bluuebunny/crossref_metadata_2025 huggingface dataset:
Automated Chamber Measurements of Methane and Nitrous Oxide Flux in a Flooded Rice Soil: I. Residue, Nitrogen, and Water Management
Authors: Bronson K. F. , Neue H.‐U. , Abao E. B. , Singh U.
Published: July 2010

Methane and N2O are gases that are several times more radiatively active than CO2. It is well known that flooded rice (Oryza sativa L.) soils are a globally important source of atmospheric CH4. Mitigation strategies for CH4 flux, such as mid‐season drainage, might have the opposite effect on N2O emissions. An automated chamber system at the International Rice Research Institute in the Philippines measured CH4 and N2O fluxes from flooded rice and fallow rice fields essentially 24 h a day between December 1992 and April 1994. This period included two irrigated dry rice‐growing seasons (DS) and one wet rice‐growing season (WS). Nitrous oxide fluxes were generally barely detectable during the growing seasons, but small peaks (maximum 3.5 mg N2O‐N m‐2 d‐1) appeared after N fertilizer applications. Methane fluxes, on the other hand, were evident throughout the rice‐growing seasons. Organic matter additions as straw (5.5 t ha‐1, dry) or green manure (GM; Sesbania rostrata L.; 12 t ha‐1, wet) stimulated CH4 flux severalfold. Seasonal CH4 flux with ammonium sulfate (AS) was one‐fourth to one‐third the flux with urea. During the DS, however, the seasonal N2O flux was 2.5 times higher with AS than with urea. Mid‐season drainage (2‐wk duration) at either mid‐tillering or panicle initiation was very successful in suppressing CH4 flux up to 60%. However, N2O flux increased sharply during the drainage period at mid‐tillering until reflooding, when it dropped back to near zero.
Automated Chamber Measurements of Methane and Nitrous Oxide Flux in a Flooded Rice Soil: II. Fallow Period Emissions
Authors: Bronson K. F. , Neue H.‐U. , Abao E. B. , Singh U.
Published: July 2010

Methane and N2O are radiatively important gases that are emitted from waterlogged soils. Automated chamber measurements of CH4 and N2O fluxes were carried out at the International Rice Research Institute in the Philippines in flooded rice (Oryza sativa L.) and fallow rice fields 24 h a day between December 1992 and April 1994. This period included three 5‐ to 11‐wk rainfed fallow periods. During the first two fallows, the soil was generally aerobic, and moderate amounts of NO3 accumulated (7–20 kg NO3‐N ha‐1). Moderately high, continuous N2O fluxes were evident during these two fallow periods. This N2O was apparently emitted during nitrification of mineralized organic N in the topsoil and possibly from denitrification in the wet subsoil. Nitrous oxide fluxes were highest (up to 80 mg N2O‐N m‐2 d‐1) immediately after rainfalls >20 mm, and following the establishment of flooding for rice at the end of the fallows. Acetylene inhibition in intact cores at these times showed that more N2 was produced than N2O. Denitrification of accumulated NO3 was therefore occurring after the wetting events. Methane emissions were generally absent during the fallow periods. Two exceptions were immediately after rice harvest and 1 to 2 wk after the establishment of the permanent flood. Following flooding and green manure (GM; Sesbania rostrata L.) incorporation, CH4 fluxes appeared within 7 d. During the third fallow period, which was unusually wet, no NO3 accumulated in the soil. Nitrous oxide emissions were not significant, and low levels of CH4 fluxes persisted throughout this fallow period. This study demonstrates that rice soils in the fallow periods can be significant sources of N2O, and during wet fallow seasons, important sources of CH4 as well.
Soil Solution Electrical Conductivity Measurements under Transient Conditions Using Time Domain Reflectometry
Authors: Persson Magnus
Published: July 2010

Several studies have shown that time domain reflectometry (TDR) can be used to measure water content and solute concentration during steady‐state experiments with constant soil water content. Few studies have been carried out to identify the relationship between bulk electrical conductivity (σa) and soil solution electrical conductivity (σw) to be used for transient conditions with varying water content. In this study, a single TDR probe was calibrated and found to accurately measure water content and σa. The σa measurements were fitted to σw using several previously published models. The accuracy of the σa and σw measurements increased with σa; however, the error in the σw estimation was <10% for σa higher than 5.7 mS m−1. The calibration was verified in a large sand column; σa measurements were fitted to σw measured in soil solution extracted with suction samplers. The calibration was also used in a transient solute transport experiment. If not accounted for, the temperature effects of the water content measurement introduce significant errors in the σw estimations.
Spatial Series Analysis of Horizontal Cores to Characterize Tracer Patterns in Soil Profiles
Authors: Powers W. L. , Shea P. J. , Troyer W. , Verhoeff R. L. , Parkhurst A. M.
Published: July 2010

Surface topography, plant canopy, plant root distribution, and soil preferential flow paths cause variability in downward movement of agrichemicals in soil. To better understand this variability, tracers are often applied to the soil surface and their signatures (concentration patterns) are examined. To characterize Br− signatures within a Crete silt loam soil (fine, montmorillonitic, mesic Pachic Argiustoll), potassium bromide was applied to the soil surface at 200 kg ha−1. Four sets (two sets in June and two sets in August) of four 6.1‐m horizontal cores were taken in a vertical plane, perpendicular to corn plant rows at 20‐, 40‐, 80‐, and 120‐cm depths. Each 6.1‐m core was divided into 120 5.1‐cm sections and analyzed for Br−. Spatial series analyses were used to identify deterministic and combined deterministic‐stochastic models that described the cyclic nature of Br− concentrations at each depth. Periods for these sinusoidal cycles generally corresponded to the distance between plant rows or the distance between the wheel tracks of field equipment.
Immobile Water Content and Mass Exchange Coefficient of a Field Soil
Authors: Casey F. X. M. , Horton R. , Logsdon S. D. , Jaynes D. B.
Published: July 2010

