bluuebunny/crossref_metadata_2025 huggingface dataset:
Bromide Transport Detection in Tilled and Nontilled Soil: Solution Samplers vs. Soil Cores
Authors: Fleming J. B. , Butters G. L.
Published: July 2010

Experimental validation of solute transport models is often met with unsatisfying or ambiguous results. Aside from the inherent difficulty in conducting these experiments, the lack of a standard set of methodologies employed in these studies may be an impediment. This study was conducted to determine if the method employed in the sampling of soil solutes could influence the results of model validation attempts. A narrow pulse of KBr was uniformly applied to two 25‐m2 plots, one tilled and the other undisturbed, on a level Ulm clay loam (fine, montmorillonitic, mesic Ustollic Haplargid). Solute movement was then monitored for the next 388 d using replicated ceramic cup solution samplers at depths of 0.15, 0.30, 0.80, 1.20, 1.60, and 2.00 m and by the periodic removal of replicate soil cores in 0.05‐m increments to a maximum depth of 3.70 m. Overall, the methods yielded consistent information regarding the mean convective transport of Br−. Both methodologies indicated deeper and more rapid movement of Br− in the nontilled soil, although the shallow solution samplers appear to miss the most rapidly moving solute. By the conclusion of the experiment, the difference between the centers of mass of the Br− plumes in the tilled and nontilled soil was at least 0.6 m. The soil core and solution sampler methods appear to yield inconsistent measures of the solute dispersion but we attribute the differences to the effects of incomplete mass recovery and, we argue, follow naturally from the instrument window of the sampling methodology relative to the observed soil heterogeneity.
Analysis of Aggregate Fractal Dimensions and Aggregate Densities Back‐Calculated from Hydraulic Conductivity
Authors: Logsdon S. D.
Published: July 2010

Recently, aggregate size distributions (ASDs) have been characterized by fractal dimension (D, i.e., slope of the log of cumulative aggregate number as a function of the log of aggregate size). To simplify the laboratory work, aggregates are often considered to have scale‐invariant density (ρ). The D describes the completely fragmented ASD after sampling and sieving the aggregates, but a bulk Dr describes an incompletely fragmented ASD in situ. The Dr requires an estimate of scale‐variant ρi for each class. Then the ASD may be used to determine pore distributions and hydraulic conductivity (K) of the soil. The objectives of this study were to start with K determinations to back‐calculate ρi, D, Dr, and scale‐invariant ρ, and compare these with measured ρ and D from measured ASDs taken near the locations of the K measurements. Back‐calculated ρi was not significantly different between class sizes for aggregates >2.8 to 5 mm, and the ρi differences between aggregate sizes 1.26 to 25.2 mm would be difficult to determine with the precision (0.03–0.1 g cm−3) of current methods for determining ρi. The calculated ρi and scale‐invariant ρ were all significantly less than measured ρ. The D from back‐calculated ASD were all greater than three, but they should have been less than the Dr from back‐calculated ASD, which were all less than three. The D and Dr from back‐calculated ASD were significantly different from D for measured ASD. Further study is needed to determine the relationship between ASD and K.
Elevated Atmospheric Carbon Dioxide Effects on Cotton Plant Residue Decomposition
Authors: Torbert H. A. , Prior S. A. , Rogers H. H.
Published: July 2010

Assessing the impact of elevated atmospheric CO2 concentration on the global environment is hampered due to a lack of understanding of global C cycling. Carbon fixed within plant biomass ultimately enters the soil via plant residues, but the effects of elevated‐CO2‐grown plant material on decomposition rates and long‐term soil C storage are unknown. The objective of this study was to determine the decomposition rate of plant residues grown under an elevated CO2 environment as affected by soil type. Cotton (Gossypium hirsutum L. ‘Delta Pine 77’) samples were collected from a free‐air CO2 enrichment (550 µL L−1) experiment. The plant residues were incubated under ambient CO2 conditions to determine decomposition rates of leaves, stems, and roots and potential N and P mineralization‐immobilization in three soil series: a Blanton loamy sand (loamy siliceous, thermic Grossarenic Paleudult), a Decatur silt loam (clayey, kaolinitic, thermic Rhodic Paleudult), and a Houston clay loam (very fine, montmorillonitic Typic Chromudert). No significant difference was observed between plant residue grown under CO2 enrichment vs. ambient CO2 conditions for soil respiration or P mineralization‐immobilization. Significantly greater net N immobilization was observed during the incubation in all soil types for plant residue grown at elevated CO2. These results indicate that while decomposition of plant residue may not be reduced by CO2 enrichment, N dynamics may be markedly changed.
Potential Soil Respiration and Relationship to Soil Properties in Ridge Tillage
Authors: Liebig M. A. , Jones A. J. , Doran J. W. , Mielke L. N.
Published: July 2010

