMO model (a subroutine of CERES‐MAIZE) for residue‐amended soils at four temperatures for up to 160 d. Model‐predicted mineralizations was close to measured Nmin for most crop‐residue‐soil mixtures tested. The use of the new temperature equations improved the precision of the model, reducing the residual sum of squares (RSS) from 9797 to 5286. However, the modified model was not able to accurately predict Nmin for crop residues with a C/N ratio of 10 and 28. Inadequate prediction at these C/N ratios may be related to how the model allocates the pool sizes for soluble and insoluble plant N and the cellulose‐hemicellulose pool size of the crop residues.
Water Table Management Effects on Denitrification and Nitrous Oxide Evolution
Authors: Kliewer B. A. , Gilliam J. W.
Published: July 2010

Previous research suggested that using controlled drainage to elevate the water table reduces NO3− contamination of surface water by enhancing denitrification. The C2H2 inhibition technique was used to study denitrification and N2O evolution using 56‐cm‐long, undisturbed cores of Cape Fear loam (clayey, mixed, thermic Typic Umbraquult) maintained in the field and subjected to three static water table levels (15, 30, and 45 cm). Results showed that intermittent C2H2 exposure did not (i) affect soil inorganic N distribution between NH4+ and NO3−, (ii) diminish inhibition of N2O reduction during subsequent C2H2 exposure, or (iii) induce C2H2 decomposition. Denitrification from 1 Nov. 1993 through 21 Apr. 1994 (172 d) was 341 kg N ha−1 for the 15‐cm water table treatment, 260 kg N ha−1 for the 30‐cm water table treatment, and 86 kg N ha−1 for the 45‐cm water table treatment. Denitrification was maximum at the lowest monitored zone (36–54 cm) for each water table treatment. Total N2O evolution was 9 kg N ha−1 for the 15‐cm water table treatment, 4 kg N ha−1 for the 30‐cm water table treatment, and 2 kg N ha−1 for the 45‐cm water table treatment. Nitrous oxide evolution was positively correlated with mean soil temperature (10‐cm depth) until low NO3− levels appeared to limit denitrification. Since steady‐state diffusion was not reached, estimates of N2 evolution, using N2O evolution in the presence of C2H2, were underestimated 12‐fold. Evolved N2O‐N represented only 2% of denitrification in the soil core (0–54 cm) for each water table treatment. Drainage control to elevate the water table enhanced denitrification and N2O evolution, reducing the potential for N transport with subsurface drainage to surface water.
Temporal Patterns in Beech Forest Soil Solutions: Field and Model Results Compared
Authors: Johnson Dale W.
Published: July 2010

The applicability of cation exchange equations and the Nutrient Cycling Model (NuCM) to a beech (Fagus grandifolia Ehrh.) forest soil in the Great Smoky Mountains, North Carolina, was tested by examining temporal variations in soil solution chemical composition. Both field data and NuCM output implied that short‐term, local variations in exchangeable Ca2+ and Al3+ occurred, causing non‐ideal plots of pAl vs. pCa in soil solution. Thus, plots of pCa vs. pAl, with their inherent assumptions of constant exchangeable cations, do not provide an adequate test of the applicability of cation exchange equations to field conditions. A large NO3− pulse caused sequential peaks in soil solution Mg2+, Ca2+, and Al3+, which became more separated with soil depth. The NuCM model did not reproduce this pattern, showing instead simultaneous peaks of all cations in response to NO3− pulses. As a consequence, NuCM underestimated peak Al/Ca ratios. The disparity was probably due to the coarse scale at which NuCM and models like it represent the soil. A chromatographic model may be more appropriate for this time scale.
Iron and Aluminum Oxide Influence on Agronomic Effectiveness of Modified Phosphate Rocks
Authors: Menon R. G. , Chien S. H. , Hellums D. T.
Published: July 2010

