bluuebunny/crossref_metadata_2025 huggingface dataset:
Phosphate‐Induced Clay Dispersion as Related to Aggregate Size and Composition in Hapludoxs
Authors: Lima Jose M. , Anderson Sharon J. , Curi Nilton
Published: July 2010

Although phosphate sorption is a well‐understood phenomenon in soils, less is known of its effect on electrophoretic mobility (EM), isoelectric point (IEP), and clay particle dispersion in Oxisols. High phosphate sorption and high stability of aggregates are characteristic of Oxisols. Phosphate sorbed as inner‐sphere complexes brings negative charge to the surface of particles, affecting EM, IEP, and clay particle dispersion. The objectives of this research were to determine the effect of residual sorbed P (after one sorption–desorption cycle in 0.015 M NaCl) on EM, IEP, and clay particle dispersion in aggregates of Oxisols with different organic matter contents and hematite/goethite ratios. Aggregates of 1 to 2 and 0.1 to 0.2 mm were fractionated from samples of A and B horizons of two Oxisols, both with 165 g kg−1 Fe2O3, that differ in their organic matter and hematite and goethite contents. Phosphate sorption decreased EM and IEP of B horizon aggregates. It also decreased the amount of dispersed clay, as the IEP decreased to values closer to the pH of the soil suspension, decreasing net positive charge. Then, P sorption increased dispersed clay as the IEP became lower than the pH of suspension. The effect was slightly higher on aggregates with higher hematite/goethite ratio. The changes on those parameters were mostly noted for B horizon samples, where phosphate sorption had a major effect on charge balance because of their lower organic matter content. Small aggregates had less dispersed clay than large aggregates.
Changes across Artificial E–Bh Boundaries Formed under Simulated Fluctuating Water Tables
Authors: Harris W. G. , Hollien K. A.
Published: July 2010

Spodosols in the coastal plain of the southeastern USA are mainly restricted to zones of fluctuating water table, but mechanisms of water table linkage are not well established. A previous study showed that Aquod‐like E and Bh horizons could be artificially formed only under simulated fluctuating water table conditions. The present study was conducted to characterize the redistribution of components during artificial podzolization and to compare the resultant distributions with those of an Aquod adjacent to the Psamment from which column materials were collected. A hypothesis tested is that the E–Bh formation involves colloidal as well as chemical translocation. Artificial E–Bh horizons were generated in three acrylic columns using an approximate cycle of 22 h of saturation and 2 h of free drainage. Distilled water was introduced from the bottom of columns, and oxalic acid from the top. Boundaries migrated downward at a progressively slower rate, and became increasingly abrupt and irregular with time. E horizons consisted mainly of stripped sand grains; coatings were retained in Bh horizons. Fine materials containing C and Al accumulated in the upper centimeters of the Bh, as documented by scanning electron microscopy (SEM) and energy dispersive x‐ray analysis (EDX). Bulk analyses verified accumulations of C in the Bh and a movement of Al and crystalline clay from E to Bh. Artificial E–Bh formation entailed redistribution of sand grain coatings initially present on sands used in columns. Crystalline as well as noncrystalline clay accumulated in the Bh horizon, supporting the idea that E–Bh formation involved colloidal as well as chemical migration. A weatherable‐mineral source of Al is not required for the artificial water table–induced formation of E–Bh sequences. Noncrystalline Al, dissolved in the grain‐stripping process, is a likely source of Al in Bh horizons of the region. Distributional trends in artificial E–Bh sequences paralleled those of the natural Aquod.
Impacts of the Real and Imaginary Components of Relative Permittivity on Time Domain Reflectometry Measurements in Soils
Authors: Topp G.C. , Zegelin S. , White I.
Published: July 2010

Time domain reflectometry (TDR) is widely used for routine field monitoring of water content and salts in soils. Most estimates of water content assume the TDR‐measured apparent relative permittivity, ϵa, is a good approximation for the real component, ϵ′r, of the soil's complex relative permittivity with the magnitude of ϵ′r being determined primarily by water content. We examine this assumption and show that ϵa is influenced by both the real and imaginary components of the relative permittivity. Increases in ϵa resulted from the dc conductivity and dielectric loss arising from the presence of ions in solution and clay content. At water contents above 0.15 m3 m−3 in soils with high clay content and/or salt, specific calibrations are needed for precise determinations of water content from TDR. We use the wave propagation equations to separate the real and imaginary component contributions to ϵa The Giese and Tiemann interpretation for dc conductivity was again shown to be within 10% of that from a conductance meter and this fact was used to propose a method using only TDR data to separate real and imaginary components of the relative permittivity. It was found that the dielectric losses and conductive losses did not differ according to the source of conductivity, whether from clay content in the soil matrix or electrolyte in the soil solution.
Portable, two‐stage sampler for “difficult” soils
Authors: Seaby D. A.
Published: July 2010