Differences in traffic and tillage intensity among positions in ridge tillage create distinctly different environments for microbial activity. This study was conducted to assess the impact of long‐term controlled wheel traffic on soil respiration in ridge‐till and to use correlation analysis to identify relationships between soil respiration and soil physical and chemical properties. Soil respiration was evaluated from 0 to 30 cm in one row, one tractor‐trafficked interrow, and one nontrafficked interrow of continuous corn (Zea mays L.) and continuous soybean [Glycine max (L.) Merr.]. Soil respiration was measured on disturbed samples at three levels of water‐filled pore space (WFPS) by gas chromatography for 25 d. Properties assessed included bulk density, soil strength, texture, aggregate‐size distribution, saturated hydraulic conductivity (Ksat), water retention characteristics, organic C, and total N. Soil respiration was greatest at 0 to 7.5 cm in each position and decreased significantly below that depth. Correlation analysis indicated microbial activity in ridge‐till varied spatially in relation to changes in the soil physical environment. Soil respiration was negatively correlated with bulk density at each WFPS. The Ksat was positively correlated with soil respiration at 0 to 7.5 cm for each WFPS. Under drier soil conditions, as exemplified by 47% WFPS, aggregates <1.0 mm and gravitational water were positively correlated with soil respiration at the 0 to 7.5 cm. Soil environments characterized by bulk density <1.4 Mg m−3 and Ksat >10 cm h−1 were associated with respiration rates >4 and 12 mg CO2‐C L−1 soil d−1, respectively.
Improved Computational Procedure for Retention Relations of Immiscible Fluids Using Pressure Cells
Authors: Liu H. H. , Dane J. H.
Published: July 2010

The capillary pressure‐saturation relation, or the capillary pressure head‐volumetric fluid content relation, is of widespread interest in modeling and predicting multiphase fluid flow. A common procedure to determine this relation is to use a pressure cell and assume uniform distributions of capillary pressure and volumetric fluid contents at static equilibrium. The pressure cell method, however, yields inaccurate results for certain combinations of multiphase fluids and porous media. The sensitivity analysis in this study shows that the pressure cell height, the positions of the pressure measuring devices, which can coincide with the in‐ and outflow ports, and the relative density difference of the fluids, can give rise to considerable errors in the determination of capillary pressure head‐volumetric fluid content relations by the standard procedure using pressure cells. We, therefore, propose a curve‐fitting method that considers nonuniform distributions of capillary pressure and volumetric fluid content of multiphase fluids. The method uses the same input data as commonly used procedures.
Dinitrogen Fixation in Pea: Controls at the Landscape‐ and Micro‐Scale
Authors: Stevenson F. C. , van Kessel C. , Knight J. D.
Published: July 2010

Topography influences the distribution of water and N, thereby exerting an indirect control on N2‐fixing activity of legumes. A study was conducted to assess the (i) variability of N2 fixation in pea (Pisum sativum L.) in a rolling field, and (ii) degree of landscape‐scale control on N2 fixation. A 90 by 100 m sampling grid, with 10‐m spacings, was established in the field. Each sampling point was classified as either footslope or shoulder. The percentage of N derived from the atmosphere (% Ndfa) was estimated using natural 15N abundance and the A‐value approaches. Spring soil water content and inorganic N were most concentrated in the footslopes, whereas mean estimates of % Ndfa showed inverted spatial distribution patterns. At flowering, natural 15N abundance estimates of % Ndfa did not differ between shoulders and footslopes. In contrast, A‐value mean estimates of % Ndfa were 84% for shoulders and 92% for footslopes. The two approaches gave similar mean estimates of % Ndfa at maturity, with values of approximately 72 and 84% for footslopes and shoulders. A random spatial pattern existed for total aboveground N accumulation, indicating that pea was able to adjust its N2‐fixing activity according to the available soil N. Despite similar spatial patterns for % Ndfa estimated using the two approaches, the correlation between the A value and natural 15N abundance approaches was poor (r = 0.213 at flowering and r = 0.377 at maturity). The poor correlation suggests that N2 fixation by pea was partially controlled at the landscape scale, whereas strong micro‐scale controls may have existed that ultimately regulated N2 fixation.
Impact of Fertilizer Placement and Tillage System on Phosphorus Distribution in Soil
Authors: Rehm G. W. , Scobbie A. J. , Randall G. W. , Vetsch J. A.
Published: July 2010

Repeated use of band applications, both surface and subsurface, of immobile nutrients applicable to a variety of tillage systems raises concerns about the collection of soil samples used for making fertilizer recommendations. This study was conducted to measure the effect of tillage system and fertilizer placement on soil test P in a major part of the root zone. Soil samples were collected from control treatments and from treatments where phosphate fertilizer supplying 29 kg P ha−1 yr−1 had been broadcast, applied as a surface band, or applied in a subsurface band for three consecutive years in a corn (Zea mays L.)‐soybean [Glycine max (L.) Merr.] rotation. These placements were used in both a chisel and a ridge‐till planting system. Soil samples were collected from depths of 0 to 7.5, 7.5 to 15.0, 15.0 to 22.5, 22.5 to 30.0, and 30.0 to 45.0 cm at distances of 0, 7.5, 15.0, 22.5, 30.0, and 37.5 cm from the row. Soil test P, to a depth of 15 cm at each position from the row, was significantly affected by fertilizer placement. Placement also had a significant effect on soil test P to a depth of 30 to 45 cm at a distance of 30 cm from the row. Tillage system had a significant effect on measured soil test P at sampling positions near the row. These results indicate that both tillage system and fertilizer placement could have a major effect on soil test values. If band locations are known, they should be avoided, if possible, during sample collection.
Phosphorus Transport during Transient, Unsaturated Water Flow in an Acid Sandy Soil
Authors: Chen J. S. , Mansell R. S. , Nkedi‐Kizza P. , Burgoa B. A.
Published: July 2010