A greenhouse experiment was conducted to study the influence of Fe and Al oxide in phosphate rocks (PR) on partially acidulated phosphate rock (PAPR) effectiveness and to evaluate compaction of PR with soluble phosphates as an alternate method of utilizing PR with high Fe and Al oxide. Five PR with Fe + Al oxide ranging from 19 to 320 g kg−1 were acidulated with 50% of the H2SO4 (PAPR, 50% H2SO4) required to fully convert the PR to triple superphosphate (TSP) or were compacted with TSP so that 50% of the total P was in TSP form (PR + TSP). They were applied to Hartsells silt loam (fine‐loamy, siliceous, thermic Typic Hapludult, pH 4.8) at rates 50, 100, 200, and 300 mg P kg−1. Two successive crops of maize (Zea mays L.) and one of rice (Oryza sativa L.) were grown. Biomass yield of the first crop from PAPR‐treated soils decreased from 86% of that from TSP to 16% and P uptake from 72 to 12% with increasing Fe and Al oxide from 1.9 to 32%. The decrease in effectiveness of PAPR was directly related to a decrease in water‐soluble P in PAPR with increasing Fe and Al oxide content. The compacted PR = TSP were 86 to 104% as effective as TSP in increasing biomass yield and 78 to 94% as effective in increasing P uptake. There were no great differences between PAPR and the compacted PR + TSP in terms of residual effect on the second crop of maize, except for Matongo PR. The difference in residual effectiveness between PAPR and compacted PR + TSP was not as pronounced for rice as for maize.
Characterizing Water and Solute Movement by Time Domain Reflectometry and Disk Permeametry
Authors: Vogeler Iris , Clothier Brent E. , Green Steven R. , Scotter David R. , Tillman Russell W.
Published: July 2010

To investigate a rapid, nondestructive way of characterizing solute transport properties, time domain reflectometry (TDR) and disk permeametry have been used in combination. Calibration measurements had previously related TDR measurements to both the volumetric water content and the pore water concentration of Cl‐. Laboratory measurements from a horizontal TDR probe were used to estimate transport parameters in a soil column by applying a one‐dimensional numerical model in an inverse sense. A vertical TDR probe was used to provide independent verification of these parameters. A repacked column of Ramiha silt loam (an Andic Dystrochrept) was used under unsaturated, transient flow conditions. The disk permeameter, set to a pressure head of −50 mm and containing a solution of 0.032 M KCl, was placed straight onto the repacked soil column, which had an initial water content of 0.32 m3 m‐3. The soil wet to 0.60 m3 m‐3. However, in the columns only an envelope of Cl‐ concentration could be obtained, due to exchange between the initially resident Ca2+ and the invading K+. This illustrates why cation exchange needs to be considered when TDR is used to infer solute dispersivity and the retardation were found to be 2.3 mm and 1.2, respectively. The retardation is shown to be due to the anion‐exchange capacity varying with the concentration of the invading soil solution.
Stability of Wetting Fronts in Dry Homogeneous Soils under Low Infiltration Rates
Authors: Yao Tzung‐mow , Hendrickx Jan M. H.
Published: July 2010

Our understanding of unstable wetting phenomena in soils is limited. Therefore, lysimeter experiments were conducted in the laboratory to validate current wetting front instability theories. Four different grades of sieved and air‐dried perlite and quartz sand were used as the experimental material. Water was applied by a sprinkler system at rates within the range of natural precipitation rates in New Mexico. Experiments were conducted in small lysimeters (diameter 30 cm, height 50 cm) as well as a large one (diameter 100 cm, height 150 cm). The experimental results show that wetting front instability will cause fingering phenomena in a homogeneous soil system. This observation confirms experimental and theoretical results of other workers. The diameter of fingers was observed to be a function of the grain size of the sand. Small fingers (3–4 cm diameter) were found in coarse sand (grain size 1.41–0.84 mm); large diameter fingers (12 cm diameter) were observed in fine sand (grain size 0.42–0.25 mm). Our experimental results in the coarse sand show that, for infiltration rates varying between 0.3 and 12 cm/h, finger diameters remain more or less constant. This observation also agrees with existing theories. However, at infiltration rates lower than 0.12 cm/h, the coarse sand experiments show that the wetting fronts became stable. For rates between 0.3 and 0.12 cm/h, the wetting is semistable; that is, there is incomplete wetting without distinct development of fingers. A similar trend was observed in the experimental results of sands with grain sizes of 0.841 to 0.594 and 0.594 to 0.42 mm. This phenomenon has not been observed in previous experimental studies and is not predicted by current wetting front instability theories. Our experimental data under infiltration rates similar to natural precipitation intensities may explain why unstable wetting has rarely been observed in wettable field soils.
Packing of Sands for the Production of Homogeneous Porous Media
Authors: Oliviera I. B. , Demond A. H. , Salehzadeh A.
Published: July 2010