A lightweight hand‐sampler is described for taking uncompacted cores up to 1.5 m long and 49 by 49 mm in section. Cores were successfully taken from tilled gravelly brown earths or had intact unshattered earthworm tunnels even in samples with a high clay content. The sampler comprises two halves, two lengths of light, high quality angle‐steel (2 m long × 50 mm wide). The two half‐lengths are inserted separately into the soil. Tips are acutely pointed and sharpened with the chamfer to the outside. These cutting edges are constricted internally to give relief to the core. Steel caps are welded to the top of each half‐length to receive hammer blows. The first half‐length to be driven into the soil has a cube of hardwood fixed at the upper end and another is positioned just above the soil surface. These serve as alignment bearings for the second half‐length. It is positioned tightly against the first half‐length and driven until the steel caps just touch and overlap. The entire sampler and soil core are then withdrawn using a lever, the tip of which keys sequentially into a series of holes bored along the first half‐length. A short steel tube attached horizontally across a piece of heavy marine‐plywood provides a fulcrum for the lever.
Artifacts Caused by Collection of Soil Solution with Passive Capillary Samplers
Authors: Goyne Keith W. , Day Rick L. , Chorover Jon
Published: July 2010

Soil solution was collected with passive capillary samplers (PCAPS) and zero‐tension samplers (ZTS) from A horizons of forested soils. The volume and chemistry of collected solutions were measured weekly during discrete seasonal collection periods. Acid‐washed PCAPS increased alkalinity (3‐fold), pH (1–3 units), and concentrations of Ca (2‐fold), Na (8‐fold), and Si (4‐fold), relative to ZTS solutions. Aluminum concentrations were dramatically reduced in PCAPS compared with ZTS samples. Differences in solution chemistry were attributed to leaching and weathering of fiberglass wicking material utilized in the PCAPS. In addition, PCAPS collected greater volumes (normalized by sampler area) of solution relative to ZTS in weak‐structured sandy loam soil due to a preponderance of matrix flow. The results indicate that the PCAPS used in this study are not suitable for aqueous geochemical studies of dilute soil solutions.
Soil Carbon and Nitrogen Dynamics Following Application of Pig Slurry for the 19th Consecutive Year I. Carbon Dioxide Fluxes and Microbial Biomass Carbon
Authors: Rochette Philippe , Angers Denis A. , Côté Denis
Published: July 2010

Agricultural soils often receive annual applications of manure for long periods. The objective of this study was to quantify the effects of 19 consecutive years of pig (Sus scrofa) slurry (PS) application on CO2 emissions and soil microbial biomass. Soil temperature, soil moisture, and extractable soil C were also determined to explain the variations in CO2 emissions and soil microbial biomass. Long‐term (19 yr) treatments were 60 (PS60) and 120 Mg ha−1 yr−1 (PS120) of pig slurry and a control receiving mineral fertilizers at a dose of 150 kg ha−1 yr−1 each of N, P2O5, and K2O. Very high CO2 emissions (up to 1.5 mg CO2 m−2 s−1) occurred during the first 2 d after PS application. Following that peak, decomposition of PS was rapid, with one‐half the total emissions occurring during the first week after slurry application. The rapid initial decomposition was exponential and was attributed to the decomposition of the labile fraction of the slurry C. The second phase was linear and much slower and probably involved more recalcitrant C material. Cumulative annual decomposition was proportional to the application rate, with 769 and 1658 kg C ha−1 lost from the 60 and 120 Mg ha−1 doses, respectively. Pig slurry application caused a rapid increase in soil microbial biomass (from ≈100 to up to 370 mg C kg−1 soil), which coincided with a peak in the concentration of extractable C and in CO2 emissions. Field estimates of the microbial specific respiratory activity suggested that the difference in soil respiration between the two slurry treatments was due to differences in the size of the induced microbial biomass rather than to differences in specific activity.
Soil Carbon and Nitrogen Dynamics Following Application of Pig Slurry for the 19th Consecutive Year II. Nitrous Oxide Fluxes and Mineral Nitrogen
Authors: Rochette Philippe , van Bochove Eric , Prévost Danielle , Angers Denis A. , C o ˆ té Denis , Bertrand Normand
Published: July 2010

Agricultural soils often receive annual applications of manure for long periods. Our objective was to quantify the effects of 19 consecutive years of pig (Sus scrofa) slurry (PS) application to a loamy soil (loamy, mixed, frigid Aeric Haplaquept) on N2O emissions. Soil surface N2O fluxes (FN2O) were measured 36 times in 1 yr. Nitrous oxide concentration profiles, soil NH+4‐ and NO−3‐N contents, denitrifying enzyme activity (DEA), and denitrification rate (DR) in soil were also determined to explain the variation in FN2O Long‐term (19 yr) treatments on continuous silage maize (Zea mays L.) were 60 (PS60) and 120 Mg ha−1 yr−1 (PS120) of pig slurry and a control receiving mineral fertilizer at a dose of 150 kg ha−1 each of N, P2O5, and K2O. Denitrifying enzyme activity, soil N2O concentrations, and FN2O (<25 ng m−2 s−1) were low in the control plots receiving mineral fertilizer. Annual applications of PS to the soil for 18 yr had positive residual effects on the DEA compared with the long‐term fertilized control plots. Following PS application, there was a strong and rapid increase of FN2O (up to 350 ng m−2 s−1) on manured plots. The PS‐induced FN2O increased with increasing quantity of PS, probably as the result of a greater availability of NO−3‐N for denitrification. The effects of PS on FN2O were mostly limited to the 30 d following application, with low fluxes (<10 ng m−2 s−1) during the rest of the measurement period. Total N2O–N emissions represented 0.62, 1.23, and 1.65% of total N applied in control, PS60, and PS120 plots, respectively. These emission factors for the PS plots agreed with values previously suggested for N‐fertilized soils (1.25%).
Carbon and Nitrogen in the Enriched Labile Fraction along a Climosequence of Zonal Steppe Soils in Russia
Authors: Rodionov A. , Amelung W. , Urusevskaja I. , Zech W.
Published: July 2010

