bluuebunny/crossref_metadata_2025 huggingface dataset:
Propagation Velocity Errors in Time Domain Reflectometry Measurements of Soil Water
Authors: Hook W. R. , Livingston N. J.
Published: July 2010

The determination of soil water content by time domain reflectometry (TDR) involves two steps: the measurement of the propagation velocity of an electromagnetic pulse along a transmission line and the conversion of this measurement to an estimate of soil water content. The objective of this study was to identify and quantify errors associated with propagation velocity measurements. We developed new TDR techniques, involving the use of remotely switched diodes and differential wave form detection, that can be used to quantify and minimize propagation velocity errors. These errors are presented in terms of time interval errors. We show that the dominant time interval error term relates to the transition time of reflected pulses and that absolute time interval errors cannot be assumed to be <200 ps. We identify the presence of dissolved ions and the use of long cables as major sources of additional transition time errors. For transition times >2 ns, the time interval error caused by transition time effects can be estimated as the root mean square sum of the basic 200‐ps error and an error equal to 10% of the transition time. We show that although instrument time base errors are the main source of precision errors, they are less important in determining the absolute accuracy of propagation velocity measurements. We show that system costs can be reduced without compromising accuracy by using 75 Ω coaxial cable, which is not only inexpensive but is superior to standard 50 Ω cable.
Using Short Soil Moisture Probes with High‐Bandwidth Time Domain Reflectometry Instruments
Authors: Kelly Shaun F. , Selker John S. , Green James L.
Published: July 2010

Time domain reflectometry (TDR) is used to measure moisture content and salinity of soils. Most TDR systems have a bandwidth of 2.5 GHz or less, limiting the precision of measurements using short probes. The primary objective of this research was to develop and test short probes for use with a high‐bandwidth (20‐GHz) TDR instrument. The secondary objective was to determine moisture content in highly conductive media by insulating the probes with Teflon heat‐shrinkable tubing. Laboratory‐packed test cells with probe lengths of 0.025, 0.05, and 0.075 m were used for the primary research objective, and 0.075‐m probes for the secondary objective. Linear relationships between the apparent refractive index and volumetric moisture content were developed for each probe length and type. The accuracy of the probes was ±0.0125, ±0.05, ±0.025, and ±0.035 m3/m3 for the probes of lengths 0.075, 0.075 (insulated), 0.05, and 0.025 m, respectively. Moisture measurements in highly conductive media were possible using insulated probes.
Reactions of Ground Magnesium Limestone and Gypsum in Soils with Variable‐Charge Minerals
Authors: Shamshuddin J. , Ismail H.
Published: July 2010

Ultisols and Oxisols in Malaysia are characterized by low pH and high Al, with their subsoils usually deficient in Ca, Mg, or both. An experiment, using polyvinyl chloride columns, was conducted to study chemical reactions in a representative Ultisol and Oxisol from Malaysia resulting from application of ground magnesium limestone (GML), gypsum, and their combinations. An increase in soil pH due to GML application in the Ultisol and the Oxisol resulted in an increase in negative charge on the soil surfaces in the zone of incorporation. Calcium was retained in that zone. In the gypsum‐treated soils, some Ca moved into the underlying horizons. Negative charge in the Oxisol increased from 2.8 to 3.7 cmolc kg−1 by an application of 8 t gypsum ha−1 due to specific adsorption of SO4 on Fe oxides. An increase in pH of the Oxisol due to SO4 adsorption and OH desorption was also observed. There was an accumulation of NO3 in the subsoils of the Oxisol and Ultisol as a result of gypsum treatment. The NO3 was adsorbed by the positively charged oxides. Leachate pH values were high at the beginning of the experiment, but the values decreased with time and were higher in the Oxisol than the Ultisol. This was attributed to a lower Al concentration in the leachate of the Oxisol. Effective alleviation of topsoil Al toxicity and subsoil Ca or Mg deficiency would require the application of 2 t GML together with 1 to 2 t gypsum ha−1.
Zinc and Cadmium Uptake by Hyperaccumulator Thlaspi caerulescens Grown in Nutrient Solution
Authors: Brown S. L. , Chaney R. L. , Angle J. S. , Baker A. J. M.
Published: July 2010

Phytoremediation of heavy‐metal‐contaminated soils can be an inexpensive means to remove hazardous metals from soil. Two metallophytes, Thlaspi caerulescents (J. & C. Presl, a Zn and Cd hyperaccumulator) from Prayon, Belgium, and a Zn‐tolerant ecotype of bladder campion [Silene vulgaris (Moench.) Garcke L.] from Palmerton, PA, were compared with tomato [Lycopersicon lycopersicum (L.) Karsten, metal intolerant] in nutrient solution to characterize Zn and Cd uptake and tolerance. Zinc and Cd were added to solutions at a 50:1 molar ratio to simulate concentrations often found on contaminated sites. Seven treatment concentrations were used, ranging (in half‐log increments) from 3.16 µM Zn + 0.063 µM Cd to 10000 µM Zn + 200 µM Cd. Thlaspi caerulescens showed much greater tolerance to Zn/Cd treatments than the other species, with toxicity stress only apparent at the 10000 µM Zn/200 µM Cd treatment. In this treatment, shoot concentrations of Zn and Cd were 33600 and 1140 mg kg−1, respectively. Thlaspi caerulescens was also more effective at translocating both Zn and Cd from solution to shoots. Zinc concentration in shoots of T. caerulescens was higher than the other species at all Zn/Cd treatments. Cadmium concentration in shoots of T. caerulescens were significantly higher than in bladder campion only at the 316 µM Zn/6.32 µM Cd treatment. This genotype of T. caerulescens may not hyperaccumulate Cd. However, extreme Zn and Cd uptake and tolerance is evident in T. caerulescens, with >25000 mg Zn kg−1 and 1000 mg Cd kg−1 before yield is reduced. Results suggest that T. caerulescens may be a candidate for the phytoremediation of Zncontaminated soils.
Effect of Cropping and Fertilization on Plant and Soil Phosphorus
Authors: Selles F. , Campbell C. A. , Zentner R. P.
Published: July 2010