Determining the preferential flow characteristics of a soil is important because agrichemicals can contaminate groundwater via preferential flow pathways. A model that predicts solute transport due to preferential flow is the mobile‐immobile solute transport model, which partitions the total water content (θ, m3 m−3) into a mobile fraction (θm) and an immobile fraction (θim). Recently, an in situ method was proposed for determining the mobile‐immobile model parameters of θim and mass exchange coefficient (α) between the fractions by using a tension infiltrometer to apply a series of four fluorobenzoate tracers. The objective of this study was to test the in situ technique at 47 sites along a transect in a ridge‐till corn (Zea mays L.) field of Nicollet soil (fine‐loamy, mixed, mesic Aquic Hapludoll). The immobile fraction (θim /θ) ranged from 0.394 to 0.952 with a median of 0.622. The mass exchange coefficient ranged from 0.000237 to 0.00481 min−1 with a median of 0.00123 min−1. These values are similar in magnitude and range to values reported by other investigators, and they follow the same relationships. The values of θim/θ and α along the transect indicated no obvious spatial trends or spatial correlations. Significant linear correlations did exist between α and soil water flux, α and θim, and θ and θim.
Soil Salinization Processes in Rice Irrigation Schemes in the Senegal River Delta
Authors: Ceuppens J. , Wopereis M. C. S. , Miézan K. M.
Published: July 2010

Soil salinization constitutes a major threat to irrigated agriculture (mainly rice, Oryza sativa L.) in the Senegal River Delta. It is generally hypothesized that salinization is caused by (i) capillary rise from a saline water table and (ii) concentration of salts in the field due to lack of adequate drainage facilities. The impact of field water management and rice cropping intensity on salinization in the Delta was determined using an electromagnetic conductivity meter (Geonics EM38). More than 4000 measurements were made in 40 rice fields on a typical heavy clay soil (Vertic Xerofluvent). Thirty EM38 measurements per field (0.25 ha) estimated average field soil salinity with a relative error of 20%. A multiple linear regression model based on EM38 readings explained 60 to 75% of the variability in conductivity of 1:5 saturation extracts at 0‐ to 5‐, 10‐ to 15‐, and 30‐ to 35‐cm depths. Higher cropping intensity limited upward salt transport from the water table. Average horizontal and vertical EM38 measurements increased in the following order: two rice crops per year with drainage: 0.73 and 0.98 dS m−1; one rice crop per year with drainage: 1.26 and 1.76 dS m−1; one rice crop per year without drainage: 2.23 and 2.98 dS m−1; and abandoned fields: 4.77 and 4.29 dS m−1. Results indicate a beneficial effect of flooded rice on salinity for this type of heavy clay soil. Irrigation development in the area needs to be accompanied by monitoring of water table depth.
Changes in Soil Organic Matter under Different Tillage and Rotation: Mathematical Modeling in ecosys
Authors: Grant R. F.
Published: July 2010

Reductions in both tillage and fallowing may increase soil organic C. The effects of tillage and fallowing on soil C could be estimated under site‐ and climate‐specific conditions by mathematical modeling. The simulation model ecosys predicts changes in soil C by calculating C added through fixation by plant communities and C lost through oxidation by microbial communities. During a 14‐yr simulation of a field experiment in a semiarid environment, ecosys predicted that 18 and 20 g m−2 yr−1 more C would be sequestered in the upper 0.15 m of soil in continuous wheat (W‐W; triticum aestivum L.) than in wheat‐fallow (W‐F) under conventional tillage (CT) and no tillage (NT), respectively. During the same period, ecosys predicted that 14 and 12 g m−2 yr−1 more C would be sequestered in the upper 0.15 m of soil under NT than under CT in W‐W and W‐F, respectively. These increases were compared with those measured from the field experiment of 9 and 19 g m−2 yr−1 in W‐W than W‐F under CT and NT, respectively, and of 13 and 4.5 g m−2 yr−1 under NT than CT in W‐W and W‐F, respectively. The sensitivity of simulated changes in soil C to doubled atmospheric CO2 and a 3 or 6°C increase in air temperature was then tested. Similarity in the direction and magnitude of simulated and measured changes in soil C lend initial support to model hypotheses. However, there are some issues concerning the modeling and measurement of soil C that require further clarification.
Computer‐Controlled Variable Intensity Rain Simulator
Authors: Lascano R. J. , Vorheis J. T. , Baumhardt R. L. , Salisbury D. R.
Published: July 2010