Breaking aggregates by plowing resulted in the decomposition of formerly physically protected soil organic matter (SOM), including the enriched labile fraction (ELF), but it was unknown to what extent such effects were controlled by climate. To investigate this question, aggregate‐size fractions were obtained from each of five native and adjacent long‐term cultivated topsoils across a climosequence in the Russian steppe. After ultrasonic dispersion of small macro‐ (250–2000 μm) and microaggregates (53–250 μm) at 100 and 500 J mL−1 (only for so‐called stable macroaggregates) and a particle‐size fractionation, density fractions <1.85 g cm−3, 1.85 to 2.07 g cm−3, 2.07 to 2.22 g cm−3 (= ELF for the small macroaggregates) and >2.22 g cm−3 were obtained from the fine silt–sized particles. In all fractions, C and N contents were determined. The stable aggregates were found only at native sites, were almost free of ELF, and showed their C maximum in the two lightest fractions. Cultivation reduced the C and N contents in all aggregates. In small macroaggregates, C losses occurred primarily as ELF, whereas microaggregates lost C in the fine silt density range of 1.85 to 2.22 g cm−3 The C partition among the fine silt density fractions was not related to climate. Losses in ELF were also not related to climate, suggesting that ELF represents a C pool that is site‐specifically influenced by cultivation. The C losses from fractions <2.07 g cm−3, however, increased as the climate became dryer and warmer, suggesting that they reveal interactive effects of climate and land use on physically stabilized SOM.
Water Drop Impact Angle and Soybean Protein Amendment Effects on Soil Detachment
Authors: Cruse R. M. , Berghoefer B. E. , Mize C. W. , Ghaffarzadeh M.
Published: July 2010

To improve soil erosion prediction technology, the mechanics of each erosion process must be understood sufficiently to predict soil loss on an event basis. The mechanics of the initial erosion process, soil detachment caused by falling raindrops, requires greater understanding to improve mechanics‐based prediction. This laboratory study addressed the effect of soil shear strength and raindrop impact angle on soil detachment. Loess (fine‐silty, mixed, superactive, mesic Typic Hapludoll) and glacial till (fine‐loamy, mixed, superactive, mesic Aquic Hapludoll) A and C horizon soil materials were used. To vary soil shear strength, soybean protein material was added to each soil material at concentrations of 0.0, 0.5, and 1.0% by weight. Soil shear strength and soil detachment were measured on preformed soil cores. Soil detachment tests were performed at water drop impact angles of 90, 80, 70, and 60°. Soil strength increased and detachment decreased with increasing soybean protein concentrations. Shear strength of the loess C horizon increased 0.61 to 1.85 Mg m−2, while that of the till C horizon material increased 0.57 to 0.98 Mg m−2 with addition of 1% soybean protein. A 1%–soybean protein addition reduced soil detachment 26% compared with unamended soil. Significant soil detachment interactions existed between waterdrop impact angle and the other variables. These interactions were due to different mechanical behavior of the soils and changing strength caused by soybean protein additions. Interactions observed are explained based on differences in the lateral jet for varying impact angles and for elastic vs. inelastic impacts.
Organic Matter Influence on Clay Wettability and Soil Aggregate Stability
Authors: Chenu C. , Le Bissonnais Y. , Arrouays D.
Published: July 2010

Soil organic matter is thought to increase aggregate stability by lowering the wettability and increasing the cohesion of aggregates. In southwest France, thick humic loamy soils (Vermic Haplubrepts) have been intensively cropped for 40 yr, decreasing the soil organic pool and lowering the soil agregate stability. This study assessed (i) the contribution of organic matter to aggregate stability by decreasing aggregate wettability and (ii) the specific role of clay‐associated organic matter. Soil samples with a C content of 4 to 53 g kg−1 were sampled and soil aggregate stability was measured. Aggregate wettability was assessed by measuring water drop penetration times on individual 3‐ to 5‐mm aggregates. The <2‐μm fractions were extracted without organic matter destruction and their wettability was determined by measuring contact angles of water on clay deposits. Aggregate stability against slaking was correlated to soil C content Water drop penetration time increased with C contents from 1 to 32 s and was very heterogeneous among individual aggregates from a given soil. The contact angle of water on the clay fraction increased linearly with the C content This change in clay wettability could partly explain the higher water stability of soils rich in C.
Crop Rotations and Nitrogen Fertilization to Manage Soil Organic Carbon Dynamics
Authors: Studdert Guillermo A. , Echeverría Hernán E.
Published: July 2010