Many studies of transport through soil assume that the laboratory columns used in the experiments are packed homogeneously. This research evaluated a variety of dry and wet packing techniques for their ability to produce uniform packings of clean sands without the use of specialized equipment. The best dry packing technique consisted of the deposition of 0.2‐cm layers followed by compaction with a metal pestle. For the sand used in this study, this technique resulted in a porosity of 0.325 ± 0.0020. The best wet packing technique consisted of the deposition of thin layers of saturated sand into water while vibrating the column. This technique resulted in a denser and more uniform packing, with the porosity equaling 0.315 ± 0.0018. No significant lateral particle‐size segregation was observed for either technique. Despite their success in producing homogeneous columns, the techniques proved time consuming, taking about 1 h per 5 cm of packed depth to pack a 15‐cm square column.
Alteration of Imogolite, Allophane, and Acidic Soil Clays by Chemical Extractants
Authors: Su Chunming , Harsh James B.
Published: July 2010

Pretreatment of soils to enable physical separation and purification of minerals may alter their structural or chemical composition and preclude meaningful surface charge characterization. A natural imogolite and allophane, a synthetic imogolite (Al/Si = 2.01) and allophane (Al/Si = 1.64), and clays from the Bs and BC horizons of a Spodosol containing imogolite were subjected to standard H2O2 treatments before and after extraction with buffered dithionite‐citrate‐bicarbonate (DCB) to determine if effects on their surface charge characteristics and bulk composition were reversible. The H2O2 slightly shifted the point of zero charge (PZC) determined by electrophoretic mobility (EM) of natural imogolite and allophane toward more positive values, probably by the partial oxidation of retained organics, whereas it had no effect on the PZC values of soil clays. Five DCB extractions removed the most Al and Si from the synthetic allophane and the least Al from the synthetic imogolite. More Si than Al was removed from the synthetic imogolite by five DCB extractions. Citrate retained on allophane and soil clays after DCB extraction was probably partially transformed to oxalate by subsequent H2O2 treatment. The EM in synthetic allophane could not be restored by oxidation of retained citrate following DCB treatment. Synthetic imogolite, after five DCB extractions, had only positive mobility up to pH 11.8. The DCB extraction, commonly used to remove Fe oxide from imogolite and allophane samples isolated from soils, irreversibly altered mineral surfaces and modified surface charge characteristics.
Modeling Competitive Adsorption of Arsenate with Phosphate and Molybdate on Oxide Minerals
Authors: Manning Bruce A. , Goldberg S.
Published: July 2010

The mobility of As in soils depends on several factors including redox potential, soil mineralogy, pH, and the presence of other oxyanions that compete with As for soil retention sites. We investigated the effects of pH and competing anions on the adsorption of arsenate [As(V)] on α‐FeOOH (goethite) and γ‐Al(OH)3(gibbsite). Batch equilibrium As(V) adsorption experiments were conducted with P and Mo as competing anions in order to produce single‐anion [As(V), P, and Mo] and binary‐anion [As(V)/P and As(V)/Mo] adsorption envelopes (adsorption vs. solution pH). Arsenate and P single‐anion adsorption envelopes were similar with substantial adsorption occurring across a wide pH range, including pH values above the points of zero charge of the oxides. Maximum Mo adsorption occurred across a narrower pH range (pH 4–6). On both oxides, equimolar P concentrations decreased As(V) adsorption within the pH range 2 to 11, whereas Mo decreased As(V) adsorption only below pH 6. The constant capacitance model was used to predict competitive surface complexation behavior between As(V)/P and As(V)/Mo using intrinsic equilibrium constants [Kanlon (int)] optimized from single‐anion data. In addition, the model was applied using one‐site (monodentate) and two‐site (monodentate + bidentate) conceptualizations of the oxide surface. The two approaches gave comparable fits to experimental adsorption data and were consistent with competitive adsorption observed in binary adsorption envelopes.
Characterization of the Hydrophobic Acid Fraction Isolated from a Wheat Straw Extract
Authors: Grossl Paul R. , Inskeep William P.
Published: July 2010