Natural rainstorms have dynamic intensities; thus, a large portion of total precipitation may be received during a short time. Natural rainstorms also have both variable drop sizes and kinetic energy. Accurate studies of sediment transport and infiltration require realistic duplication of natural rain kinetic energy and intensities. Rain simulator designs must be capable of producing dynamic multiple intensities and corresponding multiple drop sizes to simulate variable kinetic energies. Many simulators are capable of producing rain with multiple and, in some cases, dynamic intensities, but a simulator that can vary both the intensity and kinetic energy is needed. Our objective was to design and build a rain simulator for field use that realistically duplicated natural rain storms. A computer‐controlled solenoid‐valve multiple nozzle type rain simulator was developed. The computer not only controls solenoid valve activation of the rain simulator during operation but also measures runoff and records rain application and runoff data to a log file for processing. The simulator has control software that controls and monitors all processes. The rain simulator is capable of uniformly applying water (coefficient of uniformity = 84–94%) with a dynamically changing intensity of up to 200 mm h−1 and kinetic energies ranging from 15.0 to 23.5 J m−2 mm−1. Uniformity of application was achieved by oscillating the nozzles in two directions. Field operation of the simulator is enhanced by its portability. The measurement area is easily modified using in‐line handrail connectors to change support frame dimensions, thus accommodating infiltration and chemical or sediment transport studies.
Prediction of the Saturated Hydraulic Conductivity‐Porosity Dependence Using Fractals
Authors: Giménez D. , Allmaras R. R. , Huggins D. R. , Nater E. A.
Published: July 2010

Fractal theory has been used to quantify morphological properties of pore systems in soil, but predictive capabilities of the derived fractal dimensions have remained largely untested. The objective of this study was to use morphologically derived fractal dimensions to predict an exponent N in a power law relation between saturated hydraulic conductivity, Ksat, and porosity. A Kozeny‐Carman equation was used to derive N as a function of two fractal dimensions (pore volume, Dv, and pore surface roughness, Ds) and a connectivity parameter, α. The α parameter was used as a matching factor between fitted and calculated N values. Values of Dy and Ds characterizing pores in both undisturbed and packed soil were obtained from images with pixel sizes of 0.06 and 0.29 mm. Porosity was measured on the 0.29‐mm pixel images, while Ksat was measured in undisturbed cores and packed soil columns. Also, published data on porosity, Ksat, and Dv and Ds from dye‐stained patterns of four undisturbed soils were used. Lower coefficients of variation and lower absolute values of α were obtained with fractal dimensions from the 0.06‐mm pixel images. Values of α were related to parameters from probability distributions of hydraulic radii as calculated from the 0.06‐mm pixel images, and to the connectivity of pores as inferred from dye‐stained patterns. Fractal characterization of pore structure proved useful for predicting N, but predictions would probably be improved by considering only flow‐active pores in the calculation of a fractal dimension. Methods to obtain such fractal dimensions were suggested.
Phosphorus Sorption Characteristics of Bh and Bt Horizons from Sandy Coastal Plain Soils
Authors: Zhou M. , Rhue R. D. , Harris W. G.
Published: July 2010

Retention of land‐applied P is an environmental concern in sandy Coastal Plain regions. Compositional differences between Spodosol Bh and Ultisol Bt horizons are likely to have major consequences for P retention. The purpose of this study was to compare P sorption‐desorption characteristics of Bh and Bt horizons using selective chelation to evaluate the effects of organically complexed Fe and Al in Bh samples. The Bh and Bt samples sorbed comparable amounts of P but Bh samples desorbed a higher proportion of their adsorbed P. After alkaline acetylacetone extraction of metal‐organic complexes, Bh samples lost most of their P adsorption capacity, but their desorption behavior was more like that of samples whose clay fractions were dominated by inorganic metal oxides. Aluminum‐organic matter complexes appear to be responsible for the high P adsorption capacity of Bh horizons as well as for the tendency of these horizons to release P more readily at low solution P concentrations.
Accumulation and Redistribution of Fertilizer Nitrogen‐15 in Soft Red Winter Wheat
Authors: Bashir R. , Norman R. J. , Bacon R. K. , Wells B. R.
Published: July 2010

Information is lacking on N accumulation in soft red winter wheat (Triticum aestivum L.) and the fate of residual fertilizer N in soil. The objectives of this study were to determine the uptake and redistribution of fertilizer and total N within the wheat plant as well as the amount and distribution of fertilizer N in the soil profile at wheat maturation. Urea‐15N was applied at the rate of 112 kg N ha−1 at tillering. Plant samples were taken at six growth stages and partitioned into top and bottom leaf blades, stem + leaf sheaths, and spikes, and analyzed for total and fertilizer N accumulation. Soil samples were taken at wheat harvest and analyzed for total N and fertilizer N recovery in inorganic, organic, and total N fractions. Nitrogen accumulation in the wheat plants increased until flag leaf emergence and then displayed no significant change during later growth stages or at maturity. Leaf blades and stem + leaf sheaths attained maximum N accumulation at flag leaf emergence and heading, respectively. Spikes continued to accumulate N from heading until maturity. Nitrogen loss from the leaf blades and stem + leaf sheaths exceeded N gain in the spikes. A maximum fertilizer N accumulation of 74.4% in the wheat plants was found by the flag leaf stage and then the fertilizer N accumulation steadily declined to only 46.7% by maturity. Fertilizer N accumulation for plant parts was similar to total N accumulation; however, significant fertilizer N losses occurred after flag leaf emergence. Fertilizer N recovered in the 0‐ to 100‐cm soil layer following harvest was only 11.6% of that applied and was mainly concentrated in the organic fraction of the top 10 cm of soil.
Isotopically Exchangeable Phosphate in Size Fractionated and Unfractionated Soils
Authors: Sinaj S. , Frossard E. , Fardeau J. C.
Published: July 2010