Knowledge regarding the effects of long‐term cropping practices and fertilization on the fate of P applied to the soil is required to aid in the prediction of how such practices influence the quality and sustainability of the environment. A 24‐yr crop rotation experiment, conducted on a Swinton silt loam (Aridic Haploboroll) in southern Saskatchewan, was used to determine uptake of P by crops, its distribution in grain and straw, and the rate of change and nature of the residual P in the soil. Phosphorus concentration in the grain relative to that in the straw was 7:1 to 12:1 for spring wheat (Triticum aestivum L.), 6:1 for flax (Linum usitatissimum L.), and 5:1 for lentil (Lens culinaris Medikus) and fall rye (Secale cereale L.). Grain P concentration was lowest in cereal crops (3.6 g kg−1) and highest in flax and lentil (4.4 g kg−1). Phosphorus removed annually with the grain increased with P fertilization and decreased with an increase in fallow frequency; it was 4.8 to 5.3 kg P ha−1 for continuous wheat, 3.5 to 3.8 kg P ha−1 for fallow‐wheat‐wheat, and 3.1 kg P ha−1 for fallow‐wheat. Trend analysis showed that fertilizer P increased the Olsen P levels in soil at rates of 1.0 to 1.7 kg ha−1 yr−1, but in the systems receiving no P, Olsen P remained constant and did not decline as suggested by a P balance sheet. Residual fertilizer P accumulated in labile forms, dominated by sorbed P and microbial P, rather than in forms dominated by Ca phosphate precipitates or other organic forms.
Deep Tillage Effects on Compacted Surface‐Mined Land
Authors: Dunker R. E. , Hooks C. L. , Vance S. L. , Darmody R. G.
Published: July 2010

Federal law requires that surface‐mined prime farmland be reclaimed and restored to premining productivity for row crops (Surface Mining Control and Reclamation Act, Public Law 95–87). In Illinois, reclamation must include soil horizon replacement (topsoil and subsoil) to a depth of 120 cm. Poor physical condition is the most limiting factor in reclarmation in Illinois. The effects of five deep tillage implements (80–120‐cm tillage depth) and two standard agricultural chisel plows (20–35‐cm tillage depth) were evaluated for 6 yr. The seven tillage treatments were applied to a scraper‐placed minesoil that had a high soil strength (2.5–3.5 MPa) throughout the reconstructed profile. A nearby tract of undisturbed Cisne silt loam (fine, montmorillonitic, mesic Mollic Albaqualf) was used as a prime farmland comparison. Crop yields for both corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] significantly increased with tillage depth. Average soil strength decreased and net water extraction increased with increasing depth of tillage. The 1.2‐m‐deep tillage treatments significantly reduced average 23‐ to 114‐cm soil strength from 2.8 to 0.93 MPa. Significant correlation occurred between 23‐ to 114‐cm average soil strength and 6‐yr mean corn (−0.97** [significant at P = 0.01]) and 4‐yr mean soybean (−0.92**) yields. Deep tillage successfully restored productivity; however, the depth of tillage necessary was influenced by initial levels of soil strength.
Simulated Small‐Channel Bed Scour and Head Cut Erosion Rates Compared
Authors: Zhu J. C. , Gantzer C. J. , Peyton R. L. , Alberts E. E. , Anderson S. H.
Published: July 2010

Concentrated‐flow erosion is often a major part of cropland erosion. The concentrated‐flow processes of bed scour and head cut need improved characterization to better predict and prevent erosion. This study was conducted to compare the erosion rates due to simulated small‐scale bed‐scour (Db) and head‐cut (Dh) processes. A 6.4‐m‐long by 0.15‐m‐wide hydraulic flume was used to simulate concentrated‐flow erosion on five Midwestern soils: Barnes (fine‐loamy, mixed Udic Haploboroll), Forman (fine‐loamy, mixed Udic Argiboroll), Mexico (fine, montmorillonitic, mesic Udollic Ochraqualf), Sharpsburg (fine, montmorillonitic, mesic Typic Argiudoll), and Sverdrup (sandy, mixed Udic Haploboroll). For slopes of 1.5, 3.5, and 5.0%, flow rates of 3.78, 5.67, 7.65, 11.34, and 15.12 L min−1 were used to provide a range from low (0.5 Pa) to moderate (2.5 Pa) shear stresses (τ). Soil detachment rates are functions of slope, flow rate, and shear stress. Slope, flow, their squares, and the slope × flow interaction were highly significant predictors of Db. Only flow, its square, and its interaction with slope were significant predictors of Db. Nonlinear power regressions using τ as an independent variable were better predictors of detachment than simple linear regressions. Erodibility for the soils from this study does not relate well with soil erodibility calculated using the Universal Soil Loss Equation. Differences in the slope and intercept of detachment vs. τ exist among soils. The value of Db was at least four times greater than Db for all soils at equal slope and flow rate, indicating that head cutting is the main process of detachment for the conditions tested.
Rhizosphere Phosphorus Depletion Induced by Heavy Nitrogen Fertilization in Forest Nursery Soils
Authors: Teng Y. , Timmer V. R.
Published: July 2010