Sorbent‐sorbate interactions heavily retard the movement of P during water flow in most soils. The effect of soil/solution ratio on sorption kinetics and movement of P during unsteady unsaturated water flow were investigated. A series of batch experiments with soil/solution ratios ranging from 0.1 to 6.4 Mg m‐3 were conducted to obtain sorption rate coefficients. Aqueous P solutions (100–800 g m‐3) were applied at two constant fluxes (1.4 × 10‐6 and 6.9 × 10‐6 m s‐1) to columns of an air‐dry spodic soil. The experimental data were simulated with a parallel two‐site nonlinear, nonequilibrium transport model during unsteady, unsaturated water flow. Phosphorous sorption followed Freundlich‐type reversible kinetics with a very fast reaction occurring in Type I sites and a slow reaction occurring in Type II sites. The sorption reaction of P in batch experiments was satisfactorily described by the model but the rate coefficients varied with the soil/solution ratio. Experimentally determined rate coefficients described P movement in column experiments at an influx rate of 6.9 × 10‐6 m s‐1, but not for the slower influx rate of 1.4 × 10‐6 m s‐1, hence calibration of rate coefficients was necessary for describing P movement during the slower water influx. We developed a technique for determining rate coefficients under water saturated‐unsaturated conditions that provides a way to validate P transport models during transient, unsaturated water flow.
Macroscopic Capillary Length, Sorptivity, and Shape Factor in Modeling the Infiltration Rate
Authors: Nachabe M. H.
Published: July 2010

Infiltration tests in the field involve measurements of sorptivity and macroscopic capillary length. These two parameters are strongly related through the shape factor, which is a measure of the nonlinearity of the soil hydraulic diffusivity. In this study, relationships were developed between the macroscopic capillary length, the sorptivity, and the shape factor and the parameters of the Brooks and Corey and van Genuchten expressions of hydraulic conductivity and diffusivity. These relationships are important for users of numerical models who need to estimate the parameters of these expressions to predict water flow and contaminant transport in soils. Numerical simulations with a dimensionless form of Richards' equation show that the predicted infiltration rate will not be very sensitive to small variations in the shape factor, provided the macroscopic capillary length is the same. This result is encouraging because the shape factor is difficult to determine accurately in the field. The similarity of the dimensional infiltration solutions implies that the macroscopic capillary length is a scale factor, because (i) it renders predictions of infiltration rates fairly insensitive to the expressions of hydraulic conductivity and diffusivity used, and (ii) it reduces the number of parameters needed to characterize infiltration. Therefore, the infiltration curve into a particular soil can be deduced by choosing units of length and time (i.e., scaling) of a generalized infiltration solution.
Model Sensitivity to Measured and Estimated Hydraulic Properties of a Zimmerman Fine Sand
Authors: Wu L. , Allmaras R. R. , Lamb J. B. , Johnsen K. E.
Published: July 2010

Water retention and hydraulic conductivity are necessary for simulating solute transport in process‐oriented models because of the fundamental role of water flow. Accuracy and extension of soil hydraulic properties are crucial for using models to give site‐specific predictions, yet these evaluations are infrequent and often lack rigor. The objectives of this study were to compare measured and estimated hydraulic properties and their effects on simulation of field soil‐water regime. Water contents (θ) and hydraulic conductivities (K) at specific matric potentials (ψ) alternatively measured in the laboratory and field or estimated from other soil properties were fitted to water retention and hydraulic conductivity functions to obtain parameters according to the van Genuchten formulation. The estimated parameters were inputs for the Root Zone Water Quality Model to simulate water movement in a Zimmerman fine sand (mixed, frigid Argic Udipsamment). Simulated and in situ measured water contents were compared to assess methods used for measuring or estimating hydraulic properties. Simulation using hydraulic functions generated from field‐measured θ(ψ) and K(ψ) was closest to measured total water depth in the 1.5‐m profile as well as water contents at the 0.2‐m depth. Arbitrary variations of α and n in the van Genuchten equations had larger effects on θ(ψ) and K(ψ) functions compared with variations caused by methods to measure and estimate hydraulic functions. This test demonstrated the crucial role and importance of soil hydraulic properties in simulating water storage and movement even in a sandy soil.
Determining Convective Lognormal Solute Transport Parameters from Resident Concentration Data
Authors: Vanderborght J. , Vanclooster M. , Mallants D. , Diels J. , Feyen J.
Published: July 2010

Time Domain Reflectometry (TDR) has become a popular method to obtain time series of resident concentrations in solute transport studies, so effective interpretation of TDR data is very useful. In this study, we evaluated three procedures to determine the parameters of the convective lognormal transfer function (CLT) model from such time series. In a first approach, the CLT model travel time probability density function (pdf) was related to time series of time‐integral‐normalized flux concentrations, Cf*(z,t). The normalization was based on mass balance calculations performed with time series of resident concentrations measured at several depths. The Cf*(z,t) data thus obtained were very erratic leading to a high parameter uncertainty. In a second approach, the CLT model travel depth pdf was related to time series of depth‐integral‐ or mass‐normalized resident concentrations, Crm*(z,t). This approach suffered from uncertainty regarding calibration coefficients relating TDR measured impedance to resident concentrations and from uncertainty regarding the solute mass crossing the sampling volume of the TDR probe. The third approach used the relation between travel time and travel depth pdf for a stochastic‐convective transport process to formulate a new expression relating time‐integral‐normalized resident concentrations, Crt*(z,t), to the CLT model parameters. No information is needed about calibration coefficients while no a priori assumptions about the solute mass detected by the TDR probe are required. This approach yielded better CLT model fits to observed concentration data, and the variability of parameters fitted at different locations can be related to heterogeneity of solute and water flow after averaging across the TDR measurement window.
Fractal Characterization of Aggregate‐Size Distribution: The Question of Scale Invariance
Authors: Logsdon S. D. , Gimenez D. , Allmaras R. R.
Published: July 2010