The hydrophobic acid portion (WSE‐FA) was isolated from a water‐soluble extract of wheat (Triticum aestivum L.) straw and chemically characterized. This material was isolated and concentrated on a column containing Amberlite XAD‐8 resin. Chemical analysis of the WSE‐FA included elemental composition (C, O, H, N, S, P, and ash); titrametric evaluation of carboxylic acid and phenolic functional group content; molecular weight (MW) determination by vapor pressure osmometry (VPO); gel filtration and ultrafiltration; and UV‐visible, solid‐state cross‐polarization magic angle spinning 13C nuclear magnetic resonance (NMR), and Fourier‐transform infrared spectroscopy. The MW of the WSE‐FA was approximately 600 daltons. The 13C‐NMR spectrum, the low MW relative to other humic substances, high E4/E6 ratio, and the low N content suggested that the WSE‐FA was comprised of small, aromatic phenolic acids, oxidized polyphenols with exposed carboxyl groups, and saccharides that were condensed during senescence.
Sulfur Balances and Sulfur‐34 Abundance in a Long‐Term Fertilizer Experiment
Authors: Kirchmann H. , Pichlmayer F. , Gerzabek M. H.
Published: July 2010

Reduced SO2 emissions, increased use of non‐S‐containing fertilizers, and higher crop yields may lead to S deficiency in agricultural soils in the future. In a long‐term field experiment on a clay loam soil (Typic Eutrochrept) at Uppsala, Sweden, S balances were evaluated for plots under continuous fallow, plots receiving inorganic fertilizers [(NH4)2(SO4 or Ca(NO3)2; 80 kg N ha‐1 yr‐1], and plots receiving organic amendments (2000 kg C ha‐1 yr‐1) with the aim of estimating plant uptake, leaching losses, immobilization, and variations of δ34S in soil. Total N was correlated with total S concentrations in soil (R2 = 0.980), but organic C was less well correlated (R2 = 0.741). Total soil S decreased in all treatments where no organic material was added, the largest decrease occurring in the continuous fallow plots with a S mineralization rate of 6 kg ha‐1 yr‐1. Sulfur added through (NH4)2SO4 and sewage sludge was mainly leached, whereas SO4 leaching was reduced in the Ca(NO3)2‐treated plots as a result of increased crop uptake. Of the organic amendments, 26 to 54% of the S remained in the soil with a half‐life of 24 to 38 yr. Recoveries of S from organic amendments in soil were correlated with their initial C/S ratios (R2 = 0.999) excluding peat. A significant enrichment of 34S was found only in plots receiving peat. Peat was more highly enriched in 34S than the other organic materials studied and was more resistant to decomposition. The results indicate that shifts in soil δ34S cannot be used as a tool for quantitative determinations of S turnover. Nitrogen transformations were the main cause of acidification in the (NH4)2SO4‐treated plots, as opposed to S leaching in the sewage‐sludgetreated plots.
Comparing Two Approaches of Characterizing Soil Map Unit Behavior in Solute Transport
Authors: Finke P. A. , Wösten J. H. M. , Kroes J. G.
Published: July 2010