Sustainability is influenced in many production systems by the variation of soil organic C (SOC) content and dynamics, and crop rotations. We hypothesized that arable layer SOC under conventional tillage can be managed through the amount of residue C (RC) returned to the soil as affected by tillage and fertilization. Soil organic C dynamics of a complex of Typic Argiudoll and Petrocalcic Paleudoll soils under conventional tillage between 1984 and 1995 at Balcarce, Argentina was studied for 16 crop sequences. Crops included were spring wheat (Triticum aestivumL.), soybean [Glycine max(L.) Merr.], sunflower (Helianthus annuusL.), and corn (Zea maysL.). Eleven years of conventional tillage decreased SOC 4.1 to 8.8 g kg−1without supplemental N and 2.8 to 7.2 g kg−1when N fertilizer was applied. Soil organic C loss increased when soybean (1.2 Mg RC ha−1yr−1) was present in the sequence and decreased when corn (3.0 Mg RC ha−1yr−1) was present. The amount of RC returned by the sequences correlated with SOC in 1995and with SOC at equilibrium, but the sequences with two summer crops (soybean, sunflower, or corn) every 3 yr showed lower SOC in 1995 (28.9–33.8 g kg−1) and at equilibrium (24.0–34.4 g kg−1) than sequences with none or one summer crop (29.7–35.0 g kg−1either in 1995 or at equilibrium) for the same range of RC (1.4–2.6 Mg RC ha−1yr−1). The difference between sequences in the relationship between RC and SOC were attributed to tillage timing. Under conventional tillage, arable layer SOC can be managed through the selection of the crops in the rotation and N fertilization, but the timing and intensity of tillage have to be taken into account.
Soil Microclimate and Chemistry of Spruce–Fir Tree Islands in Northern Utah
Authors: Van Miegroet Helga , Hysell Molly T. , Johnson Amber Denton
Published: July 2010

The objective was to investigate differences in soil properties in high‐elevation tree islands composed of subalpine fir [Abies lasiocarpa(Hook.) Nutt.] and Engelmann spruce [Picea engelmanniiParry ex Engelm.] relative to semiarid meadows in Northern Utah. We compared snow depths, surface soil temperature and moisture, and soil chemical properties (0–30 cm) between tree islands and the surrounding meadow. Snow accumulated evenly around the tree islands. Snow depth differences developed during snowmelt when shaded areas to the north of the islands retained snow longer. Less snow accumulated inside and at the edge of the tree islands. In summer, canopy shading inside the islands slowed soil moisture loss and decreased the magnitude and fluctuation of soil temperatures relative to the exposed meadow. Tree islands affected O horizon mass and chemistry: 6 to 10 kg m−2of O material, with higher macronutrient concentrations and lower C/N ratio, had accumulated under tree canopies; 3 kg m−2in the island interior; and <1 kg m−2in the meadow. Surface soils inside the tree islands had significantly higher C and N soil concentrations and higher C/N ratio. The pH of meadow soils was constant with depth (6.22–5.95). Below the trees, the pH in the upper soil was 0.1 to 0.3 units higher than in the meadow and decreased 0.8 to 0.9 pH units with depth. The presence of tree islands significantly modified the soil microclimate and nutrient distribution relative to the surrounding meadows.
Influence of Phosphorus Loading on Organic Nitrogen Mineralization of Everglades Soils
Authors: White J. R. , Reddy K. R.
Published: July 2010

There have been recent concerns about the anthropogenic phosphorus (P) loading to the naturally oligotrophic Everglades ecosystem. We investigated the effect of P loading on the biogeochemical cycling of nitrogen (N). We investigated the distribution of the potentially mineralizable N (PMN) rate as an indicator of the influence of P loading on selected microbial activities in soil and detritus layers. Soil characteristics measured included bulk density; total C, N, and P; microbial biomass C; and N and extractable NH+4PMN rates ranged from 10.4 to 325 mg N kg−1d−1The highest values of microbial biomass C and N, total P, extractable NH+4, and PMN were observed in the detrital layer, and rates decreased with increasing soil depth. An increase in the size of the microbial pool and heterotrophic activity (organic N mineralization) was found to be related to the P‐loading rate and related to the distribution of total P content in the soil and detrital layers. Extractable NH+4was a good indicator of PMN rates and total P was found to be significantly correlated to microbial biomass C and N. The stimulatory effect of P enrichment on microbial activity, overall size of the microbial biomass pool, and the PMN rate has led to an increased availability of inorganic N, which could potentially affect macrophyte growth and water quality of the northern Everglades system.
New Dielectric Mixture Equation for Porous Materials Based on Depolarization Factors
Authors: Hilhorst M.A. , Dirksen C. , Kampers F.W.H. , Feddes R.A.
Published: July 2010

A change in the relative proportions of the constituents of a porous material like soil will cause a change in its electrical permittivity. The measured permittivity reflects the impact of the permittivities of the individual material constituents. Numerous dielectric mixture equations are published, but none of these equations are generally applicable. A new theoretical mixture equation is derived, using the principle of superposition of electric (E) fields. This mixture equation relates the measured permittivity to a weighted sum of the permittivities of the individual material constituents and includes depolarization factors to account for electric field refractions at the interfaces of the constituents. The depolarization factors are related to physical properties of the material. Most other mixture equations contain one or more empirical factors. The concept of the depolarization factor is comparable with that of the “shape factor” of particles as described by other authors. A special case of the new mixture equation, for which the depolarization factors equals one (no depolarizations), appeared equal to a mixture equation for fluids derived from using thermodynamics. The new mixture equation is compared with other mixture equations. Comparison of the new mixture equation with measured data for glass beads and fine sand was promising. Concluding, depolarization factors in the new mixture equation relate the microstructural and compositional material properties to the measured bulk permittivity of a material. Although not shown, depolarization factors can be calculated from physical material properties.
Modeling Translocation and Dispersion of Soil Constituents by Tillage on Sloping Land
Authors: Van Oost K. , Govers G. , Van Muysen W. , Quine T. A.
Published: July 2010