Aggregate‐size distributions are often described as fractal based on the power law relationship between cumulative aggregate number and aggregate size. The slope of the natural log of cumulative number of aggregates as a function of natural log of diameter is the fractal dimension. If aggregate density, shape, and relative diameter (diameter as a fraction of the smallest diameter of the size‐class) are scale invariant, then aggregate number can be calculated from mass. These three factors can be combined into a single unknown factor, Gi. The objectives of this study were to test the assumption of a scale invariant Gi and to compare the fractal dimension from calculated cumulative number of aggregates with the fractal dimension from counted cumulative number of aggregates. Numbers of aggregates were counted for each size class of two data sets. Calculations for the 48 samples of Data Set I spanned six classes ranging from sizes 1 to 32 mm, and calculations for 12 samples of Data Set 2 spanned four classes ranging from sizes 0.5 to 8 mm. The fractal dimension from counted cumulative number of aggregates was significantly smaller than the fractal dimension determined from calculated cumulative number of aggregates for Data Set 1 (2.44 vs. 2.51) but significantly larger for Data Set 2 (2.41 vs. 2.03). The Gi factor was significantly different across many of the size classes for both data sets. Since Gi consisted of subcomponents (density, shape, relative diameter), we cannot be certain which subcomponent(s) was (were) scale variant. Scale variant Gi complicates the use of fractal mathematics to describe dry soil aggregate distributions.
Respiratory Activity and Oxygen Distribution in Natural Aggregates in Relation to Anaerobiosis
Authors: Sierra Jorge , Renault Pierre
Published: July 2010

A reliable estimate of the anoxic regions in soil is essential in predicting the activity of microbial processes such as denitrification. In this study, two models were tested that have been previously proposed to estimate anaerobiosis within saturated aggregates. The first model assumes O2 consumption is constant as long as O2 is present; in the second model, O2 consumption depends on O2 concentration. The test was based on the overall respiration of natural aggregates of different sizes placed at different O2 concentrations. Some physical, chemical, and biological properties of the aggregates were also analyzed to verify the basic assumptions of the models. This analysis included direct measurements of O2 concentration within aggregates. The second model better described aggregate respiration, but some discrepancies were found between data and theory. Oxygen profiles indicate a steep fall of O2 near the aggregate surface. This suggests that the potential of respiration was not homogeneously distributed as assumed by the model, but decreased toward the center of the aggregates. The coefficient of variation of the respiration of 4‐mm aggregates placed at 0.21 m3 O2 m−3 was approximately 40%. This interaggregate variability must be incorporated into the model to prevent underestimation of the real and potential rates of respiration. Physical heterogeneities affected O2 transport within aggregates and induced irregularities in the O2 distribution. However, there was no evidence of any effect of such heterogeneities on the aggregate respiration.
Elevated Atmospheric Carbon Dioxide and Leaf Litter Chemistry: Influences on Microbial Respiration and Net Nitrogen Mineralization
Authors: Randlett Diana L. , Zak Donald R. , Pregitzer Kurt S. , Curtis Peter S.
Published: July 2010

Elevated atmospheric CO2 has the potential to influence rates of C and N cycling in terrestrial ecosystems by altering plant litter chemistry and slowing rates of organic matter decomposition. We tested the hypothesis that the chemistry of leaf litter produced at elevated CO2 would slow C and N transformations in soil. Soils were amended with Populus leaf litter produced under two levels of atmo‐spheric CO2 (ambient and twice‐ambient) and soil N availability (low and high). Kinetic parameters for microbial respiration and net N mineralization were determined on soil with and without litter during a 32‐wk lab incubation. Product accumulation curves for CO2‐C and inorganic N were fit to a first order rate equation [y = A(1 − e−kt)] using nonlinear regression analyses. Although CO2 treatment affected soluble sugar concentration in leaf litter (ambient = 120 g kg−1, elevated = 130 g kg−1), it did not affect starch concentration or C/N ratio. Microbial respiration, microbial biomass, and leaf litter C/N ratio were affected by soil N availability but not by atmospheric CO2. Net N mineralization was a linear function of time and was not significantly different for leaves grown at ambient (50 mg N kg−1) and elevated CO2 (35 mg N kg−1). Consequently, we found no evidence for the hypothesis that leaf litter produced at elevated atmospheric CO2 will dampen the rates of C and N cycling in soil.
Kinetics of Phosphate Rock Dissolution in Acidic Soil Amended with Liming Materials and Cellulose
Authors: He Z. L. , Baligar V. C. , Ritchey K. D. , Martens D. C. , Kemper W. D.
Published: July 2010