The acquisition of precise information on soil phosphate availability is a prerequisite to its sustainable management in agroecosystems. The objective of this study was to assess the exchangeability of phosphate in the particle‐size fractions (2000–50, 50–2, and <2 µm) of seven soil samples. Samples were completely dispersed with a Nasaturated cation‐exchange resin, and phosphate exchangeability was assessed by the isotopic exchange kinetic method on each particle‐size fraction and on the unfractionated samples. Isotopically exchangeable phosphate was then divided into five pools containing phosphate that is exchangeable within different time frames. In the 2000‐ to 50‐µm fraction, an average of 91.8% of the acid‐extractable (1.0 M H2SO4) inorganic phosphate could not be exchanged within 1 yr while 5.5% of it was exchangeable within 1 min. These results are consistent with the presence in this fraction of slowly exchangeable phosphate in the form of condensed Ca phosphates and rapidly exchangeable phosphate associated with plant debris. In the <2‐µm fraction, a mean of 44.8% of the acid‐extractable phosphate was exchangeable within 1 yr. This confirms that this fraction is the main source of phosphate available to plants, and the main sink for water‐soluble phosphate. A comparison between the sum of the single pools assessed in fractions with the phosphate pools of unfractionated soils showed that, on dispersion, up to 30% of the acid‐extractable phosphate was transferred from slowly to more rapidly exchangeable phosphate pools. This result strongly suggests that unfractionated soils contain phosphate that is physically protected from isotopic exchange, probably within water‐stable aggregates.
Pedologic Assessment of Radionuclide Distributions: Use of a Radio‐Pedogenic Index
Authors: Evans C. V. , Morton L. S. , Harbottle G.
Published: July 2010

Naturally occurring radionuclides in soil profiles can provide a useful pedological tool, if their distribution can be calibrated to pedogenesis. As a first step toward such calibration, this study presents a radio‐pedogenic index that compares differences between sola and parent material with respect to distributions and equilibrium relationships of 214Pb and 228Ac activity. The ratios 228Ac/214Pb were used to indicate concurrent secular equilibrium within the parent 232Th and 238U decay series. Among the 12 Typic Haplorthod pedons examined, equilibrium prevailed in C horizons, but not in A, E, Bs, and BC horizons. The C horizon levels of both 214Pb and 228Ac were significantly (P < 0.05) higher than those of E horizons, and considerably higher than those of Bs and BC horizons (P = 0.052 and 0.058, respectively). Nuclide deficiencies were not significantly correlated to podzolization indicators such as the optical density of the oxalate extract (ODOE), melanization, or rubification. Relationships of nuclide deficiencies to KCl pH were strong (r2 > 0.8, P < 0.0001), however. The 228Ac deficiencies were larger than 214Pb deficiencies and more strongly differentiated, particularly in E horizons. Profile distribution of 214Pb was relatively homogenous, compared with that of 228Ac. Analytical methodology ensured equilibrium between 228Ac and 228Ra and between 214Pb and 226Ra. Therefore, deficiencies can be interpreted as a pedogenically directed loss of Ra. Distribution differences, however, are probably due to radiogenic factors, such as half‐life differences between the two Ra isotopes or chemistry of decay‐chain precursors.
Crop‐Pasture Rotation for Sustaining the Quality and Productivity of a Typic Argiudoll
Authors: Studdert Guillermo A. , Echeverría Hernán E. , Casanovas Elda M.
Published: July 2010

Inclusion of pastures in a rotation may reverse the effects of conventional cropping and tillage on soil degradation. We hypothesized that crop‐pasture rotations could be defined with a minimum pasture component and a maximum conventional cropping component to accomplish sustainable management. Soil organic carbon (SOC), light‐fraction carbon (LFC), microbial biomass nitrogen (MBN), and aggregate stability index (ASI) were measured in the surface 0 to 0.15 m of a fine, mixed, thermic Typic Argiudoll with 2% slope under a long‐term crop rotation experiment at Balcarce, Argentina. Treatments were continuous cropping and crop‐pasture (50:50 and 75:25) rotations. All seedbeds were prepared with conventional tillage. All soil quality indicators decreased with cropping and increased with pasture. Data for each variable were fitted to an exponential model to describe their variation in time. Soil organic C decreased 4.4 g kg−1 in 6 to 7 yr under cropping and rose to the original level (37.2 g kg−1) after 3 to 4 yr under pasture. Light‐fraction C, MBN, and ASI fell 0.9 g kg−1, 39.0 mg kg−1, and 43.9, respectively (97–100% of the estimated decline), after 7 yr under cropping, while they recovered to the values at the beginning of the cropping period (1.8 g kg−1, 99.5 mg kg−1, and 76.7, respectively) in a few years under pasture. Rotations including a maximum of 7 yr of conventional cropping alternated with a minimum of 3 yr of pasture would maintain soil properties within acceptable limits and meet the goals of sustainable agriculture under conditions similar to this experiment.
Landscape Effects on Desiccation Cracking in an Aqualf
Authors: Baer J. U. , Anderson S. H.
Published: July 2010