The cause of N‐induced P deficiency in white spruce [Picea glauca (Moench) Voss] seedlings was investigated in a greenhouse pot trial testing factorial additions of ammonium nitrate (AN) and phosphoric acid (PA) to root zone and root‐free compartmentalized soils. Plant growth was significantly improved by combined N and P topdressings during the growing season. However, N‐only fertilization induced P deficiency symptoms and reduced biomass and P status in shoots, demonstrating an apparent N antagonism on P. Phosphorus availability in this treatment also was 35% lower in the rhizosphere soil than in the root‐free bulk soil, reflecting rhizosphere P depletion. Soil acidity and Al activity were increased most by AN‐only applications, probably contributing to reduced P availability. A diagnosis of induced Al toxicity by N fertilization based on plant analysis data was supported by symptoms of root injury. Phosphorus capture by ion‐exchange resin bags at the base of the pots was lower than that of N with combined N‐P applications, suggesting rapid fixation and low mobility of P in the soil compared with N. Nitrogen‐induced rhizosphere P depletion in this nursery soil was attributed to restricted root development of seedlings due to Al toxicity, reduced P availability by Al‐phosphate precipitation, and low P replenishment because of slow diffusion from the bulk soil. Topdressing with both PA and AN increased P availability in the rhizosphere (320 mg kg−1), reduced soil extractable Al by 40% compared with AN‐alone treatments, and increased plant uptake of N and P by 270%, resulting in positive N × P interactions on plant growth.
Soil Carbon Stocks of the Brazilian Amazon Basin
Authors: Moraes Jener L. , Cerri Carlos C. , Melillo Jerry M. , Kicklighter David , Neill Christopher , Skole David L. , Steudler Paul A.
Published: July 2010

We determined stocks of C and N for soils under undisturbed vegetation across the Brazilian Amazon Basin based on 1162 soil profiles of the RADAMBRASIL survey and a digitized Brazilian soil survey map. Mean basin soil C density was 10.3 kg C m−2. Forty‐seven petagrams C and 4.4 Pg N were contained in the top 1 m of soil. Forty‐five percent of total basin soil C (21 Pg C) and 41% of total soil N (1.8 Pg N) were contained in the top 20 cm across a ≈ 5 000 000‐km2 area. Mean C/N ratio for the basin to a depth of 1 m was 10.7. Because these data represent sites with forest vegetation in the absence of significant disturbances, they represent a valuable baseline for evaluating the effects of land‐use changes on soil C stocks in the Amazon Basin.
Fitting Ammonia Volatilization Dynamics with a Logistic Equation
Authors: Demeyer P. , Hofman G. , Van Cleemput O.
Published: July 2010

To improve the interpretation of the results from NH3‐volatilization experiments, the cumulative loss rates for different treatments were fitted to a simple logistic equation. This equation is a function: Y = a(1 − e−ct)i, with Y the cumulative N loss (%). The first derivative of this function represents the daily volatilization rate and is Y′ = acie−ct(1 − e−ct)i‐1. Important parameters such as the total cumulative loss (a), and the maximum (Rm) and average (Ra) volatilization rates can easily be calculated. In the case of urea applications, an estimation can be made of the time it takes to hydrolyze all applied urea (th). This parameter also corresponds to the lag phase of the cumulative volatilization curve. Parameter i determines the position of the point of inflection of the curve. For values of i between 0 and 1, volatilization rates cannot be adequately calculated. This can be encountered if the initial volatilization rate is very high, e.g., after ammonium sulphate application upon calcareous soils. In this case, volatilization rates will be estimated by fitting the results to a modified logistic equation in which i = 1. This value of i is most common for NH4NO3 application. The best applicability of the logistic equation is with i values > 1. These values are typical for the shape of cumulative volatilization curves obtained on application of urea‐containing fertilizers. Possible applications of the logistic equation are illustrated by some experimental results.
Methane and Reduced Sulfur Gas Production by Fresh and Dried Wetland Soils
Authors: Crozier C. R. , Devai I. , DeLaune R. D.
Published: July 2010

Wetland soil microorganisms are probably influenced by fluctuating water levels and by longer‐term climatic change. This study investigated production of CH4 and reduced S gases [H2S, CH3SH, (CH3)2S, COS, and CS2] by both freshly collected and air‐dried, rewet soils from four wetlands in the Mississippi deltaic plain. Soils from three depth intervals (0–10, 10–20, and 20–30 cm) from a swamp forest (Typic Hydraquent), a freshwater marsh (Terric Medisaprist), an intermediate salinity marsh (Typic Medisaprist), and a salt marsh (Typic Medisaprist) were incubated under anaerobic conditions. At repeated time intervals, the headspace gas was analyzed and then purged with N2. Peak production rates of both CH4 and H2S occurred earlier in the fresh soils than in the dried soils in almost all cases. Presumably, this time lag associated with drying was caused by an increase in potentially reducible Mn and Fe. Nevertheless, drying increased labile C levels, thus total CH4 production was similar during the entire 69‐d incubation. Production of CH4 was positively correlated with organic matter content (fresh, R2 = 0.52; dried, R2 = 0.64) and labile C (fresh, R2 = 0.83; dried, R2 = 0.89). Due to their relatively low concentrations, inclusion of soil oxidants in a model did not significantly improve the fit of a model that already contained a term for labile C. Production of S gases was positively correlated with initial SO2−4 concentrations (fresh, R2 = 0.75; dried, R2 = 0.42). Drying significantly decreased total H2S production only for the salt marsh soils. In contrast, the drying treatment increased CH3SH production; only these soils produced (CH3)2S, COS, and CS2.
Wall Material and Capping Effects on Microlysimeter Temperatures and Evaporation
Authors: Evett S. R. , Warrick A. W. , Matthias A. D.
Published: July 2010