Phosphate rock (PR), coal combustion by‐product (CCBP), limestone, and organic materials are promising amendments for low P, acid soils, and two or more of the amendments are commonly applied to improve crop yields in acid soil. However, the interactions between these amendments in acid soil are not fully understood. The effects of CCBP, limestone, and cellulose application on dissolution of North Carolina phosphate rock (NCPR) in an acid Lily loam (fine‐loamy, siliceous, mesicTypic Hapludult) from West Virginia were examined in an incubation study. Based on the ΔP‐NaOH estimation method, ≈ 30% of the added NCPR was dissolved after 60 d of incubation. The NCPR dissolution decreased drastically with increasing rate of CCBP, particularly when it was applied together with limestone, whereas cellulose addition tended to enhance NCPR dissolution. Dissolution of NCPR was well described by first‐order, Elovich, and Langmuir kinetic equations. The average r2 values between the experimental data of NCPR dissolution and the three models were 0.829*, 0.851**, and 0.930**, respectively, for 17 soil samples with and without amendments. Half‐life (t1/2) from the first‐order reaction could be employed as a simple measure of the NCPR dissolution rate. Constants A and b derived from the Elovich equation were related to the initial and average dissolution rate of NCPR. The potential maximum dissolution of PR from the Langmuir equation provided an estimate of the dissolution extent of NCPR. All four kinetic parameters, particularly t1/2 and A, were closely correlated with CCBP application rates. These parameters appeared useful in evaluating the effects of CCBP application on NCPR dissolution in the acid soil and might also be applicable to predicting PR dissolution for a wide range of soils with different properties.
Factors Affecting Phosphate Rock Dissolution in Acid Soil Amended with Liming Materials and Cellulose
Authors: He Z. L. , Baligar V. C. , Ritchey K. D. , Martens D. C. , Kemper W. D.
Published: July 2010

Coal combustion by‐product (CCBP; a wallboard‐quality gypsum), limestone, and cellulose had been found to influence the dissolution of North Carolina phosphate rock (NCPR) in an acid Lily loam soil (fine‐loamy, siliceous, mesic Typic Hapludult). Laboratory and incubation experiments were carried out in this study to elucidate the mechanisms responsible for the reduced dissolution of NCPR from CCBP application. The major factors responsible for the reduced PR dissolution were (i) an increase in exchangeable Ca + Mg derived from the CCBP, (ii) an increase in soil pH from the small amounts of CaCO3 in the CCBP material; and (iii) sulfate released from the CCBP resulting in a decrease in P sorption capacity of the amended soil due to competitive sorption between sulfate and phosphate. The further reduction in NCPR dissolution by limestone addition was probably due to the increase in pH and exchangeable Ca relative to CCBP application alone. An increased NCPR dissolution from cellulose addition probably reflected an accelerated depletion of labile P by the cellulose‐induced microbial growth and, hence, biomass production. The four kinetic dissolution parameters, i.e., the half‐life (t1/2) from a first‐order rate equation, the initial dissolution rate‐ and the average dissolution rate‐related constants (A and b) from an Elovich equation, and the potential maximum dissolution of phosphate rock from a Langmuir equation were significantly correlated with pH, the content of exchangeable (Ca + Mg), and P sorption capacity of the CCBP‐amended soil. The t1/2 and A of NCPR in the acid soil could be predicted with 95% confidence by the three soil variables, i.e., soil pH, P sorption capacity, and exchangeable (Ca + Mg). Both t1/2 and A could serve as an indicator of the NCPR dissolution potential in soils.
Pyrolysis‐Field Ionization Mass Spectrometry of the Four Phases of Cow Manure Composting
Authors: van Bochove Eric , Couillard Denis , Schnitzer Morris , Schulten Hans‐Rolf
Published: July 2010

Pyrolysis‐field ionization mass spectrometry (Py‐FIMS) was used to analyze organic matter formed by the composting of cow manure. Analyses were done on samples withdrawn during the four characteristic phases (mesophilic, thermophilic, cooling, and maturation) of the composting process. The organic components that were identified included carbohydrates, phenols, monomeric and dimeric lignins, n‐C38 to n‐C51 n‐alkyl monoesters, n‐C15 to n‐C34 fatty acids, sterols, and heterocyclic N. Sterols of animal and plant origins were detected in relatively large amounts. Among the heterocyclic N forms, indole predominated. During the thermophilic phase, carbohydrates and n‐fatty acids increased. During the cooling phase, concentrations of carbohydrates increased, while dimeric lignins degraded to monomeric lignins and complex plant waxes to n‐fatty acids. The maturation phase was characterized by decreases in concentrations of all identifiable components, indicating the formation of chemically more complex structures.
Imaging a Pseudomonad in Mineral Suspensions with Scanning Force and Electron Microscopy
Authors: Fendorf Scott E. , Li Guangchao , Morra Matthew J. , Dandurand L.‐M.
Published: July 2010