The rapid transport of water and solute through desiccation soil cracks can lead to crop water and nutrient stress as well as ground and surface water contamination. This study hypothesized that desiccation cracking varies with landscape positions. Cracking was quantified at three landscape positions in a Mexico silt loam (fine, montmorillonitic, mesic Mollic Endoaqualf). The elevation changes of seven disks placed at 15‐cm depth increments in the profile were measured to determine changes in soil layer thickness. Crack volume in the upper 90‐cm profile was determined from thickness changes of the six soil layers. Disk elevation and soil profile water content measurements were made weekly during the crop season (April–November) in 1992 and 1994 on summit, backslope, and footslope positions. During both years, the backslope position profile developed the largest amount of crack volume (4.3 cm3 cm−2 in 1992, 4.8 cm3 cm−2 in 1994) and had the least amount of water depletion (10.0 cm3 cm−2 in 1992, 9.7 cm3 cm−2 in 1994). At the backslope position, a thin, high‐clay‐content Ap horizon and a shallow, clay‐rich Bt horizon (both high in smectite clay) lead to these developments. Differences in shrinkage characteristics of aggregates from Ap, Bt, and C horizons across landscape positions were related to textural and mineralogical differences. Bt horizon clay contents as high as 64.4%, comprised of up to 75% smectite, produced the greatest volume reductions (41%) of aggregates from the Bt horizon compared with volume reductions of aggregates from the Ap and C horizons. Soil texture and mineralogy influenced the magnitude of desiccation soil cracking and water depletion.
Nitrogen‐15 Tracers Combined with Tensio‐Neutronic Method to Estimate the Nitrogen Balance of Irrigated Maize
Authors: Normand Béatrice , Vachaud Georges , Recous Sylvie , Kengni Lucas , Garino Bernard
Published: July 2010

In response to a very high increase of groundwater NO−3 pollution, largely connected with intensive agricultural practices, a long‐term experimentation has been set up close to Grenoble, France, with the following aims: first to characterize the response of maize (Zea mays L.), a predominant crop in the area, to fertilization, and second to quantify the N balance during and after the crop cycle. This study relates to results concerning the second issue; experiments were conducted on irrigated maize in 1991, 1992, and 1993 on the Experimental Farm at La Côte Saint‐André, France, in the heart of one of the most important agricultural zones between the French Alps and the Rhone Valley. The dynamics of soil and fertilizer N (NO−3 transport and N balance during cropping and intercropping periods) were continuously monitored using 15N isotopic tracing and the tensio‐neutronic method (i.e., continual measurement of soil water balance using a neutron moisture meter and tensiometers), together with porous suction cups installed at 0.3‐, 0.5‐, and 0.8‐m depths. Water drainage and leaching of NO−3‐N, with a partition between that derived from fertilizer and that produced by soil mineralization, were thus obtained during and after the crop cycle. The balance of the labeled fertilizer at harvest was also determined by conventional soil coring and plant sampling. The two methods were used successfully during the 3 yr in which there were different climatic conditions and different fertilizer application rates. The results show that the traditional fertilizer input in the area (260 kg N ha−1) could be reduced nearly 30% without any substantial loss in grain yield but with a considerable reduction of nonpoint source pollution due to NO−3 leaching. It is also shown that with the combined use of the two methods, it is possible to characterize separately fertilizer‐N uptake, fertilizer‐N leaching, and N immobilization and to estimate N‐gaseous losses.
Weathering of Mt. St. Helens Tephra under a Cryic‐Udic Climatic Regime
Authors: Dahlgren R. A. , Dragoo J. P. , Ugolini F. C.
Published: July 2010

The initial stages of chemical weathering in tephra were examined during a 10‐yr period under field conditions in a cryic‐udic climatic regime. Unleached tephra from the 1980 eruption of Mt. St. Helens was applied to the surface of soils in 5‐ and 15‐cm depths to simulate natural tephra deposition on subalpine forest ecosystems. Solid‐phase tephra samples were collected 10 yr after addition and compared chemically and mineralogically to the original, unweathered tephra. The solid‐phase pH of the tephra decreased by approximately one unit, with the 5‐cm treatment being ≈0.2 units lower than the 15‐cm treatment. The accumulation of Al and Fe as alteration products in the weathered tephra was detected by selective dissolution analyses. Aluminum accumulated as Al‐humus complexes, hydroxy‐Al polymers in the interlayer of 2:1 layer silicates, and possibly as hydroxy‐Al precipitates external to the interlayer space; Fe accumulated primarily as a noncrystalline oxyhydroxide (e.g., ferrihydrite) and Fe‐humus complexes. The increase in the active Fe and Al fraction (i.e., oxalate extractable Fe and Al) contributes to a sevenfold increase in phosphate retention. Hydroxy‐Al interlayering of detrital 2:1 layer silicates present in the tephra was suggested from x‐ray diffraction and selective dissolution analyses. There was no evidence of opaline silica, allophane, or imogolite forming in the tephra. Preferential incorporation of Al into Al‐humus complexes and hydroxy‐Al interlayers of 2:1 layer silicates appears to inhibit formation of allophane and imogolite during the initial stages of weathering.
Air Flow Measurements to Describe Field Variation in Porosity and Permeability of Soil Macropores
Authors: Granovsky A. V. , McCoy E. L.
Published: July 2010