The microlysimeter (ML) is useful for measurements of evaporation from soil but questions persist regarding correct ML design. We studied the effects of length and wall material on evaporation and the effects of wall material and capping on ML temperatures. Cylindrical steel and plastic MLs of 10‐, 20‐, and 30‐cm lengths and 8.8‐cm o.d. were used in two field experiments on a bare Pima clay loam (fine‐silty, mixed, thermic Typic Torrifluvent). Steel MLs significantly underestimated 8‐d cumulative evaporation compared with plastic MLs for 20‐cm lengths. Steel MLs conducted heat more easily and their surfaces were significantly cooler during the day and warmer at night than either plastic MLs or the adjacent field soil. Capping the bottoms with 0.6‐cm‐thick plastic disks caused accumulation of heat in the MLs. For plastic MLs, only the 20‐ and 30‐cm lengths were long enough for continuous use for 9 d under our conditions. It was unclear if cumulative evaporation varied with length for steel MLs. We recommend that walls be constructed of material with low thermal conductivity and that end caps be designed to maximize thermal transfer between the soil inside and below the ML. A length of at least 30 cm is recommended if measurements are needed at the same location for several days.
Nitrogen Effects on Conservation of Carbon during Corn Residue Decomposition in Soil
Authors: Green C. J. , Blackmer A. M. , Horton R.
Published: July 2010

Nitrogen fertilization is widely recognized for its potential to help maintain soil organic matter (SOM) concentrations by increasing plant growth and amounts of plant material decomposed in soils. We studied possible mechanisms by which annual additions of more N than needed to maximize yields of corn (Zea mays L.) could cause losses of SOM. Various amounts of stover and NO−3 were added to a Galva silty clay loam (fine‐silty, mixed, mesic Typic Hapludoll), and NO−3 concentrations and CO2 evolution were monitored for periods up to 90 d at 24°C in the laboratory. The added NO−3 suppressed mineralization of C from SOM and stimulated mineralization of C from stover. Adjustment of rates of stover decomposition to temperature regimes normally encountered in Iowa fields after harvest showed that stover decomposition would not be complete within 1 yr and that increases in NO−3 availability decreased the amount of stover C remaining in the soil. This effect of NO−3 could explain how additions of unneeded N could decrease concentrations of SOM in long‐term studies in which residues and NO−3 are added in annual cycles. These observations suggest that practices that reduce unnecessary fertilization could help conserve SOM and reduce net amounts of CO2 released to the atmosphere.
Amelioration of Degraded Rain Forest Soils by Plantations of Native Trees
Authors: Fisher Richard F.
Published: July 2010

Empirical evidence from temperate Europe and North America indicates that old‐field succession, resulting in reforestation and a protracted period of forest fallow, can improve soil condition on lands once badly degraded. However, the ability of trees to ameliorate degraded sites in the tropics is widely debated. In 1988 eight native tree species: Hyeronima alchorneoides Allemao, Inga edulis Mart., Pentaclethra macroloba (Willd.) Ktze, Pithecellobium macradenium Pittier, Stryphnodendron microstachyum Poepp. et Endl., Virola koschnyi Warb., Vochysia guatemalensis J.D. Smith, and Vochysia ferruginea Mart. and three exotic tree species: Acacia mangium Willd., Gmelina arborea L., and Pinus tecunumanii (Schw.) Equiluz et Perry were planted on abandoned pasture land in northeastern Costa Rica cleared of rain forest 25 yr earlier. Four replicates of 0.25‐ha plots of the 11 species and a control were established in a randomized complete block design. All species survived fairly well, but some grew much better than others. The soil was thoroughly sampled before trees were planted and again 4 yr after trees were established. Although bulk density decreased significantly beneath eight of the 11 species, organic C increased significantly under only three species. Significant increases in base cations also occurred beneath the majority of species. Nitrogen did not increase as much as expected beneath the N2‐fixing legumes; however, the availability of P was significantly enhanced beneath both species of Vochysia. The changes in soil properties were dramatic, and significant amelioration of these degraded soils occurred in a short time beneath the majority of species.
Interactions of Monovalent Organic Cations with Montmorillonite: Adsorption Studies and Model Calculations
Authors: Rytwo G. , Nir S. , Margulies L.
Published: July 2010

Complexes between organic cations and clays provide a research tool and have ecological applications. The purpose of this study was to elucidate details of interactions between monovalent organic cationic dyes and montmorillonite. Interactions were studied by x‐ray diffraction and ultraviolet and infrared (IR) spectroscopies, IR linear dichroism, and adsorption isotherm measurements with model calculations. The adsorption model combined electrostatic equations with specific binding and considered neutral and positively charged complexes between surface sites and organic cations in a closed system. The model was extended to account for dye aggregation in solution. The adsorption of the dyes to montmorillonite was described by binding coefficients that were at least six orders of magnitude larger than those of inorganic cations such as Na+ and Cd++. The maximal amounts of crystal violet (CV), methylene blue (MB), and acriflavin adsorbed were 200, 150, and 175%, respectively, of the cation‐exchange capacity (CEC) of the clay mineral. The model also simulated the competition between dyes for adsorption sites. The c‐spacing of montmorillonite increased by the adsorption of CV. With MB at loadings of up to 40% of the CEC, the spacing was reduced, indicating desorption of water from the interlayer space. We conclude that MB lies preferentially parallel to the clay mineral plates, whereas CV lies at a slight inclination relative to the plates.
In Situ Determination of Detrital Breakdown in Wetland Soil‐Floodwater Profile
Authors: Schipper Louis A. , Reddy K. R.
Published: July 2010