Techniques for imaging bacteria in their physical environments yield information potentially relevant to better understanding the role of soil and sediment colloids in altering microbial activity. Although cell structure and morphology of bacteria have been previously investigated using light and electron microscopies, both approaches are not without deficiencies. Our objective was to demonstrate the utility of scanning force microscopy (SFM) to image cell morphologies and to observe interactions between bacteria and clay minerals. Pseudomonas fluorescens (Migula), strain 2–79 RN10, was imaged in pure culture and in suspensions of kaolinite and montmorillonite using both SFM and, for comparison purposes, SEM (scanning electron microscopy). Largescale SFM and SEM images were similar, thus confirming the lack of artifacts introduced by SFM. Images of bacteria in kaolinite suspensions were not obtained with either technique, a result attributed to bacterial isolation in large pores of the kaolinite matrix. Imaging of montmorillonite‐bacteria suspensions showed that bacteria were closely associated with the larger mineral particles. In addition, the high resolution of SFM revealed what appeared to be small hexagonal montmorillonite platelets adhering to cell surfaces. The features of SFM allow for this unique ability to image whole cells as well as the microtopography of cell surfaces within hydrated environments. Scanning force microscopy provides valuable information concerning microbial cell morphology and the interaction of bacterial cells with montmorillonite.
Description of Atrazine Transport in Soil with Heterogeneous Nonequilibrium Sorption
Authors: Chen W. , Wagenet R. J.
Published: July 2010

Numerous studies have shown that a single sorption rate constant is insufficient to describe nonequilibrium sorption and transport of organic solutes in soil. To investigate the effect of sorption rate (or, more generally, sorption site) heterogeneity on solute transport, we proposed a model assuming a γ distribution for both sorption rate and partitioning coefficient (abbreviated as GS for gamma‐distributed sites). The model was tested by a series of column leaching and batch sorption experiments conducted with atrazine (2‐chloro‐4‐ethylamino‐6‐isopropylamino‐s‐triazine) over various time scales. Observed asymmetry and a left‐hand shift of measured breakthrough curves (BTCs) with increasing flow velocity (0.51 to 21.3 cm/h) or decreasing column length (15.5 to 6.0 cm) were successfully predicted by GS with sorption parameters obtained from an independent experiment. However, two other distinct sorption site models, the two‐site first‐order sorption (TSFO) and the two‐site spherical diffusion (TSSD), were not able to describe the observed BTCs unless parameter values were adjusted for each case. In batch sorption experiments, increasing equilibration time (ranging from 1 to 237 h) was found to generally increase the isotherm‐derived partitioning coefficient (KD). The increasing trend was independently well‐predicted using the GS model with optimized parameters from the column studies. The TSSD was less successful and TSFO the least able to reconcile the entire range of change in KD. The weak time‐scale dependency of GS found in both column and batch experiments supports the assumption that a wide range of heterogeneous sorption sites or domains have been involved in the overall sorption process but to a variable degree as time elapsed. Potential alteration of soil sorption properties by γ irradiation was also observed.
Soil Erosion Effects on Soil Properties in a Highland Area of Central Kenya
Authors: Gachene C. K. K. , Mbuvi J. P. , Jarvis N. J. , Linner H.
Published: July 2010

One of the most important features that is neglected in many soil erosion studies in Africa is the monitoring of changes to the soil during the progress of the experiment. Objectives of this study were therefore to monitor runoff, soil loss, and enrichments of eroded soil material and to assess the effect of cumulative soil loss on soil chemical properties of an Alfisol in one of the high‐potential areas of Central Kenya highlands.

Runoff, soil loss, and enrichments of eroded sediments were measured on newly opened ground from 1991 to 1992. Annual rainfall was 948 and 1125 mm for 1991 and 1992, respectively. Soil loss ranged from 0.8 to 247.3 t ha−1, and runoff ranged from 1 to 89 mm. No significant correlations were observed between enrichment ratios (ER) and soil loss. The ER were ≥ 1 and sediments were mostly enriched with P and Na. The P and Na concentrations were 4 to 10 and 2 to 3 times the source material, respectively. Sediment from the plots was 247 to 936% richer in P than the soil from which it originated. Changes in soil pH, percentage organic C, and percentage total N following erosion were significantly correlated with cumulative soil loss (r values of 0.77, 0.59, and 0.71, respectively, n = 20). The data indicated that nutrient loss due to erosion is one of the major causes of soil fertility depletion of Kenyan soils.
Effect of Growing Rice on Nitrogen Mineralization in Flooded Soil
Authors: Kundu D. K. , Ladha J. K.
Published: July 2010

Only a smally fraction of the total organic N in soils is mineralized to the plant‐available forms during a cropping season. The mineralization process and the availability of mineral N in soils are affected by various environmental factors and management practices, including the presence of growing plants. This study was conducted to determine the effect of rice (Oryza sativa L.) on mineral N availability in a flooded Maahas soil (Typic Tropaquept). Available N, extractable NH4‐ and NO3‐N in soil plus N accumulated in plants, referred to as mineral N availability, was significantly higher in planted than in unplanted soils. Increased N availability may result from reduced loss or enhanced supply of mineral N in the presence of growing plants. Average increase in mineral N supply in freshly flooded soil was 31% due to growing of rice through the first month (0–30 d) and 62% through the second month (30–60 d) of its life cycle. In soils under prolonged flooding, average increases in the mineral N supply due to 2 mo rice growing during the vegetative stage (10–70 d) were 14 to 47% depending on the initial mineral N status of the soils. The gain in mineral N achieved by growing rice plants was higher in soils with a low than a high initial mineral N content. Rice genotypes differed in their ability to increase mineral N availability in flooded soils, and the stronger influences were associated with higher dry‐matter yields of the plants (shoot plus root). Soil N availability to wetland rice, especially in soils with low mineral N supply, thus may be considerably enhanced by selection of efficient genotypes.
Modeling Organic Solutes in Peatland Soils Using Acid Analogs
Authors: White Jeffrey R. , Shannon Robert D.
Published: July 2010