Limited data are available on spatial variation in hydrologic behavior of soil macropores. This study examined field variatiou of soil water release and permeability in the pressure head range that influences macropores. One hundred twenty‐seven undisturbed soil cores were collected from a no‐tillage field containing a Lobdell silt loam (fine‐loamy, mixed, mesic Fluvaquentic Eutrochrept) soil. An air flow device that eliminates the gravitational gradient was used to measure water release, θ(h), and air permeability, ka(h), in the pressure head, h, range of 0 to ‐240 mm. Mean air‐filled porosity, εa, of 0.044 m3 m‐3 and geometric mean ka equal to 1.3 µm2 at h = 0 indicated that macroporosity in a majority of these cores failed to exhibit negative air‐entry pressure heads. Generalized scaling of water release curves and air permeabilities using water contents expressed as degree of capillary saturation ([θ(h) + εa(0)]/θs) yielded considerable scatter reduction, implying a high degree of pore geometry and flow similarity for this soil. The spherical semivariogram model of water release scaling factors, α(h)r, had a range of 14.1 m, whereas the linear semivariogram of air permeability scaling factors, α(ka)r, had a range of 21.1 m. Further, sill values for α(ka)r were 7.6 times greater than for α(h)r. The poor correlation, dissimilar statistical distributions, and dissimilar spatial dependence of scaling factors suggested that characteristic lengths related to total macroporosity and fluid‐conducting macroporosity were substantially different.
Real‐time Soil Water Dynamics Using Multisensor Capacitance Probes: Laboratory Calibration
Authors: Paltineanu I. C. , Starr J. L.
Published: July 2010

There is a continued need for better methods to perform accurate, real‐time, nearly continuous soil water measurements at specific depth intervals, with minimal soil disturbance, and covering field‐scale areas. The objectives of this research were to assess the characteristics of a newly developed multisensor capacitance probe and to calibrate the sensors against a Mattapex silt loam (fine‐silty, mixed, mesic Aquic Hapludult) soil. The soil was uniformly packed in small increments, with the aid of a large hydraulic press, in a wooden box (35.5 by 35.5 cm, 40.5 cm deep). Probe installation in the boxed soil mimicked that required for correct field installation. A highly significant (r2 = 0.992 for n = 15, and RMSE = 0.009 cm3 cm‐3 water), nonlinear (θv = 0.490 SF2.1674) relationship was found between the soil volumetric water content (θv, cm3 cm‐3) and the scaled frequency [SF = (Fa − Fs)(Fa − Fw)‐1]. The SF represents the ratio of individual sensor's frequency (inside PVC pipe) response in soil (Fs) compared with sensor responses in air (Fa) and in nonsaline water (Fw) at room temperature (≈22°C). Axial and radial sensitivity studies showed that these capacitance sensors give integrated readings over a primary depth interval of 10 cm and a radial capacitance fringe within 10 cm of the wall of the access pipe. Temperature effects of air and water were measured, and the calculated effects on assessment of θv were less than the RMSE for a temperature range of 10 to 30°C. Our calibration studies indicate that these multisensor capacitance probes can be used to accurately measure volumetric soil water contents in a soil water monitoring system.
Infiltration Models for the Disk Infiltrometer
Authors: Zhang Renduo
Published: July 2010

The disk infiltrometer is becoming a widely used device for determining soil hydraulic properties in the field. To describe the infiltration process from the disk infiltrometer, a two‐term equation was developed. The first and second terms characterize water flow out of the disk due to capillary forces and gravity, respectively. The parameter of the first term in the equation was a product of a nondimensional coefficient and soil sorptivity. The parameter of the second term was a product of another nondimensional coefficient and soil hydraulic conductivity. Time‐dependent relationships were obtained from numerical models to determine the coefficients based on physical parameters of the infiltrometer, such as the radius and tension used, and soil retention parameters. The cumulative infiltration and steady‐state infiltration rate calculated using the two‐term equation were in excellent agreement with numerically simulated results for various soils, and radii and tensions of disk infiltrometers. Absolute relative errors of predicted infiltration were <5% for soils ranging from clay to sand. The infiltration solution is an accurate, yet simple approach to estimate fluxes from an axisymmetric source. It can also be applied to estimate soil sorptivity and hydraulic conductivity using inverse methods.
UNSATCHEM: Unsaturated Water and Solute Transport Model with Equilibrium and Kinetic Chemistry
Authors: Suarez D. L. , Šimůnek J.
Published: July 2010

Numerous models have been developed for predicting major ion chemistry in the soil zone and in recharge to groundwater. Soils that contain CaCO3 are prevalent in arid and semiarid regions, as well as in humid and temperate regions that have been glaciated or contain carbonate bedrock. Under these conditions, carbonate‐solution reactions and ion exchange are the dominant chemical processes. In this model we couple one‐dimensional unsaturated water and solute transport with a major ion chemistry routine and plant water uptake. The model has several unique features, including expressions relating reductions in hydraulic conductivity to chemical factors, prediction of CO2 partial pressure in the root zone based on a CO2 production‐multiphase transport submodel, kinetic expressions for silicate weathering, calcite precipitation‐dissolution, and dolomite dissolution, representation of B adsorption using the constant capacitance model, a new method for predicting cation‐exchange selectivity, the option to use Pitzer ion interaction expressions for high ionic strength, and a plant growth submodel that includes water, salinity, and O2 stress. The chemical submodel considers equilibrium ion exchange, as well as various equilibrium and kinetic expressions for precipitation and dissolution of soil minerals, including gypsum, Mg carbonates, and sepiolite. The use of a predictive submodel for CO2 production and transport allows for the calculation of CO2 concentrations with depth and time. This enables us to avoid the assumption of constant CO2 distribution or constant pH required by previous models. Use of kinetic expressions for carbonate chemistry allows a more realistic simulation of soil and groundwater solution composition as well as simulations of carbonate redistribution and climatic change with time.
Soil Survey Input in Exploratory Modeling of Sustainable Soil Management Practices
Authors: Droogers P. , Bouma J.
Published: July 2010