The chemical and biological environment in which detritus undergoes breakdown changes markedly with depth through the wetland soil‐floodwater profile. However, it has been difficult to quantify the change in breakdown rate with soil depth due to a lack of suitable methodology. We modified porewater equilibrators to measure breakdown of cattail leaves (Typha sp.) at centimeter depth increments through a wetland soil‐floodwater profile. During the first week, up to 30% of the plant dry‐weight loss occurred at all depths presumably due to leaching, followed by a second phase of breakdown with a further loss of 20 to 30% of initial dry weight in 16 wk. Calculated exponential breakdown constants for the second phase of breakdown were highest in the floodwater (0.0068 d−1) and decreased linearly down the soil profile, with decay constants of <0.003 d−1 at 20 cm below the soil surface. The decrease in the rate of detrital breakdown was partly thought to be driven by decreases in O2 availability. This technique will allow future studies to better quantify organic matter dynamics in wetlands.
Water and Solute Movement in an Undisturbed, Macroporous Column: Extraction Pressure Effects
Authors: Phillips R. E. , Zeleznik J. M. , Quisenberry V. L.
Published: July 2010

Undisturbed soil columns are commonly taken to conduct water and solute flow experiments. The proper boundary condition to impose at the bottom boundary of a soil column is most difficult to define and control. The objective of this study was to evaluate the effects of the magnitude of extraction pressure at the lower boundary of the soil column and the water application rate on the amount, distribution, and percentage of leachate and solute drained through the soil column. Water and Cl− were applied to a 325 by 325 by 339 mm undisturbed soil column of a Nolin silt loam soil (fine‐silty, mixed, mesic Dystric Fluventic Eutrochrept). The soil column was placed on a collection chamber that allowed constant extraction pressures (0, − 1.3, or −2.0 kPa) to be maintained at the lower boundary of the soil column. Water containing solute was applied at rates of 5, 10, and 15 mm h−1. The leachate was partitioned into 100 cells. Flow of water and solute through macropores increased as the extraction pressure maintained at the lower boundary of the soil column increased, and as the application rate decreased. Extraction pressure had a greater effect on preferential flow than did application rate.
Predicting Field‐Scale Solute Transport Using In Situ Measurements of Soil Hydraulic Properties
Authors: van Wesenbeeck I. J. , Kachanoski R. G.
Published: July 2010

Predicting unsaturated solute transport using measured hydraulic parameters has been difficult due to the inherent variability of soil properties, and the difficulty in obtaining accurate estimates of hydraulic properties in situ. The objective of this study was to determine if in situ measurement of soil hydraulic conductivity, the α soil parameter, and the water content (θ) vs. pressure head (h) relationship could be used to predict field‐scale solute transport. A series of steady‐state solute transport experiments were conducted on a Fox sand (fine‐loamy over sandy or sandy‐skeletal, mixed, mesic Typic Hapludalf) soil in Ontario, Canada. The transport of Cl− under steady‐state water flux was monitored in three separate experiments using solution samplers. Steady‐state water flux densities applied at the soil surface were 9.72 × 10−6, 1.53 × 10−5, and 8.68 × 10−8 m s−1, respectively, for the three sites. After completion of the transport experiments at Site II, measurements of soil hydraulic conductivity and the α parameter were made using the Guelph pressure infiltrometer (GPI) beside each location and depth where solute breakthrough curves (BTCs) were measured, as well as at the soil surface. Undisturbed soil cores were taken at each location where GPI measurements were made for estimating the parameters in the θ(h) using van Genuchten's equations. The GPI‐ and core‐measured hydraulic parameters obtained at Site II were used to predict the field‐scale solute travel time probability density function (PDF) at the same site, and at Sites I and III using a stochastic‐convective model. Observed solute travel time PDFs were predicted quite well at high surface water fluxes, which were close to the field saturated hydraulic conductivity, Kfs by both the GPI and core methods. Both methods underpredicted the variability of the observed travel time PDF.
Determination of Rate Coefficients for Potassium‐Calcium Exchange on Vermiculite using a Stirred‐Flow Chamber
Authors: Bar‐Tal Asher , Eick Matthew J. , Feigenbaum Sala , Sparks Donald L. , Fishman Svetlana
Published: July 2010

A stirred‐flow technique has been widely used to study the rate of ion adsorption on clays and soils. However, no mathematical analysis of chemical kinetics combined with mass transport has been used to derive rate coefficients for ion exchange phenomena. Accordingly, the objectives of this study were to: (i) develop a mathematical model of the kinetics of an elementary cation‐exchange reaction combined with transport processes in a stirred‐flow chamber and (ii) determine rate coefficients based on this model for K‐Ca exchange on a vermiculite. The kinetics of exchange were described by a second‐order mechanistic rate law from which the equilibrium exchange equation was derived. The second‐order mechanistic rate law combined with an equation of mass balance describing transport were solved numerically. The equilibrium K‐Ca exchange data were described by the Gaines and Thomas approach with a thermodynamic exchange equilibrium constant Kex = 754.6 L mol−1. The values of the apparent adsorption rate coefficient (ka) ranged from 84 to 185 L2 mol−2 min−1, as influent rate increased from 0.3 to 2.0 mL min−1. The calculated K‐Ca exchange half‐life (t1/2) ranged from 0.5 to 1.5 min, in agreement with published data based on batch methods.
Earthworm (Lumbricus rubellus and Aporrectodea caliginosa) Effects on Carbon Flux in Soil
Authors: Zhang Q. L. , Hendrix P. F.
Published: July 2010