Soil maps can be used in different ways to characterize spatial patterns of soil behavior. A first approach is to establish the behavior of a pedogenetically representative profile from each soil map unit (SMU); a second is to subsample each SMU and select from the sample a characteristic profile by its behavior with respect to water and solute transport. The first approach was tested against the second with data from a SMU in the Netherlands. The use of pedogenetically representative profiles resulted in biased estimations of five studied functional properties related to water and solute transport. These properties were (i) the number of days with good workability; (ii) the number of days with sufficient aeration; (iii) the elapsed time to 10% breakthrough of an inert tracer (Cl‐); (iv) the percentage break‐through after 1 yr of an adsorbing, inert contaminant (Cd); (v) the percentage breakthrough after 1 yr of an adsorbing, degrading herbicide (isoproturon [N‐(4‐isopropylphenyl)‐N′, N′‐dimethylurea]). Soil profiles that did not fit the definition of the SMU (impurities) were responsible for the occurrence of extreme values for four out of five properties, which implies that impurities must be sampled when risk assessments have to be made. In this case, probability sampling offers a valid approach.
Soil Properties and Genesis of Pans in Till‐Derived Andisols, Olympic Peninsula, Washington
Authors: Wilson M. A. , Burt R. , Engel R. J. , Sobecki T. M. , Hipple K.
Published: July 2010

Subsoil pans regarded as cemented are present in certain Andisols of the Olympic Peninsula region of western Washington. Soils formed in basalt‐derived alluvium or colluvium over Pleistocene‐aged compact alpine till. Theories of pan formation include pedogenic cementation by silica, allophane, or humic‐metal complexes; or geogenic till compaction with no subsequent cementation. We examined the properties of three pedons with compacted and cemented subsoil horizons to better understand the genesis of these soils and pan horizons. Pan horizons, designated as Cm, are very brittle, nearly impossible to dig with hand tools, and massive in structure. Soils are acidic, with pH(H2O) for all horizons ranging from 3.7 to 6.0. Crystalline phyllosilicates identified are gibbsite, kaolinite, chlorite, vermiculite, and hydroxy‐interlayered vermiculite. Allophane was detected in most mineral horizons, including Cm horizons, and allophane Al/Si molar ratios vary from 1.2 to 3.4. Low NaOH‐extractable Si, low Si/Al molar ratios from NaOH extracts, high rainfall, and the presence of gibbsite decrease the likelihood of pedogenic opal as a cementing agent in pans. Limited organo‐metallic deposition, low organic C, and low pyrophosphate‐extractable Fe and Al in pan horizons suggest that podzolization processes related to ortstein formation are not responsible for Cm horizon cementation. Micromorphologic examination of Cm horizons detected a light brown to nearly colorless substance that appears to be allophane around mineral grains and in pore channels. Soil chemistry data support allophane as the primary cementing agent in these pan horizons, with kaolinite, gibbsite, and Fe oxides acting as possible accessory agents. Glacial compaction, parent material mineralogy, and cycles of wetting and drying are important factors affecting cementation of these pans.
Sediment and Chemical Load Reduction by Grass and Riparian Filters
Authors: Daniels R. B. , Gilliam J. W.
Published: July 2010

Vegetated filter strips help reduce non‐point source pollution from agricultural areas. Even though they are an accepted and highly promoted practice, little quantitative data exist on their effectiveness under field conditions. The objective of this research was to determine the amount of nutrients and sediment removed by natural and planted filters. This was achieved by collecting and analyzing runoff at field edges and at various locations in vegetated buffers. Total weight of sediment and nutrients in runoff from North Carolina agricultural fields showed that the grass and riparian filter strips studied reduced runoff load by 50 to 80%. Total sediment decrease through the filters was about 80% for both grass and riparian vegetation. The reduction in the chemical load depended on the nutrient and its form. Filters reduced total P load by 50%, but 80% of the soluble PO4‐P arriving at the field edge frequently passed through the filters. The filters retained 20 to 50% of the NH4 and approximately 50% of the total Kjeldahl N and NO3. High‐volume flows commonly overwhelmed both grass and riparian filters next to cultivated fields. Forested ephemeral channels had little vegetation and were effective sediment sinks during the dry season but were ineffective during large storm events because there was little resistance to flow. When possible, drainageways should be designed to hold sediment and to disperse the discharge into a riparian area.