Soil survey information combined with exploratory simulation modeling was used to define indicators for sustainable land management. In one soil series in the Netherlands (the genoform), three different phenoforms were formed as a result of different management practices. Locations were identified using a soil map and interviews with farmers. Organic matter, bulk densities, and porosities were significantly different for the three phenoforms: biodynamic management (Bio), conventional management (Conv), and permanent grassland (Perm). By applying a dynamic simulation model for water movement, crop growth and N dynamics, the three phenoforms were analyzed in terms of sustainability indicators by defining four scenarios based on productivity and N leaching to the groundwater: (i) potential production, (ii) water‐limited production, (iii) current management, and (iv) the environmental scenario. The latter was divided into EnvA: never exceeding the N‐leaching threshold of 11.3 mg L‐1; EnvB: exceedance occurring in one out of 30 yr; and EnvC: exceedance occurring in three out of 30 yr. Biodynamic management obtained the lowest yield under current management, while yields for Perm were highest. EnvA could not be reached for Perm as a result of high mineralization rates. Obtainable yields for scenarios EnvA, EnvB, and EnvC differed substantially, illustrating the importance of selecting “acceptable” risks in environmental regulation. The presented methodology demonstrates the importance of pedological input in sustainability studies.
Application Effects of Controlled‐Availability Fertilizer on Dynamics of Soil Solution Composition
Authors: Yanai Junta , Kosaki Takashi , Nakano Aiko , Kyuma Kazutake
Published: July 2010

The effect of a controlled‐availability fertilizer (CAF) on plant uptake and leaching potential of soil nutrients, in relation to the dynamics of the chemical composition of the soil solution, was evaluated using a pot experiment with three N treatments [CAF or ordinary fertilizer applied as Ca(NO3)2 and an unfertilized treatment], with or without wheat (Triticum aestivum L.) plants. Soil solutions were collected nondestructively with looped hollow fiber samplers from the root zone in situ six times during the 49‐d growing period. Wheat plants were also collected at the second, fourth, and last collection date. Concentrations of NO3 and K in the soil solution were significantly lower for the CAF treatment than for the ordinary fertilizer treatment at all sampling times. Leaching potential of N and K in the CAF treatment was accordingly reduced compared with the ordinary fertilizer treatment, especially at the early stages of plant growth. The concentration of P was low, and not affected significantly by the N treatments, and the leaching potential was negligible. Moreover, plant growth, nutrient uptake of N, P, and K, and their supplying meachanisms in soil were not affected significantly by the N treatments in this experiment. Therefore, the ability of CAF to reduce the leaching potential of soil nutrients without exerting adverse effects on the growth and nutrient uptake of plants was demonstrated for N and K, based on the dynamics of the soil solution composition in the root zone.
Modeling Ammonia Volatilization from Biosolids Applied to Semiarid Rangeland
Authors: Harmel R. D. , Zartman R. E. , Mouron C. , Wester D. B. , Sosebee R. E.
Published: July 2010

One fate of N from land‐applied biosolids is NH3 volatilization. This study was conducted to: (i) quantify NH3 volatilization from anaerobically digested, dewatered biosolids; and (ii) develop equations to predict volatilization losses. Ammonia was collected under natural field conditions with semi‐open, dynamic collection systems. Volatilization was measured using a phenol‐nitroprusside method for 210 h following biosolid applications of 0.0, 6.7, and 17.9 dry Mg ha‐1. Volatilization studies using two soils (a Ustollic Calciorthid and a Ustollic Haplargid) with similar chemical properties but with different surface gravel contents were conducted under hot (20–36°C), intermediate (6–23°C), and cool (−4–14°C) temperature regimes. Volatilization followed a diurnal fluctuating pattern for both the hot and intermediate temperature regime trials and was significantly reduced 3 d after application for all trials. Cumulative volatilization losses ranged from 11.5 kg NH3‐N ha‐1 in the cool‐season trial for the 6.7 dry Mg ha‐1 application rate to 35.5 kg NH3‐N ha‐1 in the hot‐season trial for the 17.9 dry Mg ha‐1 rate. The influence of gravel content on NH3 volatilization was not statistically significant (P = 0.01). Empirical equations were developed to predict volatilization losses as a function of site‐ and season‐specific potential evapotranspiration, biosolid NH3‐N content, application rate, and time after application. The prediction equations for NH3 volatilization from applied biosolids as affected by temperature had a range of r2 values of 0.90 to 0.97.
Nitrous Oxide Emission through Plants
Authors: Chang C. , Janzen H. H. , Nakonechny E. M. , Cho C. M.
Published: July 2010

Agricultural soils are a major source of N2O that has been linked to an enhanced greenhouse effect and the depletion of stratospheric ozone. The N2O produced in the soil is usually assumed to enter the atmosphere by diffusion. We postulated that significant amounts of N2O may also be emitted via plant transpiration, and conducted a controlled environment study to test this hypothesis. The shoots of canola (Brassica napus L.) were enclosed in chambers, and N2O emission from the foliage was quantified by measuring changes in N2O concentration following various soil treatments. At field capacity, a treatment not conducive to denitrification, there was no measurable emission of N2O from the plant foliage. At saturation, however, N2O concentration within the shoot chamber increased consistently with time. By the end of the sampling period, the N2O concentration in the shoot chamber was 0.45 L L−1, well above that in the soil head space (0.35 L L−1), demonstrating that N2O was released from the shoots and not the soil. Watering the soil with a N2O solution with a concentration of approximately 8 mL L−1 resulted in almost immediate release of N2O from the shoots, indicating that the N2O was not generated by the plants directly, but merely conveyed to the atmosphere by the transpiration stream. Experiments using barley (Hordeum vulgare L.) produced results similar to those with canola, suggesting that emission of N2O from plants may be a common phenomenon. These results demonstrate a mechanism for N2O emission from upland crops, and imply that measurement of N2O release solely from the soil surface may underestimate actual fluxes from agroecosystems.
Hydraulic Conductivity Variability for Two Sandy Soils
Authors: Bosch D. D. , West L. T.
Published: July 2010