Effects of earthworm activities on litter and soil C flux were studied in a laboratory incubation experiment using two types of isotopic tracers and two earthworms with different ecological strategies‐Lumbricus rubellus, an epigeic species, and Aporrectodea caliginosa, an endogeic species. The soil was prelabeled with 14C. Dry sorghum [Sorghum bicolor (L.) Moench] leaves labeled with 13C were applied to the soil surface. Activity of both earthworm species significantly (P < 0.001) enhanced total C efflux (479 ± 8 [standard error], 483 ± 4, and 395 ± 5 mg C jar−1 for L. rubellus, A. caliginosa, and the control, respectively) and significantly (P < 0.05) reduced total surface soil microbial biomass (251.7, 205.2, and 312.1 mg C kg−1 soil for L. rubellus, A. caliginosa, and the control, respectively) during the 30‐d incubation. Activity of A. caliginosa also reduced subsurface soil microbial biomass. The epigeic earthworms assimilated significantly more 13C from the litter and significantly less 14C from the soil than the endogeic species. In the absence of earthworms, 14C in the soil was translocated into the surface litter, as shown by a 15.5‐fold increase in 14C enrichment in the surface litter by the end of the experiment. This translocation of soil C into the litter was significantly reduced by earthworm activities (155.43, 121.11, and 240.58 kBq kg−1 litter for L. rubellus, A. caliginosa, and the control, respectively), possibly due to disruption by earthworms of fungal‐hyphal connections between litter and soil. These interactions between earthworms and soil microbial processes have important implications for soil C turnover.
Seasonal Soil Nitrate Dynamics in Corn as Affected by Tillage and Nitrogen Source
Authors: Dou Z. , Fox R. H. , Toth J. D.
Published: July 2010

A better understanding of seasonal soil NO−3 dynamics in corn (Zea mays L.) as affected by different tillage and N sources is essential to refine N management options for more profitable production and less negative impact on water resources. We conducted an experiment in central Pennsylvania to study seasonal soil NO−3 dynamics in corn managed with conventional tillage (CT) or no‐till (NT) and supplied with commercial N fertilizer or legume green manures of red clover (Trifolium pratense L.) or hairy vetch (Vicia villosa Roth) double‐cropped with winter wheat (Triticum aestivum L.). The legume cover crops and winter fallow after the wheat harvest were followed by two corn crops. Soil samples taken biweekly during the corn growing seasons were analyzed for NO−3 concentrations in the 0‐ to 25‐ (Ap horizon) and 25‐ to 45‐cm (upper B horizon) soil layers. In the first corn crop, soil NO−3 concentrations peaked about 4 wk after planting in all treatments. Where no broadcast N fertilizer was applied, mid‐ to late‐season soil NO−3‐N concentrations in the Ap horizon averaged 6, 25, and 1.3 mg kg−1 in the clover CT, vetch CT, and fallow CT treatments, respectively, while the NO−3‐N concentrations in all NT 0 N treatments were similar (1.3–1.9 mg kg−1). After the first corn harvest, the clover and vetch CT 0 N treatments had 113 and 124 kg NO−3‐N ha−1 in the 120‐cm soil profile, slightly higher than that estimated for the fallow treatment with N fertilizer at the economic optimum rate (99 kg NO−3‐N ha−1). No‐tillage also reduced the amount of NO−3 accumulated in the 0‐ to 120‐cm soil profile to one‐half of the NO−3 levels in the corresponding CT treatments, regardless of N source. Both tillage and N source strongly affected seasonal soil NO−3 dynamics in the first corn season, with the major difference appearing in the Ap horizon.
Experimental Formation of Aquod‐like Features in Sandy Coastal Plain Soils
Authors: Harris W. G. , Crownover S. H. , Comerford N. B.
Published: July 2010

Coastal‐plain Spodosols of the southeastern USA are inexplicably associated with fluctuating water tables. This study tested hypotheses that account for water‐table influence in Aquod genesis: (i) sand‐grain coatings are released when Fe (a binding agent) is dissolved by organic acids; (ii) coatings migrate to form an eluvial‐illuvial sequence; (iii) C and metals accumulate in the illuvial matrix; (iv) imperfectly drained sands are predisposed to podzolization due to low crystalline Fe; and (v) a minimum solution residence time near the site of acid release is required. Toeslope and summit positions on hillslopes were sampled and subjected to physical, chemical, and mineralogical characterization. Iron dissolution at increasing loading rates of oxalic acid was monitored by batch extractions. A column experiment was conducted to test predisposition and residence‐time effects on formation of Aquod‐like features and on effluent composition. Columns were eluted daily with oxalic acid (0.001, 0.05, and 0.01 M) under freely drained or 22‐h saturation conditions. Ratios of iron/aluminum in oxalic acid extracts were higher at higher loading rates. Iron abundance and crystallinity increased from toeslopes to summits. Aquod‐like features (light eluvial zones, darkened lamellae, etc.) developed most conspicuously in toeslope columns, and only for the 22‐h residence treatment. Columns most affected released less Fe, Al, and oxalic acid. Results supported our hypotheses for laboratory conditions. Further study is needed using in situ methods and indigenous solution components.
Soil Properties beneath Ceanothus and Pine Stands in the Eastern Sierra Nevada
Authors: Johnson Dale W.
Published: July 2010