Recent studies clearly indicated that soil tillage is a major factor affecting soil redistribution on arable land. However, existing modeling studies of the redistribution of soil constituents by tillage are largely theoretical in nature due to the lack of sufficient experimental data. This paper describes the results of field experiments carried out with a moldboard plow on different slope gradients showing that an important net downslope translocation and dispersion of soil constituents is associated with soil tillage. Both translocation and dispersion are strongly affected by slope gradient. Available experimental evidence suggests that redistribution of soil constituents can be modeled by convoluting the displacement probability distribution with the spatial distribution of the soil constituent. The convolution model presented here was found to be superior to existing models because it accounts for dispersion, directionality of tillage, and topography. A comparison of soil property data with model simulations indicated that soil tillage is a major factor when assessing the effect of erosion processes on the variability of soil properties.
Method to Quantify Short‐Term Dynamics in Carbon Dioxide Emission Following Controlled Soil Deformation
Authors: Otten Wilfred , Watts Chris W. , Longstaff Dave
Published: July 2010

Tillage of soils often decreases soil organic matter content and increases CO2 emission. Enhanced CO2 emission induced by tillage may provide an early indication of the likely consequences of soil management on soil organic C. This study was conducted to investigate a link between the mechanical properties of soil and the short‐term (24 h) dynamics in CO2 emission induced by soil deformations in controlled laboratory experiments. The degree to which soil is deformed depends on the normal and shear stresses acting upon the sample and on the stress history experienced by the soil. Respirometers were designed in which a range of combined shear and compressive forces could be applied to a soil sample while simultaneously measuring CO2 emission. Mechanical characteristics of repacked sieved aggregates of soil were first established. Following the critical state concept, we identified changes in bulk density that resulted from different combinations of normal and shear stresses, and for different stress histories of the soil. We selected treatments that resulted in permanent deformation (compression) and expansion or consolidation upon shear. The CO2 emission increased by 18%, even for samples that were severely compressed with a normal load of 430 kPa. Shearing of soil samples also resulted in greater CO2 emission, in particular when combined with a normal load. The increased CO2 emission following compression was maintained for a period longer than 7 d. As the increase did not correlate with the physical changes in pore volume, we concluded that an increase in availability of organic matter was the main mechanism responsible for enhanced CO2 emission. We discuss these findings from experiments under controlled conditions in the context of the mechanical deformations that can be expected during tillage and assess the implications for biological activity in the field.
A Pore Water Conductivity Sensor
Authors: Hilhorst M. A.
Published: July 2010

The electrical permittivity and conductivity of the bulk soil are a function of the permittivity and conductivity of the pore water. For soil water contents higher than 0.10 both functions are equal, facilitating in situ conductivity measurements of the pore water. A novel method is described, based on simultaneous measurements of permittivity and conductivity of the bulk soil from which the conductivity of the pore water can be calculated. A prototype of a pore water conductivity sensor based on this method is presented. Validation results show that the method can be used for a broad range of soils and is valid for water contents between 0.10 and saturation and for the conductivity of the pore water up to 0.3 S m−1
Soil Material, Temperature, and Salinity Effects on Calibration of Multisensor Capacitance Probes
Authors: Baumhardt R. L. , Lascano R. J. , Evett S. R.
Published: July 2010

Multisensor capacitance probes (MCAP) are an alternative to gravimetric or nuclear soil water content (θv, m3 m−3) measurements. Their θv measurements are more convenient than gravimetric, and don't carry the nuclear regulatory burdens. Previous studies noted potential salinity and temperature effects on MCAP θv determinations. Our objectives were to calibrate and verify MCAP θv measurement accuracy in two soil materials, two water salinities (1.3 and 11.3 dS m−1), and with diurnal temperature fluctuations. The surface and calcic horizons of an Olton soil (fine, mixed, superactive, thermic Aridic Paleustoll) were packed into triplicate, 0.5‐m‐tall, 100‐L columns and wetted. We compared θv determined by volumetric measurements, time domain reflectometry (TDR), and MCAPs. The TDR θv were within ±0.01 m3 m−3 of volumetric determinations for air‐dry and saturated soil. The factory supplied universal MCAP calibration provided accurate θv estimates for air dry (±0.01 m3 m−3) surface and calcic soil materials but not after wetting (≈−0.05 m3 m−3). Also, imprecise MCAP sensor positioning during water frequency parameter determination was problematic and biased initial θv measurements. After calibration against TDR, the MCAP θv varied ±0.01 m3 m−3 from measured θv for air‐dry and saturated conditions for both soil materials, which were then pooled to obtain one calibration. Column resaturation with saline water affected permittivity and elevated MCAP θv ≈0.25 m3 m−3 above the available pore space. Cyclical soil temperature fluctuations of 15°C induced similar fluctuations in indicated θv throughout the column (0.04 m3 m−3 for MCAP and 0.02 m3 m−3 for TDR), which was attributed to variations in permittivity.
Long‐Term Effects of Urea and Green Manure on Rice Yields and Nitrogen Balance
Authors: Ladha J.K. , Dawe D. , Ventura T.S. , Singh U. , Ventura W. , Watanabe I.
Published: July 2010