Reliable estimates of saturated hydraulic conductivity are a prerequisite for accurate estimations of water flow and chemical transport through soil profiles. While informative, mean estimates of flow and transport have been shown to be inadequate for modeling purposes. Distributions and variances along with mean predictions provide a better form of model input. The spatial variability of field‐measured saturated hydraulic conductivity (Kfs) was investigated on a transect across a 1‐ha field in west‐central Georgia. The field includes two soil types, a Pine Flat loamy sand (coarse‐loamy, siliceous, thermic Typic Paleudult) and a Troup sand (loamy, siliceous, thermic Grossarenic Kandiudult). Estimates of Kfs were calculated using data collected with a constant‐head permeameter. Mean, variance, and distribution data were determined for each soil and four selected horizons. Calculated values in the top 2 m of the soil profiles ranged from 0.07 to 117 mm h−1 for the Pine Flat loamy sand and from 23 to 406 mm h−1 for the Troup sand. Statistical differences between Kfs data calculated for the lower soil profiles coincide with soil textural differences observed with depth and across the plot. Semivariances for ln Kfs data were calculated and models fit to the semivariograms. Random behavior, with little spatial correlation, dominated the In Kfs data for the upper 0.5 m of the Pine Flat soil and the 2.0‐m measurement for the Troup soil. Observed correlation ranges varied from 2 to 166 m. Large statistical differences in Kfs were observed for the adjacent soils.
Retardation Coefficients of Nonionic Organic Compounds Determined by Batch and Column Techniques
Authors: Maraqa M. A. , Zhao X. , Wallace R. B. , Voice T. C.
Published: July 2010

Batch and column techniques were utilized to determine retardation coefficients of two nonionic organic compounds: benzene and dimethylphthalate. Three sandy soil materials with medium to high organic C content were used as sorbents. Batch data consistently overestimated retardation coefficients of the two compounds. Several previously reported factors that may cause discrepancy between the two techniques were experimentally investigated. Sorption nonsingularity, sorption nonequilibrium, presence of immobile water regions in the column, and reduction in particle spacing in the column were eliminated as the cause of this discrepancy. Loss of sorbent from the column is unlikely to be the cause of the discrepancy. Although rapidly mixed batch tubes are subjected to soil abrasion, it causes no apparent impact on the value of sorption distribution coefficients. Batch isotherms at two solids concentrations were identical, indicating that differences in solids concentration between batch and column setup is not a significant factor. This was also confirmed by a column study. Sorption nonlinearity was found to have a minor impact. Column residence time had a major impact on one of the cases studied, but no effect on the other cases. When column residence time was accounted for, some differences in sorption distribution coefficients, which seemed to be independent of the organic C content, were noticed. In this study, all previously suggested causes for the discrepancy between batch‐ and column‐determined retardation coefficients were investigated and rejected. It remains unclear why the values determined by the two techniques were different.
Carbon Dynamics in Corn‐Soybean Sequences as Estimated from Natural Carbon‐13 Abundance
Authors: Huggins D. R. , Clapp C. E. , Allmaras R. R. , Lamb J. A. , Layese M. F.
Published: July 2010

Carbon flow in terrestrial ecosystems regulates partitioning between soil organic C (SOC) and atmospheric CO2. Our objectives were to assess SOC dynamics using natural 13C abundance in corn (Zea mays L., a C4 species)‐soybean [Glycine max (L.) Merr., a C3 species] sequences. Fifteen treatments of continuous corn, continuous soybean, various sequences of corn and soybean, and fallow were initiated in 1981 at Lamberton, MN, on a Webster clay loam (fine‐loamy, mixed, mesic Typic Haplaquoll). In 1991, soil and aboveground shoot samples from all treatments were analyzed for total organic C and δ13C. Carbon inputs, δ13C, and SOC were integrated into a two‐pool model to evaluate C dynamics of corn and soybean. Total SOC was similar across all treatments after 10 yr; however, differences in soil δ13C occurred between continuous corn (δ13C = −17.2‰) and continuous soybean (δ13C = −18.2‰). Modeled C dynamics showed SOC decay rates of 0.011 yr−1 for C4‐derived C and 0.007 yr−1 for C3‐derived C, and humification rates of 0.16 yr−1 for corn and 0.11 yr−1 for soybean. Decay and humification rates were slightly lower than those found in other Corn Belt studies. Levels of SOC were predicted to decline an additional 7 to 18% with current C inputs from either corn or soybean, respectively. Annual C additions required for SOC maintenance averaged 5.6 Mg C ha−1, 1.4 to 2.1 times greater than previously reported estimates. Controlled variation in natural 13C abundance in corn‐soybean rotations during a 10‐yr period adequately traced C dynamics.