Soils and soil solutions were collected from adjacent ceanothus (Ceanothus velutinus Dougl.) and Jeffrey pine (Pinus jeffreyi Grev. & Balf) stands in Little Valley, Nevada. It was hypothesized that excessive N fixation by ceanothus, like red alder (Alnus rubra Bong.), causes accelerated nitrification, NO−3 leaching, and soil acidification. This hypothesis was not supported. Comparisons of soils from five paired ceanothus‐pine plots showed that ceanothus soils not only had greater C and N concentrations, but also greater cation‐exchange capacity, base saturation, and exchangeable Ca2+, Mg2+, and K+ than pine soils. There were no significant differences in extractable P or SO2−4 overall, although there were differences at some sites. Extractable P levels were quite high relative to more humid forest soils, whereas extractable SO2−4 levels were very low. Soil solutions from ceanothus, pine, and a riparian mountain alder (Alnus tenuifolia Nutt.) stand were characterized by low NO−3 concentrations, circumneutral pH, and high base cation, HCO−3, SO2−4, Cl−, and Si concentrations. The chemical composition of nearby streamwaters was similar to that of soil solutions with respect to NO−3, pH, HCO−3, and base cation and Si concentrations. Streamwater SO2−4 and Cl− concentrations (≈ 3–17 µmolc L−1) were more than an order of magnitude lower than those in soil solutions (≈ 150–270 µmolc L−1), however, indicating that soils adjacent to the stream had little influence on its chemistry. Thus, the NO−3 pulse (from <1 to ≈ 30 µmolc L−1) measured in both streamwaters and ceanothus soil solutions during snowmelt most likely originated from the melting snowpack.
Nitrite Transformations during Soil Extraction with Potassium Chloride
Authors: Stevens R. J. , Laughlin R. J.
Published: July 2010

Nitrite is a key intermediate in both NH+4 oxidation and NO−3 reduction. There is increasing concern over its origin and significance in food and water. Measuring NO−2 concentrations in soils requires extraction procedures, KCl (2 M) being the most commonly used extractant. We investigated the effect of soil/extractant ratio, extraction procedure, shaking time, and pH on the transformations of 15NO−2 from a range of grassland soils. We measured the recovery of NO−2 and 15N as 15NO−2 or 15NO−3. Nitrite destruction and pool dilution occurred at 1:10 and 1:1 soil/extractant ratios. Using a 1:1 soil/extractant ratio, average 15NO−2 recovery for six soils was 86% with a blending procedure (10 min), and 32% with a shaking procedure (70 min). About two‐thirds of the 15NO−2 lost during extraction appeared as 15NO−3. When the pH of the soil suspension was adjusted with KOH to 7.8, recovery of NO−2 was not significantly less than 100% for reaction times up to 40 min. Pool dilution and decomposition of NO−2 appeared to be due to chemical rather than microbial reactions. When 20 soils were tested using the blending procedure and extractant pH of 8.0, 14 gave recoveries >97% and an additional six gave recoveries >90%. One of the mineral soils giving >97% recovery was extracted at two pH values, and NO−2 stability measured in filtered extracts during 28 d of storage at 4°C. Nitrite was more stable in extracts at pH 5.6 than 8.0. To ensure at least 95% recovery of 15NO−2, extracts at pH 8.0 had to be filtered through a 0.7‐µm filter and analyzed within 2 d of extraction.
Influence of pH on Phosphorus Retention in Oxidized Lake Sediments
Authors: Olila O. G. , Reddy K. R.
Published: July 2010

Diel pH changes in lake waters resulting from high photosynthetic activity may regulate water‐soluble P concentration (WSP) and P sorption by suspended sediments in shallow eutrophic lakes. Laboratory studies were conducted to determine the pH effect on P fractions and P sorption kinetics in oxidized sediment suspensions from two subtropical lakes (Lake Apopka and Lake Okeechobee, Florida). The P sorption rate was calculated for sediment suspensions adjusted to various pH levels: 6.5, 7.0, 8.5, 9.5, and 10.5 for Lake Apopka and 6.5, 7.0, 8.5, 9.5, and 10.5 for Lake Okeechobee. A decrease in pH increased the WSP concentrations in Lake Apopka sediment suspensions but had no effect on WSP concentrations in Lake Okeechobee sediment suspensions. Lake Apopka sediment suspensions at pH 7.0 (ambient) and below did not show net P uptake. Phosphorus uptake for Lake Apopka occurred only when pH was increased to ≥8.5 and when P treatments were increased to ≥27 mmol P kg−1, which resulted in supersaturation with respect to octacalcium phosphate. Phosphate solubility diagrams and mineral equilibria calculations suggest that P uptake by Lake Apopka sediment suspensions at pH ≥8.5 was due to P coprecipitation with CaCO3 and probable formation of nonapatitic Ca‐P compounds. Phosphorus sorption on Lake Okeechobee sediment suspensions followed first‐order kinetics for all pH levels studied, with rate constants (k) ranging from 0.003 to 0.75 h−1. High P uptake by Lake Okeechobee sediment suspensions could be attributed to two reactive components: (i) amorphous or poorly crystalline Fe and Al oxyhydroxides at pH <7.5, and (ii) Ca/Mg carbonates and other minerals at pH ≥7.5.
Precision of Neutron Scattering and Capacitance Type Soil Water Content Gauges from Field Calibration
Authors: Evett S. R. , Steiner J. L.
Published: July 2010

Soil water content gauges based on neutron scattering (NS) have been a valuable tool for soil water investigations for some 40 yr. However, licensing, training, and safety regulations pertaining to the radioactive source in these gauges makes their use expensive and prevents use in some situations such as unattended monitoring. A capacitance probe (CP) gauge has characteristics that would seem to make it an ideal replacement for NS gauges. We determined the relative precision of two brands of NS gauges (three gauges of each) and a brand of CP gauge (four gauges) in a field calibration exercise. Both brands of NS gauges were calibrated vs. volumetric soil water content with coefficients of determination (r2) ranging from 0.97 to 0.99 and root mean squared errors (RMSE) <0.012 m3 m−3 water content. Calibrations for the CP gauges resulted in r2 ranging from 0.68 to 0.71 and RMSE of 0.036 m3 m−3 water content. Average 95% confidence intervals on predictions were three to five times higher for the CP gauges than for the NS gauges, ranging from 0.153 to 0.161 and 0.032 to 0.052 m3 m−3, respectively. Although poorly correlated with soil water content, readings were reproducible among the four CP gauges. The poor correlation for CP gauges may be due to small‐scale soil water content variations within the measurement volume of the gauge. The NS gauges provide acceptable precision but the CP gauge has poor precision and is unacceptable for routine soil water content measurements.
Phosphorus Sorption at Field Capacity and Soil Ionic Strength: Kinetics and Transformation
Authors: Agbenin J. O. , Tiessen H.
Published: July 2010