Dissolved organic carbon (DOC) dominates the chemistry of porewaters in many peatland soils. Yet modeling its contribution to acid‐base chemistry and to anionic charge has not been attempted for peatland porewaters. Using 3 yr of field data, we investigated the effectiveness of four analog organic acid models in describing the acid‐base chemistry of DOC in porewaters from two acidic (mean pH = 4.0–4.2), high‐DOC (mean DOC = 4.2–5.4 mmol CL−1) peatlands in southern Michigan. Although developed for dilute lake waters, we applied these models to porewaters of peatland soils. All sites exhibited a large deficit in anionic charge (mean = 229 µmolc L−1), which was strongly correlated with DOC (r2 = 0.82) and accounted for 70 to 80% of the total anionic charge. We, therefore, used charge balance to evaluate the models. The four models varied in effectiveness; variations in model organic anion accounted for between 50 and 84% of the variation in anion deficit. Representing organic acid anions with a triprotic acid analog model yielded the best results (r2 = 0.84, slope = 0.96), while a monoprotic acid model was least effective (r2 = 0.50, slope = 0.91). Mean charge densities of the DOC pool (44–52 εmolc mmol−1 DOC) varied little with differences in site characteristics (i.e., water levels and emergent plant communities) and were consistent with other studies. The triprotic acid model appears to be an effective analog for the acid‐base and ionic charge‐contributions of DOC in porewaters of acidic peatland soils.
Evaluation of Nonequilibrium Models for Predicting Atrazine Transport in Soils
Authors: Ma Liwang , Selim H. M.
Published: July 2010

Model evaluations are commonly undertaken using relatively sparce data, and the criteria for such evaluations are often not fully addressed. For instance, less effort has been made to evaluate models based on their prediction capability. In this study we compared several chemically and physically based nonequilibrium models for their predictions of atrazine (2‐chloro‐4‐ethylamino‐6‐isopropylamino‐s‐triazine) transport in a uniformly packed Sharkey soil (Vertic Haplaquept). The models compared were a nonlinear multireaction model (MRM), a second‐order model (SOM), and their coupled versions with a mobile‐immobile model (MIM), MRM‐MIM and SOM‐MIM. Model parameters were derived from kinetic batch experiments and were used to predict atrazine column breakthrough curves (BTCs) for different pore water velocities (0.52–2.17 cm h−1), aggregate sizes (0–2, 2–4, and 4–6 mm), flow interruptions (0, 4, and 16 d), and soil column lengths (10 and 15 cm). For each model, nine formulations reflecting various adsorption sites were evaluated. Based on root mean squared errors and sums of squared errors, considerable improvement in atrazine BTC predictions was achieved using SOM compared with MRM. Furthermore, SOM was more sensitive to different model formulations than MRM. Coupled MRM‐MIM did not improve atrazine BTC prediction. Although goodness of model predictions varied with individual columns, we concluded that SOM‐MIM with four parameters and a time‐dependent mass transfer coefficient (α) best described atrazine retention and transport in Sharkey soil.
Chemical Characterization of Soils of a Tropical Humid Forest Zone: A Methodology
Authors: Menzies N. W. , Gillman G. P.
Published: July 2010

A methodology, based on a combination of routinely performed analyses and investigation of fundamental charge and anion sorption properties, was used to characterize the soils of the humid forest zone of Cameroon. In general, the soils have about 2 cmol kg−1 permanent negative charge, with about 1 cmol kg−1 from variable‐charge sources at current soil pH values. Furthermore, they are improverished with respect to Ca, Mg, and K, while Al frequently dominates the exchange complex. Thus, the ability of these soils to retain base cations is more limited than is suggested by the cation‐exchange capacity (CEC). Therefore we propose the concept of a degradation index (DI) defined as: DI = 100(CEC5.5 − sum of basic cations)/CEC5.5, where CEC5.5 is the CEC measured at pH 5.5. This index encompasses degradation a soil may have experienced from natural or man‐made causes. Extractable PO4 concentrations are considered very low and the soils have a moderate to high capacity to fix added PO4. Surface soil SO4 concentrations are considered marginal to deficient for plant growth, though adequate reserves of SO4 are held in the subsoil by SO4 sorption. The approach used demonstrated that the five morphologically different soil profile classes identified in the zone have similar chemical characteristics. Thus, the results of experimentation conducted on one of the soil profile classes will be applicable throughout the zone. Furthermore, the approach has provided a means of identifying comparable soil types in other parts of the world and will guide technology transfer. The analytical methods used in this study are relatively simple and require no specialized equipment, and are therefore within the capabilities of many laboratories in the developing world.
Consequences of Changes in Particulate Organic Carbon in Vertisols under Pasture and Cropping
Authors: Chan K. Y.
Published: July 2010