Data from a 14‐yr double‐crop rice (Oryza sativa L.) experiment allowed comparison of the long‐term effects of N fertilizer from different sources (urea and in situ grown azolla [Azolla microphylla Kaulf.] and sesbania [Sesbania rostrata Bremek. & Oberm.]) on N balances, soil N pools (both total and available), and yields. Although data show that plant‐available N was maintained over time in both wet (WS) and dry seasons (DS), yields declined significantly, indicating a decline in physiological N use efficiency. The yield declines were generally similar regardless of N source in both seasons. The WS decline averaged 150 kg ha−1 yr−1 in the three added‐N treatments, while the DS decline averaged 185 kg ha−1 yr−1. After 27 crops, the cumulative positive N balance was estimated at 1244, 348, 646, and 1039 kg N ha−1 in control, urea, sesbania, and azolla treatments, respectively. There was no significant change in soil total N content in the control and urea treatments, whereas it increased to 344 to 541 kg after 27 crops in the sesbania and azolla treatments. Conservation of the soil N status and positive N balance, in spite of the high amounts of N removed through grain and straw, reflect the N contribution (13–46 kg ha−1 crop−1) from nonsymbiotic N2 fixation. In addition, sesbania and azolla were estimated to add ≈57 to 64 kg ha−1 crop−1 through symbiotic N2 fixation. These results demonstrate that in rice–rice cropping systems biological N fixation plays a vital role in replenishing the soil N pool. However, continuous application of green manure N (GM‐N) did not increase crop N availability, perhaps because of the presence of a recalcitrant soil organic matter fraction. Residual effects on rice grain yield and N uptake were observed only with GM‐N sources.
Nitrogen Mineralization Pattern of an Oxisol of Guadeloupe, French West Indies
Authors: Sierra J. , Marbán L.
Published: July 2010

It is generally assumed that Oxisols have a low level of soil organic matter (SOM) and a high rate of SOM turnover due to the high temperatures in the tropics. This work was carried out to test this assumption by analyzing the kinetics and the temperature–moisture response of N mineralization and nitrification in a neutral (pH 6.2, organic N 1.5 g kg−1, bare for 10 yr) and an acid (pH 4.7, organic N 2.3 g kg−1, bare for 1 yr) Oxisol. Samples of these soils were incubated using a factorial design of temperature (20, 30, 40, and 50°C) by soil water (30, 200, and 1500 kPa). The mineralization pattern was related to measurements of microbial biomass, SOM light‐fraction, and the number of nitrifiers. Mineralization increased continuously with the increase of temperature and water content. No nitrification was observed at 50°C and at 1500 kPa. Although mineralization was well described by the double exponential model, nitrification showed a linear pattern with time. The size of the mineralizable N pool and N mineralization were different between the two soils, but microbial activity and microbial biomass were almost identical in both soils. Mineralization was very slow at 20 to 30°C (constant rate ≈ 10−4–10−5 d−1) and was limited by a relatively small (light‐fraction C = 8–10% of organic C) and low active mineralizable fraction. At >40°C, the constant rate of mineralization (≈10−3 d−1) was equivalent to those found in temperate soils at 25 to 35°C. Our findings do not support the assumption of a fast SOM turnover at high temperatures. Nitrifiers constitute a small population (≈20 cells g−1) well adapted to changes in moisture and substrate availability. However, NH+4 was always present in soils, which was associated with the low number of nitrifiers. Microbial biomass decreased sharply with an increase in temperature and was not related to N mineralization. Thermal denaturation or changes in microbial population could cause this decrease.
Nitrogen Mineralization from Field‐Applied Beef Cattle Feedlot Manure or Compost
Authors: Eghball Bahman
Published: July 2010

To apply manure or compost to fulfill N requirements of a crop, the amount of N mineralized in actual field conditions needs to be determined. Nitrogen mineralization from composted and noncomposted beef cattle feedlot manure applied to no‐till and conventional tillage systems was determined under field conditions for 3 yr. Manure, composted manure, and inorganic fertilizer were applied to provide for N needs of corn. A no‐treatment check was also included. An in situ resin method was used to determine N mineralization from a soil receiving manure, compost, and no treatment during the growing season (June–October). Of the organic N applied the previous autumn, ≈11% was mineralized from composted manure and 21% from noncomposted manure during the succeeding growing season. Lower N availability from compost reflects the loss of easily convertible N compounds during composting and the presence of stable N compounds. Nitrogen mineralization was similar in the no‐till and conventional tillage systems even though manure and compost were surface‐applied in the no‐till. Nitrogen mineralization was significantly, but not closely , related to thermal unit (cumulative mean daily temperature >0°C). Mineralization rate constants indicated that availability of residual manure and compost N was less than expected. The in situ mineralization approach seems to be a good method of measuring N mineralization during the growing season or during periods when the soil is not frozen or excessively dry. Nitrogen mineralization needs to be considered when manure and compost are used for an environmentally acceptable crop production system.
Modeling Soil–Landscape and Ecosystem Properties Using Terrain Attributes
Authors: Gessler P. E. , Chadwick O. A. , Chamran F. , Althouse L. , Holmes K.
Published: July 2010