An array of kinetic equations has been used to describe time‐dependent sorption of P by soils. In these studies, soil/solution ratios and ionic strengths of the equilibrating solutions did not mimic real field conditions. We investigated the time‐dependent P sorption and transformation of five benchmark soils from the semiarid part of northeast Brazil with the aid of rival kinetic models and sequential extraction. Our objectives were to ascertain whether commonly employed kinetic models can describe slow P sorption at field moisture capacity and soil ionic strength, and to determine the changes in labile and nonlabile P with time. The time‐dependent P sorption was described by the rival kinetic models in the order: Elovich equation = fractional power function > apparent first order > parabolic diffusion, all of which are indicative of diffusion kinetics with a slow chemical reaction, involving possible ligand exchange between H2PO−4 or HPO2−4 and OH−. Sequential P extraction by the modified Hedley procedure indicated that 60 to 90% of sorbed P was in the labile form. Anion‐exchange resin extractable P (AER‐P) correlated negatively with dithionite‐citrate‐extractable AL (ALd). An improved correlation between AER‐P and Ald with the time (r = −0.53 [P = 0.05] at 10 d, r = −0.55 [P = 0.01] at 100 d, and r = −0.60 [P = 0.01] at 200 d) is consistent with diffusion kinetics. Most likely, the slow P sorption involved surface‐adsorbed P diffusing to vacant sites in the micropores of Al oxides or Al‐substituted Fe oxides. Such internal sites may not be directly accessible to soil solution P.
Heavy Metal Release from a Serpentine Soil Using a pH‐Stat Technique
Authors: Kaupenjohann Martin , Wilcke Wolfgang
Published: July 2010

Serpentine soils show high contents of Cr and Ni. Acidification may convert the metals bound in silicate structures into bioavailable forms. The effects of mineral acids on a serpentine‐derived Typic Hapludoll were examined in column studies. Cadmium and Pb were also examined. One end of the soil‐packed column was placed in 1 mM HCl solution and constant pH was maintained by pumping the acid through a H‐saturated cation‐exchange resin. The experiment was conducted for 100 d, after which the soil column was vertically sectioned. Chemical and mineralogical analyses showed steep vertical gradients in the sectioned columns. Base saturation decreased by about 60% at the contact of the column with the acid; exchangeable Fe, Cr, Pb, and Al increased while Ni decreased. Cadmium exhibited a differential behavior in that it increased in concentration in the middle of the column. Extracts using ethylenediaminetetraacetic acid (EDTA) gave similar results. X‐ray diffraction indicated chlorite dissolution by acid loading. The resin‐based pH‐stat technique is a useful method in simulating soil weathering.
CERES‐N Model Predictions of Nitrogen Mineralized from Cover Crop Residues
Authors: Quemada M. , Cabrera M. L.
Published: July 2010

Winter annual cover crops, widely used in no‐till systems, can be an important source of N for the subsequent crop. Because many factors affect net N mineralization from cover crop residues, computer models can be powerful tools to predict it. The CERES models, which are some of the most widespread models for simulating the whole crop‐soil system, have a common submodel (CERES‐N) that describes N transformations. The objectives of this study were to determine decay rate constants under nonlimiting conditions for the carbohydrates and cellulose pools (CARB and CELL) of CERES‐N for residues that decompose on the soil surface, and to test if two modifications to CERES‐N could improve the simulation of N mineralization. The two modifications were to: (i) allow the user to vary the relative size of the residue pools (CARB, CELL, and lignin), and (ii) allow stems and leaves to decompose separately, having a common point of interaction through the inorganic N pool. Results of a 6‐mo laboratory incubation experiment with four cover crop residues were used to adjust rate constants and test the effect of model modifications. The decay rates obtained were 0.14 and 0.0034 d−1 for CARB and CELL, respectively. Allowing the user to vary the relative size of residue pools greatly improved the simulation of net N mineralized (root mean square error of the model decreased from 1.0 to 0.28 g m−2), whereas modeling the separate decomposition of leaves and stems only caused a slight improvement in the prediction of net N mineralized.
Residue Decomposition Effects on Nitrogen Availability to Corn following Corn or Soybean
Authors: Green C. J. , Blackmer A. M.
Published: July 2010

Rates of N fertilization required to attain maximum yields of corn (Zea mays L.) usually are less for corn grown after soybean [Glycine max (L.) Merr.] than for corn grown after corn, but the reason for the difference in N fertilizer requirement has not been clearly established. We studied immobilization and mineralization induced by various management histories and residue treatments to learn about the causes of differences in N fertilizer requirements. Soil samples were collected at five sites in the fall of 1990 from plots that had been planted to soybean and plots that had been planted to corn receiving various N rates. The soil samples were incubated for 42 wk after treatment with 15N‐labeled NO−3 and with crop residues. All treatments showed a period of net immobilization followed by a period of net mineralization. Net amounts of N immobilization induced by soybean residue were approximately equal to those induced by corn residue from plots with the higher rates of fertilization. However, the soybean‐induced immobilization was much more rapid. Rates of mineralization of nonlabeled N were not greater from soils having soybean residue than from soils having corn residue. Because soybean produces less residue under field conditions, the results suggest that differences in N fertilizer requirement are better explained by differences in amounts of N immobilized during residue decomposition than by mineralization of biologically fixed N associated with the soybean.