Soil organic C levels are generally low in Vertisols and, under cropping, they decline rapidly, resulting in N deficiency limiting crop production. The pools of soil organic C lost during cropping of native pasture soils need to be identified so that management strategies can be developed to reverse this loss. Changes in particulate (>53‐µm) organic carbon (POC) under different histories of cropping and pasture were monitored in two Vertisols with contrasting C and N status in the semiarid subtropical area of Australia and the changes were related to N availability and aggregate stability. Results indicate POC was the form of organic C that was preferentially lost under cropping; on average it made up 70% of the difference in organic C between native pasture and cropped soils in the 0‐ to 5‐cm layer. It was also the form of organic C regained when cropped soils were returned to pasture. Particulate organic C was significantly related to N mineralization, indicating the importance of this fraction to N availability in the Vertisols. Particulate organic C was more significantly related to the water stability of the >250‐µm size fraction than that of the <50‐µm fraction and thus indicates the importance of POC in maintaining the stability of the macroaggregates of these soils. Regression analyses indicated that, for Vertisols, POC is equally sensitive to changes in these important soil properties under different management practices as TOC. Results also demonstrated the effectiveness of pasture in restoring POC and therefore its usefulness in maintaining and restoring the fertility of Vertisols.
Iron Oxide Sink Method for Extracting Soil Phosphorus: Paper Preparation and Use
Authors: Myers R. G. , Pierzynski G. M. , Thien S. J.
Published: July 2010

Since the FeO‐sink method to estimate plant‐available P in soils was first introduced in 1983, modifications have continued to improve the procedure. Recently FeO‐coated paper circles (5.5‐cm diam.) were used in place of the traditional 2 by 10 cm strips. The objective of this study was to more clearly describe the preparation and use of the FeO‐coated circles. We used Whatman no. 50 hardened filter paper (2.7‐µm pore openings). Standard methods for FeCl3 treatment of the papers were used. After the papers had been air dried, one paper was secured between two 10 by 10 cm fiberglass screens. By simultaneously immersing stacks of 25 of these paper‐screen units in 2.7 M NH4OH for a 30‐s immersion time, 100 papers could be uniformly coated with FeO in about 10 min. Immersion times of 30, 60, and 180 s in NH4OH solution showed that papers immersed for 30 s in NH4OH solution extracted P as effectively as those immersed for longer times. One 5.5‐cm FeO circle was as effective as three circles in extracting P from most soils. A desorption model for soil P was used that can relate soil‐test P to desorption kinetics. In soils with maximum reversible P ranging from 25.1 to 106.5 mg kg−1, desorption constants ranged from 0.141 to 0.232 h−1 with a mean of 0.191 h−1. Low‐P soils released P slower than did high‐P soils. Such information may be of interest in assessing P availability in soils.
Estimating Long‐Term Drainage at a Regional Scale Using a Deterministic Model
Authors: Ould Mohamed S. , Bruand A. , Raison L. , Bruckler L. , Bertuzzi P. , Guillet B.
Published: July 2010

This study discusses the use of a deterministic model, at a regional scale, when the soils have small heterogeneity. Long‐term soil drainage was simulated on 3500 ha in Beauce (France), taking into account variation in soil thickness, agricultural practices, and climate characteristics for a 32‐yr period. The model was based on a numerical solution of Richards' equation, using a finite element method. Hydraulic properties were determined at four sites that are 4 km apart. The results show a small variability of hydraulic parameters, with coefficient of variation (CV) ranges from 1 to 16% for the α parameter, the empirical constant n, the saturated water content θs, and the residual water content θr, and from 60 to 75% for the saturated hydraulic conductivity Ks. Hence, the spatial and temporal variability of drainage are mainly related to soil thickness, agricultural practices, and climate characteristics. Assigning small spatial variability to hydraulic properties, we analyzed the role of soil thickness and initial water content (IWC), induced by agricultural practices and climate characteristics, on drainage prediction. Five IWC were defined: 100, 90, 80, 70, and 55% of field capacity (FC). Four soil thicknesses were studied: 30, 60, 100, and 150 cm. The IWC appeared as a significant factor in drainage prediction for 60% of the 32 yr, regardless of soil thickness. Soil thickness became the most significant factor (20% of the 32 yr) for low IWC. The model was spatially extrapolated by combining the calculated drainage (which varied with soil thickness and IWC) and the spatial distribution of crops and soil thickness classes. Simulated drainage was closely related (r = 0.90) to groundwater recharge during the 32 yr, indicating that the model could be applied to other areas with appropriate parameters.
On The History of the Ellipse Equation for Soil Drainage
Authors: van der Ploeg R. R. , Marquardt Maria , Kirkham Don
Published: July 2010

Colding from Denmark is considered to be the first who derived (in 1872) a drainage equation based on Dupuit‐Darcy theory. Because of the shape of the water table between the drains that is calculated, this equation is frequently called the ellipse equation. It has been believed that Rothe in 1924 in Germany, Kozeny in 1932 in Austria, Hooghoudt in 1937 in the Netherlands, and Aronovici and Donnan in 1946 in the USA independently developed the same equation. Such a coincidence, although possible, appears to be unlikely. Therefore, we decided to study the older European literature that formerly was not always easily accessible. The literature review shows that both Kozeny and Hooghoudt referred to Rothe in previous publications and hence were familiar with Rothe's work. Although less obvious, it appears that also Rothe, as well as Aronovici and Donnan, had knowledge of earlier work on the subject. Hence, it seems that the current view about a multiple, independent derivation of the Colding equation needs a revision. However, since each of the later researchers extended or modified the original Colding equation, all deserve some credit, especially Hooghoudt, who appears to be the first to use Dupuit‐Darcy theory to derive a ditch drainage equation.