Soil–landscape patterns result from the integration of short‐ and long‐term pedogeomorphic processes. A 2‐ha hillslope catena in California shows short‐distance variation in A horizon depth from 8 to 80 cm and in soil depth from 8 to >450 cm in convex to concave positions. Similar variations in net primary productivity (NPP) and soil C represent significant information often not captured by soil survey maps. Strong correlations between these measured soil–landscape variables and explanatory digital terrain attributes are used to develop quantitative soil–landscape models. We were able to account for between 52 and 88% of soil property variance using easily computed terrain variables such as slope and flow accumulation. Spatial implementation of the models suggest lateral redistribution processes resulting in differential accumulation of C and soil mass in convergent and divergent landscape positions. The models are explicit and quantitative, which enables their use for testing hypotheses about the spatial distribution of fine‐scale landscape and ecosystem processes and for parameterizing spatially distributed hydrological and ecosystem simulation models.
Pedogenic Transformation of Fractured Granitic Bedrock, Southern California
Authors: Frazier C. S. , Graham R. C.
Published: July 2010

Joint fractures in granitic bedrock are known to influence patterns of weathering and landform evolution, but their influence on pedogenic processes has not been thoroughly studied. This research was designed to elucidate dominant pedogenic processes and the role of joint fractures in the transformation of a soil–weathered bedrock profile in the San Jacinto Mountains of southern California. The granitic bedrock is sufficiently weathered to meet paralithic materials criteria, but is not so intensively weathered to be considered saprolite. A lateral sequence of five morphologic zones exists within each weathered bedrock horizon: the matrix, matrix rind, fracture rind, fracture coating, and fracture fill. Distributions of pedogenic clay and citrate‐bicarbonate‐dithionite extractable Fe (Fed), and micromorphologic observations of mineral grain alteration indicate that the effects of chemical weathering increase upward in the profile and laterally towards joint fractures. Illuvial clay was detected throughout the profile, but is most abundant in the shallowest weathered bedrock horizons and in morphologic zones near fracture margins. Shrink–swell activity of clay deposited in planar voids, microcracks, and root channels is responsible for mineral grain fragmentation and localized disruption of rock‐controlled fabric. Stresses generated by swelling clay are also responsible for fabric reorganization and development of incipient subsoil structure in the shallowest weathered bedrock horizon. Illuviation and shrink–swell processes play critical roles in the pedogenic transformation of this granitic bedrock.
Identification of Regional Soil Quality Factors and Indicators I. Central and Southern High Plains
Authors: Brejda John J. , Moorman Thomas B. , Karlen Douglas L. , Dao Thanh H.
Published: July 2010

Appropriate indicators for assessing soil quality on a regional scale using the National Resource Inventory (NRI) are unknown. Our objectives were to (i) identify soil quality factors present at a regional scale, (ii) determine which factors vary significantly with land use, and (iii) select soil attributes within these factors that can be used as soil quality indicators for regional‐scale assessment. Ascalon (fine‐loamy, mixed, superactive, mesic Aridic Argiustoll) and Amarillo (fine‐loamy, mixed, thermic Aridic Paleustalf) soils were sampled from a statistically representative subset of NRI sample points within the Central and Southern High Plains Major Land Resource Areas (MLRA) and analyzed for 20 soil attributes. Factor analysis was used to identify soil quality factors, and discriminant analysis was used to identify the factors and indicators most sensitive to land use within each MLRA. In the Central High Plains, five soil quality factors were identified, with the organic matter and color factors varying significantly with land use. Discriminant analysis selected total organic C (TOC) and total N as the most sensitive indicators of soil quality at a regional scale. In the Southern High Plains, six factors were identified, with water stable aggregate (WSA) content, TOC, and soil salinity varying significantly with land use. Discriminant analysis selected TOC and WSA content as the most sensitive indicators of soil quality in the Southern High Plains. Total organic C was the only indicator that consistently showed significant differences between land uses in both regions.
Association of Cadmium in Durum Wheat Grain with Soil Chloride and Chelate‐Extractable Soil Cadmium
Authors: Norvell W. A. , Wu J. , Hopkins D. G. , Welch R. M.
Published: July 2010

Cadmium uptake by food crops needs to be understood in order to limit Cd accumulation in the food chain. Cadmium is a potentially toxic heavy metal with no known benefit to humans, and plant foods are the predominant sources of Cd in human diets. In this study, 124 paired samples of soil and grain were collected from a field of durum wheat [Triticum turgidum L. subsp. durum (Desf.) Husn.] cultivar Munich in northeastern North Dakota. This field on the Langdon Research Extension Center was selected for study because it provided a range in soil pH and salinity. Cadmium in the durum grain ranged widely from 0.025 to 0.359 mg kg−1 Accumulation of Cd in grain was strongly and positively associated with soil salinity as represented by soluble chloride, soluble sulfate, or extractable Na, and also with chelate‐extractable Cd. Relationships to salinity were curvilinear. Concentrations of Cd in grain were not closely related to soil pH. The relationship of Cd in grain to the logarithm of water‐extractable soil Cl− (Clw) was especially close. A predictive model based on chelate‐extractable Cd and logCl−w in soil accounted for 66% of the variability of Cd in grain. Based on these results, and published work for other crops, we believe that the accumulation of Cd in durum wheat grain is enhanced by Cl− in the soil. Although the mechanism is not clear, it is likely to involve increased solubility or availability of soil Cd resulting from the formation of chloro‐complexes in soil solution.
Methane and Nitrous Oxide Emissions from a Rice Field in Relation to Soil Redox and Microbiological Processes
Authors: Hou A. X. , Chen G. X. , Wang Z. P. , Van Cleemput O. , Patrick W. H.
Published: July 2010

Paddy rice fields provide an environment for production of two important greenhouse gases, CH4 and N2O, because of variations in soil characteristics, moisture content, and microbial activity during the cropping season. Emissions of CH4 and N2O from a paddy rice field in northern China were measured in situ by static chamber technique during March to December in 1995 and 1996. Factors affecting gas emission, including soil temperature, pH, and redox potential (Eh), were measured as well. Emissions of CH4 and N2O were strongly correlated with changes in soil redox potential. Significant CH4 emission occurred only at soil redox potential lower than approximately −100 mV, while the emission of N2O was not significant below +200 mV. A significant inverse relationship between CH4 and N2O emissions was observed (r = −0.49, n = 16, 5% confidence level). The results suggest the possibility of using management practices to maintain the redox potential in a range where both N2O and CH4 emissions are low. The activities of six related bacteria groups (zymogenic bacteria, acetic acid and hydrogen‐producers, methanogens, CH4 oxidizers, and nitrifiers and denitrifiers) in the soil were also measured in an effort to explain the relationship between gas emission and soil microbiological processes. Methane emission was significantly related to the logarithm number of zymogenic bacteria (r = 0.76, n = 12, 1% confidence level), as well as to soil redox potential (r = −0.72, n = 12, 1% confidence level). Both zymogenic bacteria number and soil redox potential appear to be predicators of CH4 emission potential.
Tortuosity of an Unsaturated Sandy Soil Estimated using Gas Diffusion and Bulk Soil Electrical Conductivity
Authors: Weerts A. H. , Kandhai D. , Bouten W. , Sloot P. M. A.
Published: July 2010

Prediction of gas and ion diffusion in unsaturated soils has been obstructed by the inability to independently measure tortuosity parameters. Analogies between transport processes are often suggested to assess these nonmeasurable parameters. However, it is unclear whether these analogies are valid in unsaturated soils. Therefore, to obtain a more fundamental insight into the differences between ion and gas diffusion in an unsaturated sandy soil a different approach than current empirical models is required. In this study, fitted tortuosity parameter values of analogy‐based models describing the gas diffusion coefficient and bulk soil electrical conductivity (ion diffusion) of a sandy soil are compared. Gas diffusion in unsaturated soil is most affected by connectivity in contrast to electrical conductivity that is among other factors primarily determined by tortuosity. Measured gas diffusion and electrical conductivity are also compared with lattice–Boltzmann simulated gas and ion diffusion as a function of water content in a thin section (two dimensional) of a sandy soil. Results of the lattice‐Boltzmann simulations are in qualitative agreement with measured diffusional ratios and support the findings with the analogy‐based models. Lattice‐Boltzmann simulations may be a valuable tool to overcome empirism of current models and increase our insight in transport properties of unsaturated soils in the coming years.
Isothermal and Nonisothermal Evaporation from Four Sandy Soils of Different Water Repellency
Authors: Bachmann J. , Horton R. , van der Ploeg R. R.
Published: July 2010

Soil water evaporation is an important component of the surface energy balance. Evaporation also affects plant available water content of soil. Soil wettability is known to affect water‐holding capacity and water flow, but the impact of soil water repellency on evaporation has not been thoroughly investigated. The objective of the study was to measure and analyze isothermal and nonisothermal evaporation rates for four sandy soils showing different degrees of water repellency. Eight vertical soil columns (two of each soil) were exposed to isothermal conditions (20 ± 1°C), and another eight soil columns were exposed to nonisothermal conditions. During the nonisothermal experiment, the top boundary temperature was held constant at 21°C, and the bottom boundary temperature was constant at 55°C, thus maintaining a constant thermal gradient. After 195 d of evaporation, each column was sectioned to determine the soil water content distribution along its axis. A numerical model based on the Philip‐de Vries theory was used to predict soil water flow. Deviations between predicted and measured values increased with increasing contact angle. Isothermal cumulative evaporation was 25% lower for water repellent soil than for the most wettable soil. Nonisothermal cumulative evaporation was 75% larger than isothermal evaporation for the wettable soil but only 14% larger than isothermal evaporation of the most water repellent soil. Evaporation and residual water contents, especially in the surface layer, were found to be strongly affected by soil wettability.